Micro-review Element reducing oxidizing environment environment Fe Fe 2+ (high) Fe 3+ (low) Cu Cu sulfides Cu 2+ (moderate) (low) S HS - (high) S4 2- (high) Mo [MonS4-n] 2- MoS2 (low) V V 3+, V 4+ sulfides Mo4 2- (moderate) V4 3- (moderate) ote switch in relative availability of Cu and Fe. 1 Frausto da Silva & Williams Table 1.6
Inorganic Chemistry Concepts for Bio. Thermodynamics Hard-soft acids and bases. The chelate effect. Ligand pka depression. Redox potential tuning. Kinetic considerations. Ligand exchange rates. Electron transfer. Electronic and geometric structures. Reaction of coordinated ligands. 2
Hard and Soft Metal Larger, more polarizable metal ions gain extra stabilization from this capability if the ligands also share this possibility. Harder smaller metal ions electrostatic interactions are stronger, so bonds with similarly hard ligands are stronger. 3
Soft and hard metals and ligands Hard Intermed. Soft Metal ions H +, a +, K + Mg 2+, Mn 2+, Ca 2+, Al 3+, Cr 3+, Co 3+, Fe 3+ Fe 2+, Co 2+, i 2+, Cu 2+, Zn 2+ Cu +, Au +, Cd 2+, Pb 2+, Hg 2+ Ligands P4 3-, C3 2-, RP3 2-, H -, CH3C2 -, Cl -, R -, 3 -, H2, H3, 2 -, S3 -, Br -, 3 -, 2, H H 2 RS -, C -, SC -, H -, R2S, RSH, R3P, C, 4 Lippard & Berg, Table 2.1
Metallothionein: a soft-ligand protein. 1/3 of amino acids are Cys. Binds Cd 2+, Hg 2+, Pb 2+, thus protecting the cell from them. Those metal ions otherwise bind to critical SH groups and displace other metal ions from soft ligands. 5 Ag-bound metallothionein. Armitage et al. 1A.pdb
Calmodulin, a hard-ligand protein 6 Ad Bax 2HF5.pdb
The chelate effect M + L ML Kb = e -ΔGb/RT, ΔGb = ΔHb -TΔSb ΔSb is -ve M + 2L L L M K b = e -ΔG b/rt, ΔG b =2ΔHb-2TΔSb M + L~L L~L K b = e -ΔG b/rt, ΔG b =2ΔHb-TΔSb M 7 ΔG b is more favourable by TΔSb
ethylenediaminetetraacetic acid = EDTA H H H H 2 C CH 2 H 2 C C H 2 CH 2 C H 2 H 8 www.3dchem.com/molecules.asp?id=89
EDTA Based on "YMCA" by The Village People I Ligands, there s no need to feel down, I said ligands, when you re floating around, You don t have to stay there, free and unbound There s no need to be uncomplexed! Ligands, you ve got electron pairs, They re not bonded - and they re just sitting there; A cation - if you re willing to share Could accept your spare electrons! Chorus I You ve got to complex like EDTA, You ve got to complex like EDTA; It s got everything to be hexadentate! It s got six lone pairs to donate! You ve got to complex like EDTA, You ve got to complex like EDTA! It s ethylene-dia-mine-tetra-acet-ate! It s a ligand that can chelate! II Ligands you might bond to class b Metals - if you re polarised easily (As are sulphur, phosphorus, iodine) And form more covalent compounds; Ligands, if you re hard (like fluorine) You re electronegative, so you ll be Bound to harder metals like Al (III) 9 With elec-tro-stat-ic bonding! Chorus II You ve got to complex like EDTA, You ve got to complex like EDTA; It replaces all six H2s separately, So the entropy must increase! You ve got to complex like EDTA, You ve got to complex like EDTA; It s the chelate effect! It s a favoured process! It s a positive delta S! III Ligands can you act as a pi Donor? They can even stabilise high xidation states they re weak field and high Spin the delta value s smaller; But if the pi* are empty They re acceptors lowering t2g And increasing the gap in energy: The ligand field splitting s larger! Repeat Choruses I and II until bored Aimee Hartnell, February 2002 http://www.geocities.com/le_chatelier_uk/song_index.html
Deprotonation of ligands The metal ion competes with protons, both are cations. Ligand & rxn. Metal ion pka (25 C, 0.1 M) H2 + M 2+ M-H +H + H3 + M 2+ M-H2 + H + CH3CH + M 2+ H M-CCH3 +H + H 10 + M 2+ H + H + none Ca 2+, Mn 2+, Cu 2+, Zn 2+ none Co 2+, i 2+, Cu 2+ none Mg 2+, Ca 2+, i 2+, Cu 2+ none Co 2+, i 2+, Cu 2+ Lippard & Berg, Table 2.2 14.0 13.4 11.1 10.7 10.0 35.0 32.9 30.7 32.2 4.7 4.2 4.2 4.0 3.0 7.0 4.6 4.0 3.8
Formation of Fe clusters coupled to deprotonation of coordinated H - - H Fe 3+ - H H - H - Fe 3+ pka ~ 6 - H Fe 3+ 2- Fe 3+ 2-2- Fe 3+ + Fe 3+ 11
Coordination to a metal ion also makes ligands more susceptible to nucleophilic attack. M H 2 R H 2 H - M H M R H 2 + RH His- His- C Zn 2+ 12 -His H + - His- H His- C - Zn 2+ + H + H -His Proximity or template effect.
Reduction Midpoint Potentials Cu 2+ (-sal)2en + e - Cu + (-sal)2en Em = -1.21 V Em = -ΔGreduction/nF, F is Faraday s constant 96.5 J/V mol, n is the number of e - so Em corresponds to ΔG/electron transferred. -0.74 V Cu 2+ (ipr-sal)2 + e - Cu + (ipr-sal)2 Em -1.21 V Cu 2+ (-sal)2en + e - Cu + (-sal)2en Cu + (-sal)2en + Cu 2+ (ipr-sal)2 Cu 2+ (-sal)2en + Cu + (ipr-sal)2 Em = -0.74 V + 1.21 V = 0.47 V 13
Ligands tune the metal s Em: sterics and hard-soft effects Compound Cu(-sal)2en Cu(Me-sal)2 Cu(Et-sal)2 Cu(S-sal)2en Cu(i-Pr-sal)2 Cu(t-Bu-sal)2 Em vs. HE* -1.21 V -0.90 V -0.86 V -0.83 V -0.74 V -0.066 V Cu R X Cu R Cu(R-sal)2 X Cu(X-sal)2en Lippard and Berg, Table 2.4 14 *HE: normal H electrode: 2H + + 2 e - H2
Redox tuning tools Coordination geometry Ligand natures Polarization of ligands by H-bonds Local dielectric (in the event of net charge change). Proteins can impose a coordination geometry / ligand ID on a metal ion, paying the energetic cost of doing so from the energetic stability of the overall protein structure. 15