Selection of Prebiotic Molecules in Amphiphilic Environments

Transcription

1 life Artile Seletion Prebioti Moleules in Amphiphili Environments Christian Mayer 1, *, Ulrih Shreiber 2 María J. Dávila 2 1 Institute Physial Chemistry, CENIDE University Duisburg-Essen, Essen, Germany 2 Department Geology, University Duisburg-Essen, Essen, Germany; ulrih.shreiber@uni-due.de (U.S.); maria.davila@uni-due.de (M.J.D.) * Correspondene: hristian.mayer@uni-due.de, Tel.: Aademi Editor: William Bains Reeived: 1 Deember 2016; Aepted: 4 January 2017; Published: 7 January 2017 Abstrat: A basi problem in all postulated pathways prebioti hemistry is low onentration whih generally is expeted interesting reatants in fluid environments. Even though ompounds, like nuleobases, sugars or peptides, prinipally may m spontaneously under environmental onditions, y will always be rapidly diluted in an aqueous environment. In addition, any suh reation leads to side produts whih ten exeed desired ompound generally hamper first steps a subsequent moleular evolution. Theree, a mehanism seletion aumulation relevant prebioti ompounds seems to be ruial moleular evolution. A very effiient environment seletion aumulation an be found in fluid ontinuum irulating in tetoni fault zones. Vesiles whih m spontaneously at a depth approximately 1 km present a seletive trap amphiphili moleules, espeially peptides omposed hydrophili hydrophobi amino aids in a suitable sequene. The aumulation effet is shown in a numeri simulation on a simplified model. Furr, possible mehanisms a moleular evolution in vesile membranes are disussed. Altoger, proposed senario an be seen as an ideal environment onstant, undisturbed moleular evolution in on ell-like ompartments. Keywords: origin life; seletion; aumulation; prebioti hemistry; moleular evolution; vesiles 1. Introdution In past researh dealing with origin life, numerous natural soures prebioti hemistry have been identified [1]. This inludes potential reations in early terrestrial atmosphere [2,3], geormal hemistry in Earth s rust [4,5] as well as extraterrestrial soures [6 8]. In pratially all se ases, interesting moleular speies are med in very low quantities in an extremely diluted state. Often, y are also aompanied large varieties huge amounts side produts whih generally are meaningless or even obstrutive furr steps moleular evolution. So basially, two onditions had to be fulfilled in order to allow primary steps an evolution proess: (i) seletion (ii) aumulation prebioti ompounds whih med original basis life. Among those two problems, aumulation seems to be easier one to solve. In simplest ase, a phase transition (e.g., evaporation or freezing water) leads to preipitation dissolved omponents. The degree aumulation during suh a step depends on original solubility may easily inrease onentration a given speies over several orders magnitude. The ondition moleular seletion is muh harder to fulfill. Side produts ten exhibit similar moleular properties as desired omponent. Theree, it is not easy to imagine a physial proess whih separates tiny amount relevant produt from large amount waste. In many ases, a omplex mixture undesired omponents may even preipitate ause a dead-end senario Life 2017, 7, 3; doi: /life

2 Life 2017, 7, Life 2017, 7, ases, a omplex mixture undesired omponents may even preipitate ause a dead-end senario loal evolution proess. This event has been referred to as asphalt problem in literature loal evolution [9] proess. presents This a serious event has drawbak been referred many to asproposed asphaltmehanisms problem in literature moleular [9] evolution. presents a serious drawbak many proposed mehanisms moleular evolution. A possible solution to toboth problems is isa natural environment whih ats in insome respet similar to a hromatographi olumn: olumn: system system hannels hannels avities avities being being med med tetoni tetoni fault zones fault zones in ontinental ontinental rust (Figure rust (Figure 1). 1). surfae -1 km liquid water + sub-ritial CO 2 onstant material flow fed ontinuous rise hydrormal hemistry moleular evolution liquid water + super-ritial CO 2 seletion separation prebioti maromoleules Figure 1. Shemati depition moleular evolution in tetoni fault zone. onstant supply Figure 1. Shemati depition moleular evolution in a tetoni fault zone. A onstant supply hydrormal produts pass through zone vesile mation (h 1 km). In this vesile zone, hydrormal produts pass through a zone vesile mation (h = 1 km). In this vesile zone, prebioti prebioti moleules moleules whih whih integrate integrate into into vesile vesile membranes membranes are are seleted seleted aumulated. aumulated. A periodi periodi proess proess vesile vesile mation mation presents presents perfet perfet senario senario subsequent subsequent moleular moleular evolution evolution (right). (right). The fluid ontent, primarily onsisting a mixture liquid water gaseous (g) or The fluid ontent, primarily onsisting a mixture liquid water gaseous (g) or superritial superritial (s) arbon dioxide ats like a mobile phase, while organi preipitates mesosopi (s) arbon dioxide ats like a mobile phase, while organi preipitates mesosopi strutures strutures (med at depths where s-g transition ours) represent relatively immobile (med at depths where s-g transition ours) represent relatively immobile ounterpart. ounterpart. Regarding seletion aumulation prebioti ompounds, this system Regarding seletion aumulation prebioti ompounds, this system ombines some ombines some intriguing properties: intriguing properties: (a) It is onstantly supplied with a huge amount a huge variety hydrormally med (a) ompounds It is onstantly from supplied geologial with soures. a huge amount a huge variety hydrormally med (b) It ompounds is well proteted from geologial against potentially soures. destrutive influenes suh as variable wear onditions (b) or It is UV well radiation. proteted against potentially destrutive influenes suh as variable wear onditions () It orprovides UV radiation. onstant onditions over long periods time (suh as thouss years), here () allowing It provides onstant undisturbed onditions moleular overevolution. long periods time (suh as thouss years), here (d) It allowing generates strutures undisturbed whih moleular are highly evolution. seletive amphiphili ompounds suh as lipids or (d) peptides It generates omposed strutures hydrophili whih are highly hydrophobi seletive amino amphiphili aids in ompounds a suitable sequene. suh as lipids or (e) The peptides system omposed has aess hydrophili to surfae allowing hydrophobi produts amino aids in a suitable moleular sequene. strutural (e) evolution The system to spread has aess out into new surfae environments, allowingespeially produts on near moleular terrestrial surfae. strutural With evolution se to advantages spread out into new mind, environments, we want to espeially propose ona possible near senario terrestrial surfae. seletion, aumulation evolution whih starts with hydrormal hemistry in a highly diluted state, With se advantages in mind, we want to propose a possible senario seletion, aumulation inludes mation mesosopi strutures whih absorb aumulate a ertain fration evolution whih starts with hydrormal hemistry in a highly diluted state, inludes intermediate produts, allows highly produtive periodi proesses. mation mesosopi strutures whih absorb aumulate a ertain fration intermediate produts, allows highly produtive periodi proesses. 2. Tetoni Fault Zones A Desription Environment 2. Tetoni Fault fault Zones A zones onsist Desription a huge network Environment interonneted raks avities [10,11]. Well known Tetoni examples fault zones tetoni onsist fault zones a huge are low-temperature network interonneted hydrormal raks systems avities with extensive [10,11]. gas Well flux known in examples ontinental rust. tetoni Their fault fluid zones ontent are low-temperature inludes superritial hydrormal subritial systems water with as well extensive as superritial gas flux in subritial ontinental gases, rust. primarily Their fluid arbon ontent dioxide. inludes In addition, superritial y are onstantly subritial supplied water as well with as organi superritial building bloks subritial hydrormal gases, primarily soures. arbon An example dioxide. In suh addition, a proess y is are

3 Life 2017, 7, onstantly supplied with organi building bloks hydrormal soures. An example suh a proess is synsis amino aids whih is very likely to our under given irumstanes [12 14]. Regarding reation onditions, tetoni fault zones fer a large range temperatures pressures depending on depth reation environment. Below a depth approximately 1 km (with T > 304 K p > 74 bar), arbon dioxide will neessarily exist in superritial state [15]. At levels above 8 km, water is expeted to our in liquid state. In between, both omponents toger neessarily m a two-phase system fering a non-polar phase (superritial arbon dioxide saturated with water) a polar one (liquid water saturated with arbon dioxide). This two-phase system provides exellent onditions synti organi hemistry, espeially mation amphiphili ompounds at phase boundary. At upper limit this area (near 1 km), rising arbon dioxide undergoes phase transition from superritial to gaseous state. At this point, ability arbon dioxide to at as a solvent will drastially derease most less polar or amphiphili organi onstituents will preipitate. This region an be seen as an aumulation zone suh ompounds, leading to loations with elevated onentrations interesting prebioti building bloks. 3. Vesile Formation Reently, we established a mehanism vesile mation whih is expeted to our in tetoni fault zones in presene water CO 2 [11]. At a depth approximately 1 km, pressure temperature onditions indue a loal phase transition between superritial CO 2 (sco 2 ) subritial gaseous CO 2 (gco 2 ). The unertainty regarding preise depth derives from variable omposition gas mixture from fat that density liquid filling vertial hannels depends on volume fration CO 2 bubbles whih an vary over time. Various amphiphili produts, eir med Fisher-Tropsh like hemistry under oxidation terminal end group aliphati hains or ondensation reations with glyerol [16], are expeted to aumulate at this point due to solubility drop in CO 2 presene large transition-indued interfaes. With additional periodi pressure variations resulting from tidal influenes or geyser phenomena [17], a yli proess an our in whih transition sco 2 gco 2 indues mation water droplets overed a monolayer amphiphili ompounds [18]. When migrating through interfae to aqueous domain (whih itself is overed a layer amphiphiles), droplets turn into vesiles with a bilayer membrane [19]. Being rmodynamially unstable, vesiles are expeted to disintegrate release ir organi ontents into bulk water phase over time. During transition gco 2 sco 2, organi onstituents water again beome soluble in CO 2 phase yle an start again [11]. This leads to a ontinuous proess repetitive ompartmentalization subsequent remixing water-soluble organi omponents whih ours at a well-defined depth inside Earth s rust. The ompartments would likely have lifetimes between minutes days allow extended reations to our under variable internal onditions. The transient state enapsulation is espeially interesting as re are experimental indiations that vesiles failitate mation peptides [20,21] ause ir seletive immobilization [22]. The desribed proess vesile mation may also solve onentration problem (whih basially onsists in onstant dilution water soluble organi ompounds): while an inreased onentration prebioti moleules is already expeted at aumulation zone, proess droplet mation leads to furr aumulation prebioti moleules due to low solubility organi ompounds in gco 2 [11,23]. Experimental evidene vesile mation was gained from systemati studies using a high-pressure ell filled with water/sco 2 in presene phospholipid as a model amphiphile. A pressure yling results in mation vesiles whih were haraterized optial mirosopy field-gradient nulear magneti resonane spetrosopy (PFG-NMR) [11]. The size vesiles differs between one 60 µm. Some vesiles obviously show multilamellar struture

