Structures of Fibrous Supramolecular Assemblies Constructed by Amino Acid Surfactants: Investigation by AFM, SANS, and SAXS

Abstract

Aqueous gel-like solutions of N-acyl-L-aspartic acids (C(n)Asp, n=14, 16, 18) and N-dodecanoyl-beta-alanine (C(12)Ala) were prepared at pH 5-6 at room temperature. Structures of supramolecular assemblies in the solutions were investigated by atomic force microscopy (AFM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). The cross-sectional radii, 22-30 Å, of helical, fibrous assemblies were obtained from analysis of SANS for 1% gel-like C(n)Asp solutions. Three Bragg spacings were observed in a SANS spectrum for a 6% C(16)Asp solution. C(n)Asp molecules are associated into the unit chain of a helical bilayer strand with a diameter of 50-60 Å. Unit chains where linear bilayers twist form a double strand with helical sense of approximately 650-Å pitch. It was confirmed from AFM images that cylindrical fibers in a gel-like C(12)Ala solution had a circular cross-section. The SAXS spectrum showed characteristic Bragg spacings. Cylindrical C(12)Ala fibers consist of multilamellar layers of period approximately 34-Å. The fibers are laterally organized with period 365-380 Å. Copyright 2000 Academic Press.