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 beta-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 farce field, and a discussion of qualitative successes and failures is provided. The results display both the successes and limitations of the force fields studied and can be used as benchmark data in the development of new and improved force fields. In particular, comparisons of hydrogen-bonding energetics as a function of geometry suggest that future force fields will need to employ a representation for electrostatics that goes beyond the use of atom-centered partial charges.