Pressure-induced structural changes of alanine oligopeptides in aqueous solutions

ABSTRACT

Short alanine (Ala) oligopeptides in aqueous solution adopt polyproline II [PPII; (φ, ψ)?=?(?60°, 150°)] and extended β conformations [(φ, ψ)?=?(?150°, 150°)], whose conformers are related to the denatured state of proteins. In this study, we investigated pressure-induced conformational changes of penta- and hexa-alanines (Ala₅ and Ala₆, respectively) in aqueous solutions using Fourier-transform infrared (FTIR) spectroscopy. A remarkable observation was that two peaks at 1620 and 1690 cm^?1 in Ala₆ assigned to the intermolecular β-sheets were generated with increasing pressure. These peaks were not observed in Ala₅. Our analyses of absorbance changes and frequency shifts further suggested that pressure was responsible for the PPII?→?β conformational change of Ala₅, and the PPII?→?intermolecular β-sheet structure of Ala₆, respectively. These results indicated a differing conformational stability of Ala₅ under high pressure as compared with Ala₆.