Sequence-dependent conformations of short polypeptides in a hydrophobic environment

Surfactants, which provide a hydrophobic environment, may induce an ordered conformation in polypeptides and proteins that contain a sequence with helix-or beta-forming potential. This hypothesis has been illustrated in circular dichroic studies of oligopeptides and short polypeptides. These peptide-surfactant complexes can form (1) a helix, (2) a beta-form, (3) either form (depending on experimental conditions), or can remain in (4) an ordered form. The induced helix is stable in a surfactant solution below or above its critical micellar concentration, whereas the induced beta-form is usually converted back to an unordered form when the surfactant used is above its critical micellar concentration, or it is transformed into a helix in excess surfactant solution if the peptide has both the helix- and beta-forming potential. In most cases the observed conformations agree with those predicted from the amino acid sequences of the peptides. The induced conformation of a peptide can be destabilized by charges on the side groups having the same sign as that of surfactant ions. Disulfide bonds can inhibit the formation of induced conformation because of steric hindrance. The terminal effect can prevent a peptide from forming an ordered conformation near the NH2- and COOH-terminus.