Vibrational Spectra of N-Acyl-l-Glutamic Acid Oligomers and the Long N-Acyl Chain Effect

Abstract

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.