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| - Conformation of the Dipeptide Cyclo(L-Pro-L-Pro) Monitored by the Nuclear MagneticResonance and Raman Optical Activity Spectra. Experimental and ab Initio ComputationalStudy
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| - Nuclear magnetic resonance (NMR) and Raman optical activity (ROA) spectra of the cyclic dipeptide weremeasured and analyzed with respect to their ability to sense molecular structure and conformation. Dataobtained by both techniques were simulated using ab initio quantum mechanical computations. Calculatedchemical shifts, hydrogen−hydrogen spin−spin coupling constants and ROA intensities agreed well with theexperimental values. The spin−spin NMR coupling constants were found to be most suitable for estimatingof the conformational ratio. The ROA intensities provided additional information about the absoluteconfiguration. The relation of the NMR chemical shifts to molecular structure was obscured by the solventeffect. The experimental results and calculated relative conformer energies suggest that equilibrium of threeconformations takes place in the solution at the room temperature with a prevalence (∼80%) of the conformationpresent in the crystalline state.
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