Conformational preferences and prolyl cis-trans isomerization of phosphorylated Ser/Thr-Pro motifs

The conformational study on Ac-pSer-Pro-NHMe and Ac-pThr-Pro-NHMe peptides has been carried out using hybrid density functional methods with the implicit solvation reaction field theory at the B3LYP/ 6-311++G(d,p)//B3LYP/6-31+G(d) level of theory in the gas phase and in solution (chloroform and water). For both pSer-Pro and pThr-Pro peptides in the gas phase and in chloroform, the most preferred conformation has the alpha-helical structure for the pSer/pThr residue, the down-puckered polyproline I structure for the Pro residue, and the cis prolyl peptide bond between the two residues, in which two hydrogen bonds between the phosphate oxygens with the backbone N--H groups seem to play a role. However, the trans conformations that have a single hydrogen bond of the phosphate oxygen with either of two backbone N--H groups become most preferred for both peptides in water. This is because the hydration free energy of the anionic oxygen of the phosphate group is expected to dramatically decrease for the cis conformation upon formation of the hydrogen bond with the backbone N--H groups. These calculated results are consistent with the observations by NMR and IR experiments, suggesting the existence of hydrogen bonds between the charged phosphoryl group and the backbone amide protons in solution. The calculated cis populations of 14.7 and 14.2% and rotational barriers of 19.87 and 20.57 kcal/mol to the cis-to-trans isomerization for pSer-Pro and pThr-Pro peptides in water, respectively, are consistent with the observed values for pSer-Pro and pThr-Pro containing peptides from NMR experiments. However, the hydrogen bond between the prolyl nitrogen and the following amide N--H group, which was suggested to be capable of catalyzing the prolyl isomerization, does not play a role in stabilizing the preferred transition state for the pSer/pThr-Pro peptides in water. Instead, the amide hydrogen of the NHMe group is involved in a bifurcated hydrogen bond with the anionic oxygen and phosphoester oxygen of the phosphate group.