Fluid Mozaic Model of Membranes

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1 Replacement for the 1935 Davson Danielli model Provided explanation for Gortner-Grendel lack of lipid and permitted the unit membrane model. Trans membrane protein by labelling Fry & Edidin showed that proteins diffused in the membrane. Not until 1978 was the sequence of glycophorin determined and modeled to span the membrane with a-helices. Generally a-helix internally satisfies the hydrogen bonding of a peptide and the trans-membrane segments of proteins are almost all a-helical physical measurements including x-ray neutron diffraction and simulation confirm this Fluid Mozaic Model of Membranes Fluid Mozaic Model of Membranes Fluid Mozaic Model S nger & Nicolson Sc ence (1972) Understanding has resu ted from an ncreas ngly detailed analysis of movement in memb anes using many methods

2 Membrane dynamics ncludes many time domains Fluidity is defined as the inverse of viscosity (kg/(m-sec)) Therefore it represents the viscous drag in membranes but the link between viscous drag and physical measurements like lateral diffusion and membrane permability was not always clear and the experiments lacked predictibility. To these authors it seems that compatison should be to order parameters and correlation times The assembly of multiple parameters may be required to generate predicitive data for membranes.

3 From Concepts and Methods of Solid-State NMR Spectroscopy Applied to Biomembranes Fig. 11 [13] introduction At NIST they have observed a number of membrane movement modes that have more predictable relationships to structure using neutron spin echo methods.

4 P π P P π P Effect of Melittin on the Bending Elasticity and Thickness Fluctuation of the Lipid Bilayer Influence of peptide incorporation on lipid bilayer dynamics

5 Motion and Order Within Lipid Atoms Lipid structures that could influence fluctuations Chapter 4 Gawrisch in Yeagle 2nd edition [12] This has been extensively studied by NMR snc 1 -C 2 bond stays in gauche isomers which aligns the hydrocarbon chains The C 2 C 3 bond is parallel to membrane normal and has significant rotational mobility. Lipid structures that could influence fluctuations Glycerol backbone This would seem to promote the gel phase L o version of lamellar lipids with the chains approximating the crystalline state but the membrane remaining in a more liquid state Speculate on the asymmetry at the C 2 position

6 Motion and Order Within Lipid Atoms Alignment with Membrane Normal Chapter 4 Gawrisch in Yeagle 2nd edition [12] In gel phase PE and PC the P N dipole aligns perpendicular to the membrane normal. This resembles the crystal structure but it is dependent upon hydration and surface electrostatic potentials. In L d states this is highly variable while in the L 0 state this perpendicular alignment is more dominant and influences the membrane structure.

7 Motion and Order Within Lipid Atoms Alkane Chain Conformation Chapter 4 Gawrisch in Yeagle 2nd edition [12] Inference from crystal packing The saturated chain is usually at the C1 position. sn-1 chain has high order to middle of chain length which decays to the terminal methylene sn-2 less ordered at all positions and is at an angle to the bilayer normal This results from anchoring of the glycerol headgroup Relaxation times increase parallel to order

8 Motion and Order Within Lipid Atoms POPC Chains Chapter 4 Gawrisch in Yeagle 2nd edition [12] sn-1 Palmitate chain has high order to middle of chain length which decays to the terminal methylene sn-2 Oleate less ordered at all positions and is at an angle to the bilayer normal This results from anchoring of the glycerol headgroup Relaxation times increase in the unsaturated parallel to order parameter decrease Lower order parameters do not correlate directly with increased motional order

9 Summary of Bilayer Structure Bilayer Structure Bilayer Structure Chain Order Summary cyan phospholipid, blue +cholesterol Carbon movement in the bilayer is dramatically affected by unsaturations There is also a positional distinction since there is reduced order in chains at sn-2 position The length of the chain seems decreased

10 Summary of Bilayer Structure Distribution of Membrane Components Describe the distribution of the components These have been constructed from scattering data Especially neutron scattering which can detect the densities.

11 Diffusion in the Membrane Lateral Anisotropy in Membranes Originally a concept, annular lipids Detergent resistant membranes (DRMs) Enriched in sphingolipids and cholesterol Atomistic to continuum model Diffusion in Membranes Lateral Anisotropy in Membranes Membrane Microdomains PG Sa fman and M De br k 197 ) B own an mo on in b o og cal memb anes P oc Nat Acad Sci USA

12 Diffusion in the Membrane Saffman-Delbruk D ffusion in the membrane plane I Einstein Smoluchowski Equation D = kt (2) x I Stoke s Law x = 6pha (3) I Stoke s Einstein Law D = kt (4) 6pha Einstein established that this is a relationship between diffusion and friction x frictional coefficient [14] [15]

13 Diffusion in the Membrane Saffman-Delbruk D T = k BT 4pµh og µmh µwa g, Consider both water and membrane viscosity Viscosity µ 1/fluidity a - particle radius µ - viscosity h - membrane thickness g constant

14 Diffusion in the Membrane Saffman-Delbruk D T = k BT 4pµh og µmh µwa This model has been validated experimenta ly and has been used to charaterize many aspects of d ffusion in membranes g, microphotolysis [16] single and continuous Optical Tweezers [17]. GPI-anchored tether behave as though they weretethered to a 26 nm membrane particle.

15 Diffusion in the Membrane FCS Measurement of Movement in Membranes Fluorescent Correlation Fluorescent Correlation Spectroscopy diffusion coefficients hydrodynamic radii average concentrations kinetic chemical reaction rates singlet-triplet dynamics

16 Diffusion in the Membrane FCS Measurement of Movement in Membranes The data reflects diffusion into the measuring volume Average number of molecules is between 0.1 and 1000 per volume to provide sufficient signal differentiation This is to 10 6 [18, 19]

17 Diffusion in the Membrane FCS Measurement of Movement in Membranes G(t)= hf(t) F (t + t)i hf(t)vi The continuous fluctuations of the signal can be quantitated by Temporal Autocorrelation The lateral diffusion time, t D, can be expressed in terms of the diffusion coefficient D. It is also possible to relate D to the hydrodynamic radius and estimate the size of the particle. The dependence on R b,i µ p MW which can limit accuracy However the amplitude of the autocorrelation curve is an accurate measure of the particle