4 Life 2017, 7, ontain internal vesiles, both features possibly deriving from fusion droplets in gas phase Life 2017, 7, (Figure 2). The vesile struture med bilayers amphiphili moleules ould furr be supported PFG-NMR measurements whih showed presene enapsulated water moleules inside membranes relatively low permeability [11]. Figure 2. Vesiles med in high-pressure ell filled with two-phase mixture water sco2 Figure 2. Vesiles med in a high-pressure ell filled with a two-phase mixture water sco 2 ontaining small amounts phospholipid as model amphiphile. The pressure temperature ontaining small amounts phospholipid as a model amphiphile. The pressure temperature onditions as well as omposition bulk medium orrespond to those expeted in tetoni onditions as well as omposition bulk medium orrespond to those expeted in a tetoni fault zone in depth km. fault zone in a depth 1 km. 4. Aumulation Seletion Moleules in Vesile Membranes 4. Aumulation Seletion Moleules in Vesile Membranes With desribed mehanism vesile mation inside tetoni fault zone, we an With desribed mehanism vesile mation inside tetoni fault zone, we an postulate postulate ourrene a layer with a relatively high onentration vesiles at a depth 1 km ourrene a layer with a relatively high onentration vesiles at a depth 1 km at at temperatures between K pressures near 70 bar. In following, this region will temperatures between K pressures near 70 bar. In following, this region will be be referred to as vesile zone. referred to as vesile zone. In some respet, vesiles in this layer an be ompared to stati phase hromatographi In some respet, vesiles in this layer an be ompared to a stati phase a hromatographi olumn: bilayer membranes dispersed vesiles speifially absorb moleules with olumn: bilayer membranes dispersed vesiles speifially absorb moleules with amphiphili properties, re seleting aumulating m. Moleules whih are apable amphiphili properties, re seleting aumulating m. Moleules whih are apable integrating into bilayer struture will ombine with present olletion amphiphiles, ming integrating into bilayer struture will ombine with present olletion amphiphiles, ming heterogeneous membrane inreasing omplexity. Even though individual vesiles will a heterogeneous membrane inreasing omplexity. Even though individual vesiles will disintegrate disintegrate after a given period time, bilayer-ompatible moleules will remain in vesile zone after a given period time, bilayer-ompatible moleules will remain in vesile zone as y as y have potential to esape into bulk phase to rapidly integrate into anor intat have potential to esape into bulk phase to rapidly integrate into anor intat vesile. vesile. Consequently, vesile zone ms a stati environment, even though individual vesiles Consequently, vesile zone ms a stati environment, even though individual vesiles may be may be mobile or unstable. mobile or unstable. Within this environment, potential prebioti moleules will undergo hemial reations will Within this environment, potential prebioti moleules will undergo hemial reations will hange ir properties. As soon as y lose ir amphiphili harater, y will also lose ir hange ir properties. As soon as y lose ir amphiphili harater, y will also lose ir ability ability to remain in vesile zone. If y turn into a hydrophili speimen, y are likely to be to remain in vesile zone. If y turn into a hydrophili speimen, y are likely to be eluted eluted aqueous phase. If instead y gain hydrophobi properties, ir solubility in aqueous phase. If instead y gain hydrophobi properties, ir solubility in superritial superritial arbon dioxide phase will inrease y will be eluted in those periods time with arbon dioxide phase will inrease y will be eluted in those periods time with inreased inreased pressure when sco2 ours. pressure when sco 2 ours. The potential this seletion mehanism is espeially high in ase peptides whih are The potential this seletion mehanism is espeially high in ase peptides whih are being being med from raemi amino aids under hydrormal onditions [24]. The seletivity med from raemi amino aids under hydrormal onditions [24]. The seletivity hydrophobi hydrophobi peptides has already gained early reognition [22]. However, seletion effet may peptides has already gained early reognition [22]. However, seletion effet may even be stronger even be stronger those peptides whih, with a suitable omposition sequene amino aids, those peptides whih, with a suitable omposition sequene amino aids, are amphiphili are amphiphili reflet spatial distribution hydrophiliity hydrophobiity reflet spatial distribution hydrophiliity hydrophobiity bilayer membrane. bilayer membrane. Preliminary experiments have shown that peptides are being med from amino aids in Preliminary experiments have shown that peptides are being med from amino aids in same environment where vesile yle ours. Under se irumstanes, an aumulation same environment where vesile yle ours. Under se irumstanes, an aumulation amphiphili peptides will our aording to sheme shown in Figure 3. amphiphili peptides will our aording to sheme shown in Figure 3.

