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-N^α-(6-trimethylammoniohexanoyl)-L-hemicysteinamide] bromide [(N⁺C₅CysC₁₂)₂], Ndodecyl-N^α-(6-trimethylammoniohexanoyl)-S-benzyl-L-cysteinamide bromide [N⁺C₅Cys(Bzl)C₁₂], and N-dodecyl-N^α-(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.