Site-Specific Hydrogen-Bonding Interaction between N-Acetylproline Amide and Protic Solvent Molecules: Comparisons of IR and VCD Measurements with MD Simulations

The effects of solute-solvent interactions on solution structures of small peptides have been paid a great deal of attention. To study the effect of hydrogen-bonding interactions on peptide solution structures, we measured the amide I IR and VCD spectra of N-acetylproline amide (AP) in various protic solvents, i.e., D2O, MeOD, EtOD, and PrOD, and directly compared them with theoretically simulated ones. The numbers of protic solvent molecules hydrogen-bonded to the two peptide bonds in the AP were quantitatively determined by carrying out the molecular dynamics (MD) simulations and then compared with the spectral analyses of the experimentally measured amide I bands. The two peptides in the AP have different propensities of forming H-bonds with protic solvent molecules, and the H-bond population distribution is found to be strongly site-specific and solvent-dependent. However, it is found that adoption of the polyproline II (PII) conformation by AP in protic solvents does not strongly depend on the hydrogen bond network-forming ability of protic solvents nor on the solvent polarity. We present a brief discussion on the validity as well as limitation of the currently available force field parameters used for the present MD simulation study.