Many-Body Effects in Systems of Peptide Hydrogen-Bonded Networks and Their Contributions to Ligand Binding: A Comparison of the Performances of DFT and Polarizable Molecular Mechanics

We compute the stabilization energies of the complexes formed between formate or water and a linear array of n = 2−5 N -methylformamide (NMF) molecules. We perform density functional theory (DFT), ab initio SCF and MP2, and SIBFA molecular mechanics computations. A very significant amount of cooperativity is found by DFT in the formate--(NMF) n complexes, amounting to −17 kcal/mol with n = 5. The SIBFA computations with fixed internal geometries for the monomers recover up to 80% of the DFT values. Single-point SCF/MP2 computations at the SIBFA-optimized geometries give binding energies and cooperativities very close to the SIBFA values. Solvation effects as represented by a continuum reaction field procedure are found to affect cooperativity to a modest extent. The implications for oligopeptides are discussed.