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The exact structure of an arginine-carboxylate salt bridge in different chemical environments remains a controversial problem. In the present work, the zwitterionic and neutral forms of arginine-carboxylate salt bridge were studied by the B3LYP/6-311G(d,p)//PM3 method. It turns out that the neutral forms are more stable than the zwitterionic counterparts in gas phase. However, when bound by α-cyclodextrin, the zwitterionic forms become more stable than the corresponding neutral ones. It is suggested that the hydrophobic environment provided by the cyclodextrin cavity leads to such behavior. Therefore, the salt bridge still could be in a zwitterionic form in the hydrophobic interior of the real proteins.
The exact structure of an arginine-carboxylate salt bridge in different chemical environments remains a controversial problem. In the present work, the zwitterionic and neutral forms of arginine-carboxylate salt bridge were studied by the B3LYP / 6-311G (d, p) / It turns out that the neutral forms are more stable than the zwitterionic counterparts in gas phase. However, when bound when α-cyclodextrin, the zwitterionic forms become more stable than the corresponding neutral ones. It is suggested that the hydrophobic environment Provided by the cyclodextrin cavity leads to such behavior. Therefore, the salt bridge still could be in a zwitterionic form in the hydrophobic interior of the real proteins.