FORMATION AND STABILITY OF MODEL MEMBRANES IN GEOCHEMICAL ENVIRONMENTS

Minerals have played important roles in various stages of prebiotic chemical evolution. It is assumed that the first life forms (protocells) may have originated at the mineral/water interface. The effects of minerals relevant to early Earth geochemical environments was studied on the critical vesicle concentration (CVC) of a decanoic acid/decanol (DA/DOH) mixture. The CVC of DA/DOH mixture (2:1) was determined by measuring fluorescence intensity of a membrane-soluble dye, naphthopyrene and by dynamic light scattering. The effect of minerals on the membrane integrity of DA/DOH was also studied by encapsulating a negatively-charged dye, calcein, in DA/DOH. Oleic acid (OA) and palmitoyl-oleoyl-phosphatidylcholine (POPC) vesicles were used for comparison.

All negatively charged rocks and minerals did not affect CVC, whereas positively charged γ-alumina shifted the CVC to higher concentration range relative to DA/DOH alone. The negatively charged minerals may interact with DA/DOH vesicles by H-bonding, but adsorption is insufficient to affect CVC. A greater amount of lipid is adsorbed on γ-Al₂O₃ because of electrostatic attraction between its positively charged surface and lipid vesicles in addition to H-bonding. Hence, more lipid has to be added to solution in order to form vesicles in the presence of γ-Al₂O₃ thus shifting the CVC value to higher concentration range relative to DA/DOH alone. It was also found that DA/DOH vesicles are much more permeable (only ~60-70% remains encapsulated at 16 h) compared to OA and POPC vesicles (~98-100% remains encapsulated). OA and POPC are more impermeable than DA/DOH, because OA has a longer chain and a double bond in the tail, which creates closer tail-tail interactions in the bilayer, and phospholipid membranes are more stable as the tail has two fatty acid chains and is more hydrophobic. However, there is no apparent effect of minerals on the calcein leakage from the membranes. Thus, the self assembly of vesicles from SCAs in different geochemical environments is a robust phenomenon and serves as a model for protocell formation during the origins of life.