Characterizing the Intramolecular H-bond and Secondary Structure in Methylated GlyGlyH(+) with H-2 Predissociation Spectroscopy

We report vibrational predissociation spectra of the four protonated dipeptides derived from glycine and sarcosine, GlyGlyH(+)center dot(H-2)(1,2), GlySarH(+)center dot(D-2)(2), SarGlyH(+)center dot(H-2)(2), and SarSarH(+)center dot(D-2)(2), generated in a cryogenic ion trap. Sharp bands were recovered by monitoring photoevaporation of the weakly bound H-2 (D-2) molecules in a linear action regime throughout the 700-4200 cm(-1) range using a table-top laser system. The spectral patterns were analyzed in the context of the low energy structures obtained from electronic structure calculations. These results indicate that all four species are protonated on the N-terminus, and feature an intramolecular H-bond involving the amino group. The large blue-shift in the H-bonded N-H fundamental upon incorporation of a methyl group at the N-terminus indicates that this modification significantly lowers the strength of the intramolecular H-bond. Methylation at the amide nitrogen, on the other hand, induces a significant rotation (similar to 110 degrees) about the peptide backbone.