Aggregation behaviour of synthetic peptide surfactants
Abstract
Single-chain surfactants with an amino-acid residue as part of a hydrophobic chain, SS′-bis-[N-dodecyl-Nα-(6-trimethylammoniohexanoyl)-L-hemicysteinamide] bromide [(N+C5CysC12)2], Ndodecyl-Nα-(6-trimethylammoniohexanoyl)-S-benzyl-L-cysteinamide bromide [N+C5Cys(Bzl)C12], and N-dodecyl-Nα-(6-trimethylammoniohexanoyl)-L-aspartamide bromide (N+C5AspC12) have been synthesized and the structures of the aggregates formed in aqueous media investigated. The peptide surfactants form tighter aggregates than ordinary micelles formed with octadecyltrimethylammonium chloride. The weight-average molecular weights of the aggregates in aqueous media were determined by the low angle laser light scattering technique: N+C5Cys(Bzl)C12, 7.04 × 106; (N+C5CysC12)2, 1.49 × 106; N+C5AspC12, 5.06 × 104. Consequently, N+C5AspC12 forms tight spherical micelles while (N+C5CysC12)2 and N+C5Cys(Bzl)C12 constitute tight and large cylindrical micelles. This aggregation behaviour was also confirmed by electron microscopy. The introduction of an amino-acid residue into the hydrophobic chain of a surfactant results in tightening of the aggregate structure in aqueous media, probably due to intermolecular hydrogen bonding among amino-acid moieties in the micelle.