Proton transfer in arginine-carboxylate interactions

Arginine-carboxylate interactions in proteins have aroused much interest due to the important role they play in the stability of biological systems. These interactions have usually been interpreted as being associated with a zwitterionic state as opposed to a neutral one. In this work, ab initio (6–31G∗∗ basis set) calculations were carried out in vacuo on appropriate models, methylguanidinium-acetate and methylguanidine-acetic acid, to simulate the zwitterionic and neutral forms, respectively. The results obtained reveal that the neutral form is more stable than the zwitterion, i.e. proton transfer should occur with the consequent annihilation of charge in some environments, possibly hydrophobic ones.