Puckering transition of proline residue in water

The puckering transition of the proline residue with trans and cis prolyl peptide bonds was explored by optimizations along the torsion angle chi1 of the prolyl ring using quantum-chemical methods in water. By analyzing the potential energy surfaces and local minima in water, it is observed that the puckering transition of the proline residue proceeds from a down-puckered conformation to an up-puckered one and vice versa through the transition state with an envelope form having the N atom at the top of the envelope and not a planar one, as seen in the gas phase, although the backbone conformations are different in the gas phase and in water. The barriers to the puckering transition DeltaGup-->down are estimated to be 3.12 and 3.00 kcal/mol for trans and cis conformers at the B3LYP/6-311++G(d,p) level of theory in water, respectively, which are about 1.7 kcal/mol higher than those in the gas phase. Out of 2197 prolines from the 241 high-resolution PDB chains, four transition-state-like structures with the envelope ring puckering are identified. Three of them have the trans prolyl peptide bonds and one has the cis one. The favorable or steric interactions by neighboring residues may be responsible for the stabilization of these transition-state-like ring structures in the proteins.