Discrimination between diastereoisomeric dipeptides by IR-UV double resonance spectroscopy and ab initio calculations

We studied diastereoisomeric dipeptides, containing two chiral centers, by comparing ab initio calculations with laser desorption jet-cooling experiments. We studied the hetero-dipeptides LL-VF (L-Val-L-Phe) and DL-VF and the homo-dipeptides LL-FF (L-Phe-L-Phe) and LD-FF. Changing one of the chiral centers in each molecule leads to changes in the spectra that can be used to distinguish between diastereoisomeric pairs. We observed three different conformers for LL-VF, four for DL-VF, two for LL-FF, and one for LD-FF. By comparing the results from IR-UV double resonant spectroscopy with ab initio calculations, we can draw conclusions about the conformational structures. At the same time, the experimental data serve as a test for the computational results. We discuss the possibilities and limitations of the interplay between theory and experiment. (c) 2005 Wiley Periodicals, Inc. Int J Quantum Chem 105: 437-445, 2005.