Two β-forms and the α-helix of N-octanoyl-L-glutamic acid oligomers

Abstract

N-Octanoyl-L-glutamic acid oligomers (residue number, N= 3–6, 8, 10, 12, 14, 16, 18, 20 and 22) have been synthesized in order to study their molecular conformations in the solid state. The X-ray diffraction powder patterns and the vibrational spectra of these oligomers have been investigated and compared with those of two β-forms (β₁ and β₂) and the α-helix of poly(L-glutamic acid).

The results are summarized as follows. These oligomers take up a β₁- or β₂-like structure, similar to that of the two β-forms of poly(L-glutamic acid). The methods used to precipitate the samples are related to the conformational preferences in the solid state. β₁↔β₂ interconversion is possible by reprecipitation of the sample. Preferential stabilization of the β₁- or β₂-forms is strongly dependent on the residue number. Films of the oligomers, made by casting them from a dimethylformamide solution onto NaCl or KBr plates, take up an α-helical structure, and the α→β₁ or α→β₂ transition depends on the residue number of the cast film.