Infrared evidence for the α-helical-type structure of very simple N-octanoyl-L-glutamic acid oligomers (residue number, N = 1-4) and for the α-CO2- group effect on their α → β-like transition

Very simple N-octanoyl-L-glutamic acid oligomers (residue number, N = 1-4) have been synthesized and the vibrational spectra of these oligomer films, made by casting them from a dimethylformamide solution onto an NaCl plate, have been investigated together with their time dependence. The results have been compared with those of two β-forms (β1 and β2) and the α-helix of the longer N-octanoyl-L-glutamic acid oligomers (N = 6-14) and with those of poly(L-glutamic acid). Even the N-octanoyl monomer and the dimer as well as the trimer and the tetramer take up an α-helical-type structure, in which the hydrogen-bonding network formed by the α,γ-CO2H and the peptide groups are very similar to that in α-helical poly(L-glutamic acid). For the cast films of these simple oligomers, a transition similar to the α → β1 or α → β2 transition in poly(L-glutamic acid) is observed at 20°C. In particular, ionization of the α-CO2H groups in the cast films is promoted at a higher relative humidity. The α → β-like transition is promoted by the α-CO2- group so formed