Structure of the gas-phase glycine tripeptide.

The structural preferences of the neutral gas-phase glycine tripeptide have been investigated using a variety of strategies including a hierarchy of electronic structure theory (encompassing HF/3-21G single-point energy calculation and geometry optimisation, B3LYP/6-31G(d) geometry optimisation and MP2/6-31+G(d) single-point energy calculation and/or geometry optimisation). The structures and relative stabilities of the 20 most stable conformers identified were verified by M05-2X and mPW2-PLYP-D calculations. The most stable conformer located has a folded gamma-turn structure, with an NH...N interaction between the N-terminal nitrogen and the amide hydrogen of glycine (2) and an NH...O interaction between the amide hydrogen of glycine (3) and the carboxyl oxygen of glycine (1). The results show a clear preference for folded over extended structures.