It is known that a C-terminal lysine stabilizes helix formation in polyalanine peptides that have seven or more residues. Using a combination of cold ion spectroscopy and DFT calculations, we demonstrate that even a three-residue peptide, Ac-Phe-Ala-LysH+, adopts a structure in which the lysine side chain forms three hydrogen bonds with backbone carbonyls, reproducing the capping motif of larger polyalanine helices. This is confirmed by comparison with Ac-Phe-(Ala)5-LysH+, which forms a 310 helix containing the same structural feature. In both molecules, we identified the vibrational bands of the N- and C-terminal amide NH stretches, which lack strong hydrogen bonds with carbonyls and consequently appear in a characteristic region above 3400 cm?1. A similar pattern is also present in the even longer peptide Ac-Phe-(Ala)10-LysH+, illustrating the generality of this capping motif. The two longer peptides contain additional, characteristic amide NH stretch bands below 3400 cm?1, which form the core of the helix.
Federico De Biasi, Paolo Costa
Nako Nakatsuka, Xinyu Zhang, Haiying Hu