5 Life 2017, 7, Life 2017, 7, sco 2 H 2 O onstant supply highly diluted hydrormal produts hydrolysis aumulation hydrophili amphiphili hydrophobi removal sco 2 onstant removal rejeted produts Figure Figure Mehanism Mehanism peptide peptide seletion aumulation in in presene vesiles. Left: Left: Peptide Peptide hains hains med med hydrophili hydrophili amino amino aids aids (blue irles) will will undergo undergo little little interation interation with with vesiles vesiles remain remain in in aqueous aqueous phase phase where where y y undergo undergo hydrolysis. hydrolysis. Right: Right: Peptide Peptide hains hains med med hydrophobi amino aids (red irles) will eventually be eluted sco2. Center: Amphiphili hydrophobi amino aids (red irles) will eventually be eluted sco 2. Center: Amphiphili peptides will aumulate in bilayer membrane remain partially proteted against hydrolysis peptides will aumulate in bilayer membrane remain partially proteted against hydrolysis elution. elution. Basially, three different types oligomers an be lassified aording to ir behavior in system: Basially, three different types oligomers an be lassified aording to ir behavior in system: (1) Hydrophili amino aids primarily hydrophili oligopeptides (left in Figure 3) will (1) Hydrophili primarily remain aminodissolved aids in primarily aqueous hydrophili phase. Here, y oligopeptides are part (left a series in Figure onneted 3) will primarily ondensation hydrolysis remain dissolvedequilibria in aqueous where oligomers phase. Here, with y inreasing are parthain a series length our onneted in ondensation hydrolysis dereasing onentration. equilibria Regarding where long-term oligomers ontribution with inreasing this fration hain length moleules, our it in dereasing an be onsidered onentration. as a material Regarding reservoir long-term amphiphili ontribution amino aids. this Regarding fration moleules, mation it longer hains, this fration suffers from water problem [9]. an be onsidered as a material reservoir amphiphili amino aids. Regarding mation (2) Hydrophobi amino aids primarily hydrophobi oligopeptides (right in Figure 3) will have longer hains, this fration suffers from water problem [9]. a higher tendeny to dissolve in sco2 phase. Even though its solubility in sco2 may be (2) Hydrophobi amino aids primarily hydrophobi oligopeptides (right in Figure 3) will have a very small in terms numbers, huge amount arbon dioxide passing at water higher tendeny to dissolve in sco interfae an arry a substantial amount 2 phase. Even though its solubility in sco over time. With high mobility superritial 2 may be very small medium, in terms se frations numbers, are likely huge to amount be separated arbon dioxide to end passing up at positions at where water arbon interfae an dioxide arry is a substantial undergoing amount transition over towards time. With gaseous highstate. mobility The removal superritial this fration medium, is se important frations as it are prevents likely to bemation separated insoluble to end up hydrophobi at positions residue wherewhih arbonorwise dioxide is undergoing ould blok transition furr reations. towardswithout gaseous a given state. seletivity, The removal this produt this mixture fration islikely important to gain as it prevents in omplexity mation to lead anto insoluble a dead end hydrophobi whih has residue been referred whih orwise to as asphalt ould blok problem furr reations. Steven Benner Without [9]. a given seletivity, this produt mixture is likely to gain in omplexity to (3) lead Peptides to a dead med endfrom whih a ombination has been referred hydrophili to as asphalt hydrophobi problem amino Steven aids Benner may develop [9]. (3) Peptides a tendeny med to integrate from a ombination into bilayer membranes hydrophili existing hydrophobi vesiles (enter amino in aids Figure may3). develop This a requires tendeny a streth to integrate hydrophobi into amino bilayeraids membranes whih is long existing enough to vesiles fit into (enter aliphati in Figure hain 3). This region requires a bilayer streth toger hydrophobi with at amino least one aids hydrophili whih is long part whih enoughreahes to fit into into aliphati polar hain head region group region. bilayer It would toger be inorporated with atin least bilayer one hydrophili membrane part whih hydrophobi reaheseffet: into polar any head disloation group region. peptide It would towards be inorporated inside or in outside bilayer membrane vesile would hydrophobi result in a steep inrease free energy system. In this state, peptide is partially proteted effet: any disloation peptide towards inside or outside vesile would result against hydrolysis as aess water moleules to peptide bonds is hampered. in a steep inrease free energy system. In this state, peptide is partially proteted against As a onsequene hydrolysis as se aess three water mehanisms, moleules amphiphili to peptide peptides bondswill is hampered. be seleted aumulate in vesile ming region. It has to be noted at this point that all amino aids are expeted As a onsequene to our in raemi semixtures. three mehanisms, Theree, amphiphili onmational peptides struture will be seleted resulting aumulate oligopeptides in strongly vesile differs ming from region. biogeni It has versions, to be noted example at thisin point lak that all ir amino tendeny aidsto are expeted m alpha to our helies. inneverless, raemi mixtures. ir basi Theree, affinity towards onmational bilayer region struture based on resulting amino oligopeptides aid sequene strongly will be differs similar from to existing biogeni biogeni versions, versions. example In fat, insuh a lak seleted ir prebioti tendeny to oligopeptide m alpha ould helies. resemble Neverless, typial ir peptide basi anhor affinity region towards a reent bilayer membrane region bound based protein. on amino aid sequene will be similar to existing biogeni versions. In fat, suh a seleted prebioti

