Journal of Physical Chemistry A, Vol.106, No.32, 7321-7327, 2002
Conformation of the dipeptide cyclo(L-Pro-L-Pro) monitored by the nuclear magnetic resonance and Raman optical activity spectra. Experimental and ab initio computational study
Nuclear magnetic resonance (NMR) and Raman optical activity (ROA) spectra of the cyclic dipeptide, were measured and analyzed with respect to their ability to sense molecular structure and conformation. Data obtained by both techniques were simulated using ab initio quantum mechanical computations. Calculated chemical shifts, hydrogen-hydrogen spin-spin coupling constants and ROA intensities agreed well with the experimental values. The spin-spin NMR coupling constants were found to be most suitable for estimating of the conformational ratio. The ROA intensities provided additional information about the absolute configuration. The relation of the NMR chemical shifts to molecular structure was obscured by the solvent effect. The experimental results and calculated relative conformer energies suggest that equilibrium of three conformations takes place in the solution at the room temperature with a prevalence (similar to80%) of the conformation present in the crystalline state.