Characterization of cooperative effects in linear α-glycylglycine clusters

The aspects of N-H center dot center dot center dot O=CNH, N-H center dot center dot center dot O=CO and C-H center dot center dot center dot O=CNH interactions are analyzed by applying ab initio and DFT methods as well as Bader theory. We investigated geometry, binding energies, O-17, N-15 chemical shift tensors. and Atoms in Molecules (AIM) properties of alpha-glycylglycine (alpha-glygly) clusters, via MP2, B3LYP and PW91(XC) methods. Dimer stabilization energies and equilibrium geometries are studied in various levels of theory. MP2 and DFT calculations reveal that for alpha-glygly clusters, stability of N-H center dot center dot center dot O and C-H center dot center dot center dot O hydrogen bonds are enhanced significantly as a result of cooperativity effects. Furthermore, a covalent nature is also detected for some hydrogen bondings. The n-dependent trend of O-17 and N-15 chemical shift tensors was reasonably correlated with cooperative effects in hydrogen-bond interactions. Regarding the various N-H center dot center dot center dot O=CNH, N-H center dot center dot center dot O=CO and C-H center dot center dot center dot O=CNH hydrogen bondings, capability of the alpha-glygly clusters for electron localization at the N-H center dot center dot center dot O and C-H center dot center dot center dot O bond critical points, depends on the cluster size. This leads to cooperative changes in the hydrogen-bond length and strength as well as O-17 and N-15 chemical shift tensors. (C) 2009 Elsevier B.V. All rights reserved.