Amino Acids and Peptides Stabilize Fatty Acid Membranes against Salt-Induced Flocculation

Abstract

The prebiotic formation of biopolymers (specifically DNA, RNA, and proteins) has long been a mystery and is important for understanding the origin of life on earth. These bio-molecules are composed of building blocks that would have been dispersed in early oceans. Our previous work has shown that RNA bases and ribose bind to and stabilize fatty acid vesicles [Black et al. PNAS 110, 13272 (2013)]. Our results implied that the building blocks of a biological polymer could have spontaneously associated with components of the first membranes to form stable structures. We have now shown that protein building blocks, too, stabilize fatty acid vesicles against salt-induced flocculation. Using spectrophotometry, we measured the presence of flocs (and other structures) in fatty acid solutions, with and without amino acids and over a range of temperatures. Using fluorescence microscopy, we identified the structures that caused changes in absorbance in our spectrophotometric assays. We found that the two most hydrophobic prebiotic amino acids, leucine and isoleucine, prevent salt-induced flocculation. Moreover, although alanine and glycine, which are less hydrophobic, had little effect on flocculation, dipeptides composed of these amino acids preserved vesicles in the presence of salt even at 60 degrees C. These vesicles appeared to be primarily multilamellar structures, which may promote reactions between components of biopolymers more effectively than unilamellar vesicles. Thus prebiotic membranes could have facilitated the formation of peptides by bringing amino acids together, and peptides could have increased the formation of stable membranes. Such an auto-amplifying system, combined with selection for more effective peptides, could have led to the first cells.