Let's assume that abiological precursors would react to form polymer-like molecules that might be complex enough to fold to structures that would allow binding, catalysis, and rudimentary replication. All this would be worthless unless they could be sequestered in a small volume which would limit diffusion and increase their local concentration. What is required is a membrane structure. Amphiphilic molecules, like lipids, with which we started this book, would be prime candidates since they spontaneously assembly to form micelles and bilayers, as shown in the review diagram below.
As mentioned in Chapter 1, alternative lipid phases are possible. Bilayers can also be formed from single chain amphiphiles, such as certain fatty acids, as illustrated in the equilibrium shown below. This occurs more readily at pH values close to the the pKa of the fatty acid, at which the fatty acids are not all deprotonated with full maximal negative charges. Single chain amphiphiles like fatty acids, which were more likely to formed in abiotic conditions, have been found in meteorites.
Clay surfaces, which have been shown to facilitate the formation of nucleic acids polymers, can also promote the conversion of fatty acid micelles to bilayers (Szostak). One such clay surface, montmorillonite, whose structure is shown below, promotes bilayer formation. It has an empirical formula of Na0.2Ca0.1Al2Si4O10(OH)2(H2O)10.
The effect of montmorillonite on vesicle formation can be shown by simple measurements of turbidity with time. Microscopy of fluorophore-encapsulated vesicles also shows encapsulated montmorillonite. The fatty acids presumably absorb to the cation layer of the clay particles. Time studies using light scattering also indicate that the vesicles grow in the presence of fatty acid micelles. To differentiate between the formation of new vesicles and the increase in size of pre-existing vesicles (which couldn't be done by simple light scattering without separation of the vesicles), investigators used two different fluorescent molecules to label fatty acid vesicles. The two probes were selected such that if the two probes came in close contact, energy transfer from the excited state of one fluorophore to the other fluorophore could occur, an example of fluorescence resonance energy transfer (FRET). FRET is observed when emission of the second dye occurs after excitation of the first dye, at a wavelength outside of the excitation wavelength of the second dye. If unlabeled vesicles were added to either labeled vesicles, no changes in FRET were observed, suggesting that the dyes did not move between vesicles. If fatty acid micelles were added, a decrease in FRET was observed, suggesting that new fatty acids were transferred to the doubly-labeled vesicles, effectively diluting the dye concentrations in the bilayer and their relative proximity, both which would decrease FRET. Most of the new fatty acid was incorporated into pre-existing vesicles which grew. The vesicles could also divide if extruded through a small pore. Later we will see that the energy to grow the vesicles can derive in part from a transmembrane proton concentration collapse. Division of vesicles might be promoted by bilayer assymetries associated with addition of substances to the outer leaflet, causing membrane distortion.
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