Nita Sahai (University of Akron - Dept. of Polymer Science)
Minerals, because of their reactivity and ubiquity, likely contributed to the transformation from prebiotic geochemistry to biochemistry. We propose two central hypothesis: that mineral surfaces acted as (1) an “evolutionary selection stress” in selecting lipid or amphiphile membrane compositions that were stable (survival of the fittest protocells) and (2) as “prebiotic enzymes” for photocatalyzed trans-membrane energy transduction in the emergence of metabolism. We use phospholipid and single chain amphiphile vesicles and planar bilayers as model protocells, and metal oxides and sulfides as model minerals to examine our hypotheses. Results of this work have implications for prebiotic chemistry as well as for other fields, such as biomedical sciences and energy-related technologies.
The plasma membranes of most extant organisms are composed predominantly of zwitterioinc lipids with embedded negatively-charged proteins and other biomolecules, which evolved to facilitate metabolic and genetic functions. We examine the effects of mineral and lipid head-group charge and solution conditions on vesicle stability and integrity. The combined results of quantitative adsorption isotherms, Atomic Force Microscopy, Neutron Reflectivity and Fluorescence methods show that bilayers of the zwitterionic lipid, dipalmitoyl phosphocholine (DPPC) have a greater affinity for and membrane integrity at the surface of the positively-charged mineral, corundum (a-Al2O3) than at the quartz (a-SiO2) surface with a negative-charge. Bilayers of charged lipids were unstable at both types of mineral surfaces. Thus, charged mineral surfaces could provide an environmental stress survival and selection of zwitterionic lipids over negatively-charged lipids in protocell membranes. The preferred stability of neutral vesicles with with or without negatively charged head-groups is also confirmed by bulk selection experiments using single-chain amphiphiles under variable environmental solution conditions, including pH, ionic strength, and multivalent cations.
Metal sulfide clusters at the active core of many metabolic enzymes have been proposed as “molecular fossils” reflecting a role for photocatalytic sulfide minerals in early trans-membrane energy transduction. Preliminary results show that CdS trapped in a DPPC membrane can promote a trans-membrane redox reaction involving methylviologen as oxidant and methanol as reductant.