Liposomes as a model of prebiotic cell like structures

Abstract

The stable membranes are prerequisite to the assembly of the earliest selfreplicatingsystems. Phospholipids, which are common in biological membranes and whichself-assemble in aqueous environment into stable lipid bilayers and vesicles, are obviouscandidates for prebiotic membrane components.Phosphate is the most probable chemical group to mediate the transition betweenlight trapping and chemical changes in nutrient molecules that lead to growth. In order toactivate phosphate so that it can undergo transfer reactions, pyrophosphate is consideredto be an intermediate. The formation of pyrophosphate from carbamyl phosphate and inirradiated photosystems is described.The photo redox reaction is presented which, if enclosed inside liposomes orplaced in the membrane can create a pH difference between interior and exterior of thevesicles. Such a system may be used as a model for the early prebiotic cell-like structureswith membrane-embedded reactions. Light-dependent pH gradients offer a plausibleenergy source in the prebiotic environment, and the system described here provides auseful model system for further experimental tests.A new method is described for determining the aqueous internal volume of smallunilamellar liposomes. The method is based on an equilibrium uptake of hydrogen iodideby liposomes. After the chromatographical separation of liposomes from external solution,the membrane is solubilized and the amount of captured iodide determined using an ionselective electrode.