IR Study of Cross-Strand Coupling in a β-Hairpin Peptide Using Isotopic Labels

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Abstract
Model β-hairpin peptides can be used to develop understanding of fundamental elements of β-sheet secondary structure formation and stability. We have studied two 13C-labeled variants of a β-hairpin peptide modified from a design originally proposed by Gellman: Arg-Tyr-Val-Glu-Val-Aib-Gly-Lys-Lys-Ile-Leu-Gln. (In this peptide, the two italicized residues form a β-turn, while 13C-labels are on the amide CO of Val3, Lys8 in HBG-L and Val3, Ile10 in HBG-S.) Both these peptides are labeled on opposite strands of the hairpin, but differ in the labeling pattern. One (HBG-L) forms a large (14-atom) H-bonded ring of labeled COs, while the other (HBG-S) forms a small (10-atom) H-bonded ring. These impact the amide I infrared spectra, with HBG-L having a 13C frequency and intensity higher than that of HBG-S, in good agreement with our spectral simulations based on quantum mechanically derived force fields. The thermal behavior of both peptides yields a broad thermal transition and lacks an isosbestic point. The 13C band for HBG-L has the largest intensity change with temperature, distinct from the 12C change and the HBG-S13C change.