6 Life 2017, 7, oligopeptide ould resemble typial peptide anhor region a reent membrane bound protein. Based on this hyposis, one ould assume that those anhor regions may be oldest portions funtional proteins. 5. Numeri Simulation Peptide Formation Aumulation Proess The rate peptide mation an be visualized in a numeri alulation using a number simple preonditions. We assume that rate onstants ondensation reation k as well as one hydrolysis a single peptide bond k h are equal all amino aids all peptides. Limiting maximum length peptides to six amino aid units, following set oupled differential equations is obtained: d 1 = 2 k dt h ( ) 2 k 1 ( ) (1) d 2 dt = 2 k k h( ) 2 k 2 ( ) k h 2 (2) d 3 dt = 2 k k h ( ) 2 k 3 ( ) 2k h 3 (3) d 4 dt = 2 k k k h( ) 2 k 4 ( ) 3k h 4 (4) d 5 dt = 2 k k k h 6 2 k 5 1 4k h 5 (5) d 6 dt = 2 k k k 2 3 5k h 6 (6) where n sts onentration a peptide med from n amino aid units ( 1 being onentration single amino aids). The various pre-fators eir aount fat that a reation an our at two or more positions moleule or that two moleules are partiipating in reation. The equilibrium onstant a single peptide bond mation is n given relation between k k h. Preliminary experiments amino aid mixtures treated in a high-pressure ell (p = 70 bar, T = 350 K) analyzed NMR show that a value k /k h = 0.1 is plausible. This is expressed fat that average onentrations amino aids 1 average onentration dipeptides 2 in rmal equilibrium exhibit an approximate relation 10:1 whih, in a kineti interpretation equilibrium onstant, should also roughly orrespond to relation k /k h. Furr, a starting onentration 1 = 1 a.u. is assumed amino aids ( n (0) n > 1 is set to zero). Under se onditions, differential Equations (1 6) an be developed over time to yield time dependent onentrations amino aids ( 1 ) as well as oligopeptides ( n ) up to hexapeptides ( 6 ). The result is shown on a logarithmi sale in Figure 4. While amino aid onentration is only slightly redued, all peptide onentrations grow rapidly at beginning level f at ir orresponding equilibrium values. In first approximation, final levels 1, 2, 3, 4, 5, 6 settle near values 10, 10 1, 10 2, 10 3, 10 4, 10 5 a.u. limit t (Figure 4b). The inreasingly low level onentrations peptides with higher hain lengths is basially numeri representation water problem [9]. In seond step, we introdue presene vesiles in reation mixture. We assume that a ertain fration hexapeptides is amphiphili, i.e., it onsists primarily hydrophili amino aids at one end primarily hydrophobi aids on or. With given seletion hydrormally med amino aids, we estimate that approximately 30% hexapeptides will fulfill this ondition. We assume that this fration hexapeptides will integrate into vesile membrane with an equilibrium onstant K = k in /k out = 1000, a value whih is plausible an amphiphile this kind. Inside membrane, it is likely to be proteted against hydrolysis. The resulting plot onentration integrated hexapeptide fration 6 (i) is shown in Figure 5 (red line).

7 Life 2017, 7, Life 2017, 7, Life 2017, 7, time (a.u.) time (a.u.) x 50 onentration onentration (a.u.) (a.u.) (a) (a) (b) Figure 4. (a) Numeri simulation peptide assuming k/kh Figure 4. (a) Numeri simulation development peptide onentration assuming k /k h = Figure a starting a 4. starting (a) amino Numeri amino aidsimulation onentration onentration development 1(0) = 1 a.u. The right image (b) shows same result 1 (0) = 1 a.u. The right peptide image onentration (b) shows assuming same result k/kh on = a saled on 0.1 a saled time a starting axis. time axis. amino aid onentration 1(0) = 1 a.u. The right image (b) shows same result on a saled time axis. onentration onentration (a.u.) (a.u.) time (a.u.) time (a.u.) time (a.u.) (i) 6 6 (i) (a) (b) Figure 5. (a) Numeri (a) simulation visualizing integration amphiphili (b) fration hexapeptides Figure 5. (a)(red Numeri line) into simulation vesile membranes. visualizing The right integration image (b) shows amphiphili same result fration a saled Figure 5. (a) Numeri simulation visualizing integration amphiphili fration hexapeptides time hexapeptides axis. (red line) into vesile membranes. The right image (b) shows same result on a saled (red line) into vesile membranes. The right image (b) shows same result on a saled time axis. time axis. Initially, given amphiphili fration 30% all hexapeptides is rapidly integrated, leading to a Initially, 7:3 ratio between given given 6 amphiphili 6(i) (6 fration sts 30% residual all hexapeptides is is in rapidly solution). integrated, Over time, leading leading tonon-amphiphili a 7:3 a 7:3 ratio between fration 6 6 hexapeptides 6(i) 6 (i)(6 ( 6 sts will undergo residual residual hydrolysis hexapeptide will in solution). deliver in solution). raw Over material time, Over time, new non-amphiphili ( n potentially fration amphiphili) hexapeptides hexapeptides. will will undergo undergo hydrolysis For hydrolysis that reason, will deliver will fration deliver raw material raw integrated material hexapeptide new ( n keeps potentially growing amphiphili) soon exeeds hexapeptides. values For For 6 that that 5 reason, (Figure 5b). fration Depending integrated on integration onstant kin/kout hexapeptide keeps growing on soon overall exeeds amount values vesiles, 6 it an easily reah signifiant levels (Figure5b). 