Accordion-like Oscillation Mode of Helical Peptides Analyzed by Terahertz Time-domain Spectroscopy

A series of Boc-(l-Ala-Aib)n-OCH3 (n = 2, 4, 6, 8) and Boc-(l-Leu-Aib)n-OCH3 (n = 2, 4, 6, 8) peptides were synthesized and subjected to terahertz time-domain (THz-TD) spectroscopy. Signals specific to helical peptides were observed around 2 and 3 THz, which were assignable to accordion-like oscillation modes, based on DFT calculations. Boc-(l-Ala-Aib)8-OCH3 showed intensified signal when compared with Boc-(l-Leu-Aib)8-OCH3, which is due to the large amplitude of the accordion-like oscillation. The accordion-like oscillation modes are discussed in relation with the higher ability of (l-Ala-Aib)8 than (l-Leu-Aib)8 as an electron-transfer mediator.