Theoretical study of arginine-carboxylate interactions

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 thes e 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 occurren ce of H-bond interactions. However, in some environments proton transfer, f rom 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 indi cate that, in solvent-free hydrophobic environments, the neutral form shoul d be more stable than the zwitterionic one. (C) 1999 Elsevier Science B.V. All rights reserved.