5b). Depending on on integration This result onstant may be a k rough approximation real onditions, but it shows atual power in kin/kout /k out on overall amount vesiles, it it an an easily easily reah reah signifiant levels. levels. This given This result result seletion may may be mehanism be a a rough approximation ombined with hydrormal real onditions, polymer but mation. it it shows atual power power given given seletion seletion mehanism ombined with hydrormal polymer mation. 6. Possible Mehanisms Vesile-Indued Moleular Evolution 6. Possible 6. Looking Mehanisms at mehanism Vesile-Indued peptide seletion Moleular in detail, Evolution one an differentiate between three possible Looking riteria at favoring different types amphiphili peptides: Looking at mehanism peptide seletion in detail, one onean an differentiatebetween between three three possible (1) possible Integration. riteria riteria favoring favoring In analogy different different to biologial types types ounterparts, amphiphili this peptides: effet ould also be named a parasiti (1) behavior Integration. as In analogy advantage to biologial is limited ounterparts, to peptide this effet itself. ould Amphiphili also be named peptides a parasiti with an (1) Integration. amphiphiliity behavior as In analogy prile advantage fitting to biologial into is limited bilayer ounterparts, to ross peptide setion this effet itself. integrate ould Amphiphili into also be membrane. named peptides a There, parasiti with an behavior y amphiphiliity gain as an individual prile advantage fitting seletive is into limited advantage bilayer to ross as peptide y setion are itself. partially integrate Amphiphili proteted into membrane. against peptides hydrolysis There, with an amphiphiliity y as gain y an beome individual prile less fitting easily seletive into eluted advantage from bilayer ross as vesile y setion zone. are partially integrate For this proteted seletion into riterion, against membrane. hydrolysis There, effet y on gain as stability y an individual beome less vesile seletive easily is eluted negligible. advantage from as y vesile arezone. partially For this proteted seletion against riterion, hydrolysis effet (2) asstabilization. y beome stability In less this easily ase, vesile eluted amphiphili is negligible. from peptides vesile are zone. being For proteted this seletion vesiles riterion, but in turn effet also on (2) stabilize Stabilization. stability vesile In this vesile struture. ase, isamphiphili negligible. Again, in analogy, peptides one are being ould proteted ategorize this vesiles mutual but advantage in turn also as stabilize vesile struture. Again, in analogy, one ould ategorize this mutual advantage as onentration onentration (a.u.) (a.u.) onentration onentration (a.u.) (a.u.) (b) time (a.u.) x 50 time (a.u.) x 50 time (a.u.) x (i) 6 (i)

8 Life 2017, 7, (2) Stabilization. In this ase, amphiphili peptides are being proteted vesiles but in turn also Life 2017, stabilize 7, 3 vesile struture. Again, in analogy, one ould ategorize this mutual advantage 8 as 10 symbioti. Very effiient idates speies ausing vesile stabilization ould be peptides symbioti. Very effiient idates speies ausing vesile stabilization ould be peptides onsisting long hydrophobi part with two hydrophili ends. In this ase, peptide ould onsisting a long hydrophobi part with two hydrophili ends. In this ase, peptide ould fully integrate into bilayer onnet two hydrophili layers membrane in analogy fully integrate into bilayer onnet two hydrophili layers membrane in to a bola-amphiphile [25]. Suh bola-amphiphiles lead to a signifiantly higher rmal stability analogy to a bola-amphiphile [25]. Suh bola-amphiphiles lead to a signifiantly higher rmal surrounding bilayer membrane our in rmophili organisms. With inreased stability surrounding bilayer membrane our in rmophili organisms. With stability, vesiles gain an advantage from hosting peptide moleules while latter in inreased stability, vesiles gain an advantage from hosting peptide moleules while turn enjoy an inreased degree protetion. Altoger, this symbioti effet would at more latter in turn enjoy an inreased degree protetion. Altoger, this symbioti effet would at effiiently than parasiti integration effet alone. more effiiently than parasiti integration effet alone. (3) Funtion. This part possible evolution proess is most speulative, but also most (3) Funtion. This part possible evolution proess is most speulative, but also most exiting one. The potential is obvious: peptides with new funtions beyond simple mehanial exiting one. The potential is obvious: peptides with new funtions beyond a simple mehanial stabilization may develop. A possible example may be an ion hannel