Structures of fibrous supramolecular assemblies constructed by amino acid surfactants: Investigation by AFM, SANS, and SAXS

Aqueous gel-like solutions of N-acyl-L-aspartic acids (C(n)Asp, n=14, 16, 1 8) 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 scatteri ng (SANS), and small-angle X-ray scattering (SAXS), The cross-sectional rad ii, 22-30 Angstrom, of helical, fibrous assemblies were obtained from analy sis of SANS for 1% gel-like C(n)Asp solutions. Three Bragg spacings were ob served in a SANS spectrum for a 6% C16AsP solution. C(n)Asp molecules are a ssociated into the unit chain of a helical bilayer strand with a diameter o f 50-60 Angstrom. Unit chains where linear bilayers twist form a double str and with helical sense of similar to 650-Angstrom pitch. It was confirmed f rom AFM images that cylindrical fibers in a gel-like C(12)Ala solution had a circular cross-section. The SAXS spectrum showed characteristic Bragg spa cings. Cylindrical C(12)Ala fibers consist of multilamellar layers of perio d similar to 34-Angstrom. The fibers are laterally organized with period 36 5-380 Angstrom. (C) 2000 Academic Press.