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Title: Theoretical analysis of co-solvent effect on the proton transfer reaction of glycine in a water–acetonitrile mixture

The co-solvent effect on the proton transfer reaction of glycine in a water–acetonitrile mixture was examined using the reference interaction-site model self-consistent field theory. The free energy profiles of the proton transfer reaction of glycine between the carboxyl oxygen and amino nitrogen were computed in a water–acetonitrile mixture solvent at various molar fractions. Two types of reactions, the intramolecular proton transfer and water-mediated proton transfer, were considered. In both types of the reactions, a similar tendency was observed. In the pure water solvent, the zwitterionic form, where the carboxyl oxygen is deprotonated while the amino nitrogen is protonated, is more stable than the neutral form. The reaction free energy is −10.6 kcal mol{sup −1}. On the other hand, in the pure acetonitrile solvent, glycine takes only the neutral form. The reaction free energy from the neutral to zwitterionic form gradually increases with increasing acetonitrile concentration, and in an equally mixed solvent, the zwitterionic and neutral forms are almost isoenergetic, with a difference of only 0.3 kcal mol{sup −1}. The free energy component analysis based on the thermodynamic cycle of the reaction also revealed that the free energy change of the neutral form is insensitive to the change of solvent environmentmore » but the zwitterionic form shows drastic changes. In particular, the excess chemical potential, one of the components of the solvation free energy, is dominant and contributes to the stabilization of the zwitterionic form.« less
Authors:
; ;  [1]
  1. Department of Chemistry, Graduate School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581 (Japan)
Publication Date:
OSTI Identifier:
22415857
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 142; Journal Issue: 20; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ACETONITRILE; CONCENTRATION RATIO; FREE ENERGY; GLYCINE; MIXED SOLVENTS; NITROGEN; POTENTIALS; PROTONS; SELF-CONSISTENT FIELD; SOLVATION; STABILIZATION; THERMODYNAMIC CYCLES; TRANSFER REACTIONS; WATER; ZWITTERIONIC COMPOUNDS