No abstract

Vibrational spectroscopic evidence for conformational change of the anions of N-acylglycine oligomers (trimers, tetramers, and pentamers) in aqueous solution is reported. The infrared absorption spectra of sample solutions diluted below the critical micelle concentration (cmc) can be explained by the coexistence of several conformations containing the polyglycine I-like extended form and the polyglycine II-like helix. However, above the cmc the infrared bands arising from the polyglycine II-like helix are intensified. This result is due to preferential stabilization of the helical structure, promoted by intermolecular association of the anions. The helical structure is also stable in the hydrophobic environment of micelles.

No abstract

Trimers and tetramers of N-acyl-l-glutamic acid oligomers (Glu-oligomers) with various acyl chains have been prepared. For these Glu-oligomer molecules the vibrational spectra were measured and compared with those of two β-forms (β1 and β2) and the α-helical form of poly(l-glutamic acid). For these samples the molecules take up a β2-type structure in the solid state. The long acyl chain in the oligomers induces stronger hydrogen bonds between the side-chain COOH and the amide linkages; this effect is due to the close packing of the long acyl chains accompanying the reduction of the intersheet spacing. Moreover, the long acyl-chain effect strongly depends upon the residue number of the oligomer. For cast films of Glu-oligomers made onto an NaCl plate, these oligomer molecules assume an α-helical type structure. For the α-helical type oligomers, it is also found that the long acyl chain induces stronger hydrogen bonds in the side-chain COOH groups, and that the effect depends on the residue number.

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 /?-forms (fil and /?*) and the a-helix of poly(L-glutamic acid). The results are summarized as follows. These oligomers take up a plor /?,-like structure, similar to that of the two /?-forms of poly(L-glutamic acid). The methods used to precipitate the samples are related to the conformational preferences in the solid state. P1 t) p2 interconversion is possible by reprecipitation of the sample. Preferential stabilization of the P1or /?,-forms is strongly dependent on the residue number. Films of the oligorners, made by casting them from a dimethylformamide solution onto NaCl or KBr plates, take up an a-helical structure, and the a -+ P1 or a -+ p2 transition depends on the residue number of the cast film.