Comprehensive studies on the tautomerization of glycine: a theoretical study

• Published in: Organic & biomolecular chemistry 2013-02, Vol.11 (8), p.147-1413
• Main Authors: Kim, Chang Kon, Park, Byung-Ho, Lee, Hai Whang, Kim, Chan Kyung
• Format: Article
• Language: English
• Subjects: Glycine - analogs & derivatives; Glycine - chemical synthesis; Glycine - chemistry; Quantum Theory
• ISSN: 1477-0520; 1477-0539
• DOI: 10.1039/c2ob26602d

The tautomerization process of glycine between the neutral (NE) and zwitterionic (ZW) forms in aqueous solution was explored theoretically using the conductor-like polarizable continuum model (CPCM) by adopting the PAULING cavity model at the B3LYP, MP2 and CCSD levels with the 6-311+G(d,p) basis set. The tautomerization of glycine is unable to be predicted satisfactorily within the equilibrated framework of the CPCM method. Instead, in this study, three plausible non-equilibrated solvation situations were assumed: (S-1) one water molecule attached to the transferring proton in the ZW moves together with the transferring proton; (S-2) one water molecule attached to the transferring proton in the ZW remains motionless at a fixed position near the NH 2 fragment at the TS structure; and (S-3) proton transfer occurs without changing the position of the surrounding water molecules from their initial state, the ZW form, in the eight water clusters. Although the calculation of (S-3) failed, the Gibbs free energies of activation for tautomerization from the ZW to NE, Δ G ≠ (ZW → NE), was well consistent with the experimental findings in the hypothetical non-equilibrated solvation states of (S-1) and (S-2). This suggests that non-equilibrium solvation is essential to explain the observed experimental data. The tautomerization process of glycine between the zwitterionic (ZW) and neutral (NE) forms in aqueous solution was explained using the non-equilibrium solvation models in the CPCM calculations.