based on a trans-membrane stabilization may develop. A possible example may be an ion hannel based on a transmembrane polypeptide [26]. In vesile yle, vesiles are generated with a natural ioni polypeptide [26]. In vesile yle, vesiles are generated with a natural ioni onentration gradient aross membrane whih destabilizes ir struture resulting osmoti stress. onentration gradient aross membrane whih destabilizes ir struture resulting An ion hannel ould ause a rapid relaxation this gradient, leading to a longer vesile osmoti stress. An ion hannel ould ause a rapid relaxation this gradient, leading to a longer lifetime resulting in a orresponding seletion advantage hannel-ming peptide. vesile lifetime resulting in a orresponding seletion advantage hannel-ming Or seleted funtions ould inlude ability to bind to a speifi surfae, to adopt a speifi peptide. Or seleted funtions ould inlude ability to bind to a speifi surfae, to adopt harge, to seletively bind to or vesiles, or even to make membrane permeable speifi a speifi harge, to seletively bind to or vesiles, or even to make membrane permeable moleules. More omplex versions peptides ould even make use onentration speifi moleules. More omplex versions peptides ould even make use gradient as a soure free energy. In any ase, suh funtional peptides ould be seen as initial onentration gradient as a soure free energy. In any ase, suh funtional peptides ould be primitive predeessors funtional membrane proteins. seen as initial primitive predeessors funtional membrane proteins. Of ourse, desribed mehanism peptide seletion misses an important preondition atual moleular evolution: apability idential reprodution. However, mehanism seletion itself an anbe be quite quite effiient effiient as long as long as aspool pool moleules moleules to hoose to hoose from is from large is enough large enough exists over existslong overperiods long periods time. time. Both Both onditions onditions are are definitely fulfilled peptides in hydrormal soures. Even if evolution proess is driven seletion only ( if a possible template effet is exluded), it ould beome remarkably omplex: on integration seleted peptides, vesiles may hange ir affinity towards or amphiphili moleules, resulting in ina ahange membrane omposition. This Thisin inturn may affet seletion riterion furr peptides, so on (Figure 6). peptide seletion amphiphile seletion peptide seletion Figure Figure Symboli Symboli depition depition a possible possible strutural strutural evolution evolution vesiles vesiles repeated repeated optimization optimization steps. steps. Peptide Peptide seletion seletion integration integration (left) hanges (left) hanges riteria riteria seletion seletion or amphiphiles, or leading amphiphiles, to an alteration leading to an alteration membrane omposition membrane (enter) omposition whih (enter) in turn hanges whih in turn riteria hanges peptide riteria seletion peptide (right). seletion (right). With integration amphiphili peptides, vesile membrane will m a very speifi template whih again influenes seletion furr peptides. Theree, one ould regard amphiphiliity prile membrane as something like a primitive memory moleular struture (i.e., sequene hydrophili hydrophobi amino aids) whih evolves optimizes over

9 Life 2017, 7, With integration amphiphili peptides, vesile membrane will m a very speifi template whih again influenes seletion furr peptides. Theree, one ould regard amphiphiliity prile membrane as something like a primitive memory moleular struture (i.e., sequene hydrophili hydrophobi amino aids) whih evolves optimizes over time. Furrmore, as a prinipal rule, all moleules whih are an advantage mation stable vesiles will remain in vesile zone ree aumulate in this environment. The result this evolution proess would be optimized, stable maybe funtional vesiles. Even though majority vesiles is immobilized in a yle mation destrution at vesile zone, a small fration espeially stable vesiles ould be transported to terrestrial surfae into different loal environments (e.g., sediment basins, ponds). Being rmally stable, y ould survive over longer periods time ould m versatile ompartments furr developments in moleular evolution. Experimentally, inreased stability se vesiles will be tested in a two-phase environment med water superritial arbon dioxide. Artifiial pressure yling indues periodi mation vesiles in aqueous phase. Without furr stabilization, vesiles generally survive a single pressure yle, as y are destroyed ontat with superritial arbon dioxide via phase boundary. As soon as aementioned stabilization step is reahed, integrated peptide will signifiantly inrease lifetime vesiles. Under se irumstanes, stabilized vesiles will persist aumulate. The stabilizing peptides will be analyzed artifiially reprodued. Using synti peptides, orresponding vesiles will be re-assembled submitted to furr strutural analysis, inluding field gradient NMR spetrosopy. At this stage, rmal stability optimized vesiles an be diretly ompared to original vesiles without integrated peptides. The prinipal apability membrane vesiles early steps life has been desribed on numerous oasions [27 29]. Reently, it was shown that a possible prebioti amphiphile (deanoi aid) undergoes distint binding interations with nuleobases proteins [30,31]. So, at this stage, y may serve as