Abstract
The importance of the guanidinium–carboxylate interactions has sprung from the observed salt bridges often present in biological systems involving the arginine–glutamate or arginine–aspartate side chains. The strength of these interactions has been explained on the basis of a great coulombic energy gain, due to the closeness of two charges of opposite sign and the occurrence of H-bond interactions. However, in some environments proton transfer, from guanidinium to carboxylate, can occur with the consequent annihilation of charge. In this work, both ab-initio (6-31G** and MP2/6-31G**) and semi-empirical (AM1) calculations were performed in vacuo on appropriate models, methylguanidinium–acetate and methylguanidine–acetic acid to simulate the zwitterionic and the neutral forms, respectively. The results obtained indicate that, in solvent-free hydrophobic environments, the neutral form should be more stable than the zwitterionic one.
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