Pressure-induced structural changes of alanine oligopeptides in aqueous solutions

Short alanine (Ala) oligopeptides in aqueous solution adopt polyproline II [PPII; (phi, psi) = (-60 degrees, 150 degrees)] and extended beta conformations [(phi, psi) = (-150 degrees, 150 degrees)], whose conformers are related to the denatured state of proteins. In this study, we investigated pressure-induced conformational changes of penta- and hexa-alanines (Ala(5) and Ala(6), respectively) in aqueous solutions using Fourier-transform infrared (FTIR) spectroscopy. A remarkable observation was that two peaks at 1620 and 1690 cm(-1) in Ala(6) assigned to the intermolecular beta-sheets were generated with increasing pressure. These peaks were not observed in Ala(5). Our analyses of absorbance changes and frequency shifts further suggested that pressure was responsible for the PPII -> beta conformational change of Ala(5), and the PPII -> intermolecular beta-sheet structure of Ala(6), respectively. These results indicated a differing conformational stability of Ala(5) under high pressure as compared with Ala(6).