| Abstract
| - Reverse micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in isooctane (IO) and water have longbeen used as a means to provide a confined aqueous environment for various applications. In particular, AOT reversemicelles have often been used as a template to mimic membrane−water interfaces. While earlier studies have shownthat membrane-binding peptides can indeed be incorporated into the polar cavity of AOT reverse micelles where theymostly fold into an α-helical structure, the underlying interactions leading to the ordered conformation are howevernot well understood. Herein, we have used circular dichroism (CD) and infrared (IR) spectroscopies in conjunctionwith a local IR marker (i.e., the C⋮N group of a non-natural amino acid, p-cyano-phenylalanine) and a global IRreporter (i.e., the amide I‘ band of the peptide backbone) to probe the conformation as well as the hydration statusof an antimicrobial peptide, mastoparan x (MPx), in AOT reverse micelles of different water contents. Our resultsshow that at, w0 = 6, MPx adopts an α-helical conformation with both the backbone and hydrophobic side chainsmostly dehydrated, whereas its backbone becomes partially hydrated at w0 = 20. In addition, our results suggest thatthe amphipathic α-helix so formed orients itself in such a manner that its positively charged, lysine-rich, hydrophilicface points toward the negatively charged AOT head groups, while its hydrophobic face is directed toward the polarinterior of the water pool. This picture is in marked contrast to that observed for the binding of MPx to phospholipidbilayers wherein the hydrophobic surface of the bound α-helix is buried deeper into the membrane interior.
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