The structure of the gas-phase tyrosine-glycine-glycine tripeptide

The structure of the neutral gas-phase tripeptide Tyr-Gly-Gly has been investigated using different strategies that employ a hierarchy of electronic structure theory (single-point HF/3-21G* energy calculation, HF/3-21G* geometry optimization, B3LYP/6-31+G* geometry optimization and MP2/6-31+G* single-point energy calculation). All 20 most stable conformers according to the single-point MP2 calculations adopt a folded structure. In the most stable structure found the C-terminal carboxylic acid group interacts with both the carboxyl C=O of glycine (2) and the tyrosine OH (forming an OH (.) (.) (.) O=C-OH (.) (.) (.) O=C hydrogen-bonding chain), with an additional NH (.) (.) (.) N interaction involving the NH of glycine (2) and the N-terminal amino group. Harmonic vibrational frequencies (N-H, C-H and C=O stretch frequencies and NH and OH in-plane bending frequencies) are reported to aid future spectroscopic studies on this peptide.