ontainments initial metaboli steps, most prominently, as an ensemble enapsulated environments repliation RNA moleules. Author Contributions: Christian Mayer developed ory peptide seletion ombined with vesile evolution wrote paper. Ulrih Shreiber ontributed ory on tetoni fault zones as a senario prebioti hemistry protoell development. Maria J. Dávila is presently perming simulation experiments delivered preliminary results. Conflits Interest: The authors delare no onflit interest. Referenes 1. Dyson, F.J. Origins Life; Cambridge University Press: Cambridge, UK, Trainer, M.G. Atmospheri prebioti hemistry organi hazes. Curr. Org. Chem. 2013, 17, [CrossRef] [PubMed] 3. Airapetian, V.S.; Gloer, A.; Gronf, G.; Hébrard, E.; Danhi, W. Prebioti hemistry atmospheri warming early Earth an ative young sun. Nat. Geosi. 2016, 9, [CrossRef] 4. Simoneit, B.R.T. Prebioti organi synsis under hydrormal onditions: An overview. Adv. Spae Res. 2004, 33, [CrossRef] 5. Matsuno, K. Hydrormal reation. In Enylopedia Astrobiology; Springer: Berlin, Germany, 2011; pp Lawless, J.G. Amino aids in Murhison meteorite. Geohim. Cosmohim. Ata 1973, 37, [CrossRef] 7. Guillemin, J.C.; Bouyahyi, M.; Riague, E.H. Prebioti, planetary interstellar hemistry starting from ompounds deteted in interstellar medium. Adv. Spae Res. 2004, 33, [CrossRef] 8. Martins, Z.; Botta, O.; Fogel, M.L.; Sephton, M.A.; Glavin, D.P.; Watson, J.S.; Dworkin, J.P.; Shwartz, A.W.; Ehrenfreund, P. Extraterrestrial nuleobases in Murhison meteorite. Earth Planet. Si. Lett. 2008, 270, [CrossRef] 9. Benner, S.A.; Kim, H.J.; Carrigan, M.A. Asphalt, water, prebioti synsis ribose, ribonuleotides, RNA. A. Chem. Res. 2011, 45, [CrossRef] [PubMed]

10 Life 2017, 7, Shreiber, U.; Loker-Grütjen, O.; Mayer, C. Hyposis: Origin life in deep-reahing tetoni faults. Orig. Life Evol. Biosph. 2012, 42, [CrossRef] [PubMed] 11. Mayer, C.; Shreiber, U.; Dávila, M.J. Periodi vesile mation in tetoni fault zones: An ideal senario moleular evolution. Orig. Life Evol. Biosph. 2015, 45, [CrossRef] [PubMed] 12. Marshall, W.L. Hydrormal synsis amino aids. Geohim. Cosmohim. Ata 1994, 58, [CrossRef] 13. Andersson, E.M.; Holm, N.G. The stability some seleted amino aids under attempted redox onstrained hydrormal onditions. Life Evol. Biosph. 2000, 30, [CrossRef] 14. Kitadai, N. Energetis amino aid synsis in alkaline hydrormal environments. Orig. Life Evol. Biosph. 2015, 45, [CrossRef] [PubMed] 15. Nelson, S.T.; Mayo, A.L.; Gilfillan, S.; Dutson, S.J.; Harris, R.A.; Shipton, Z.K.; Tingey, T.G. Enhaned frature permeability aompanying fluid flow in footwall a normal fault: The Hurriane fault at Pah Tempe hot springs, Washington County, Utah. Geol. So. Am. Bull. 2009, 121, [CrossRef] 16. Rushdie, A.I.; Simoneit, B.R. Abioti ondensation synsis glyeride lipids wax esters under simulated hydrormal onditions. Orig. Life Evol. Biosph. 2006, 36, [CrossRef] [PubMed] 17. Kümpel, H.J.; Greksh, G.; Lehmann, K.; Rebsher, D.; Shulze, K.C. Studies in situ pore pressure flutuations. Oil Gas Si. Tehnol. 1999, 54, [CrossRef] 18. Tuk, A. The role atmospheri aerosols in origin life. Surv. Geophys. 2002, 23, [CrossRef] 19. Dobson, C.M.; Allison, B.G.; Tuk, A.F.; Vaida, V. Atmospheri aerosols as prebioti hemial reators. Pro. Nat. Aad. Si. USA 2000, 97, [CrossRef] [PubMed] 20. Tsukahara, H.; Imai, E.I.; Honda, H.; Hatori, K.; Matsuno, K. Prebioti oligomerization on or inside lipid vesiles in hydrormal environments. Orig. Life Evol. Biosph. 2002, 32, [CrossRef] [PubMed] 21. Furuuhi, R.; Imai, E.I.; Honda, H.; Hatori, K.; Matsuno, K. Evolving lipid vesiles in prebioti hydrormal environments. Orig. Life Evol. Biosph. 2005, 35, [CrossRef] [PubMed] 22. Luisi, P.L.; Walde, P.; Oberholzer, T. Lipid vesiles as possible intermediates in origin life. Curr. Opin. Colloid Interfae Si. 1999, 4, [CrossRef] 23. Williams, L.L.; Rubin, J.B.; Edwards, H.W. Calulation Hansen solubility parameter values a range pressure temperature onditions, inluding superritial fluid region. Ind. Eng. Chem. Res. 2004, 43, [CrossRef] 24. Lemke, K.H.; Rosenbauer, R.J.; Bird, D.K. Peptide synsis in early Earth hydrormal systems. Astrobiology 2009, 9, [CrossRef] [PubMed] 25. Fuhrhop, J.H.; Wang, T. Bolaamphiphiles. Chem. Rev. 2004, 104, [CrossRef] [PubMed] 26. Futaki, S. Peptide ion hannels: Design reation funtion. Biopolymers 1998, 47, [CrossRef] 27. Deamer, D.W. The first living systems: A bioenergetis perspetive. Mirobiol. Mol. Biol. Rev. 1997, 61, [PubMed] 28. Segré, D.; Ben-Eli, D.; Deamer, D.W.; Lanet, D. The lipid world. Orig. Life Evol. Biosph. 2001, 31, [CrossRef] [PubMed] 29. Chen, I.A.; Walde, P. From self-assembled vesiles to protoells. Cold Spring Harb. Perspet. Biol. 2010, 2, a [CrossRef] [PubMed] 30. Blak, R.A.; Blosser, M.C.; Stottrup, B.L.; Tavakley, R.; Deamer, D.W.; Keller, S.L. Nuleobases bind to stabilize aggregates a prebioti amphiphile, providing a viable mehanism emergene protoells. Pro. Nat. Aad. Si. USA 2013, 110, [CrossRef] [PubMed] 31. Blak, R.A.; Blosser, M.C. A self-assembled aggregate omposed a fatty aid membrane building bloks biologial polymers provides a first step in emergene protoells. Life 2016, 6, 33. [CrossRef] [PubMed] 2017 authors; liensee MDPI, Basel, Switzerl. This artile is an open aess artile distributed under terms onditions Creative Commons Attribution (CC-BY) liense (