ACCURATE AB INITIO QUANTUM CHEMICAL DETERMINATION OF THE RELATIVE ENERGETICS OF PEPTIDE CONFORMATIONS AND ASSESSMENT OF EMPIRICAL FORCE FIELDS

Correlated ab initio calculations have been carried out with a parallel version of the PSGVB electronic structure code to obtain relative energetics of a number of conformations of the alanine tetrapeptide. The highest level of theory utilized, local MP2 with the cc-pVTZ(−f) correlation-consistent basis set, has previously been shown to provide accurate conformational energies in comparison with experiment for a data set of small molecules. Comparisons with published and new canonical MP2 calculations on the alanine dipeptide are made. Results for ten gas-phase tetrapeptide conformations and a β-sheet dipeptide dimer are compared with 20 different molecular mechanics force field parametrizations, providing the first assessment of the reliability of these models for systems larger than a dipeptide. Comparisons are made with the LMP2/cc-pVTZ(−f) results, which are taken as a benchmark for the tetrapeptides. Statistical summaries with regard to energetics and structure are produced for each force field, and ...