Comprehensive Studies on the Free Energies of Solvation and Conformers of Glycine: A Theoretical Study

Abstract

The stable conformers of glycine and the inter-conversions between them were studied theoretically at various levels of theory, B3LYP, MP2, CCSD and CCSD(T), in the gas phase and in aqueous solution. In aqueous solution, the structures examined by use of the conductor-like polarizable continuum model (CPCM) with various cavity models, UA0, UAHF, UAKS, UFF, BONDI and PAULING, and by use of a discrete/continuum solvation model with eight water clusters. The Gibbs free energy differences between the neutral (NE) and zwitterionic conformers (ZW), ΔG Z-N [= G ZW -G NE ], in aqueous solution were well reproduced by using the BONDI and PAULING cavity models. However the ΔG Z-N values were underestimated in other cavity models, although the ZW conformers existed as stable species in aqueous solution. In the studies of a discrete/ continuum solvation model with eight water clusters, gas phase results are still insufficient to reproduce the experimental findings. However the ΔG Z-N values calculated by use of CPCM method in aqueous solution agreed well with the experimental ones.

Temperature driven transformations of glycine molecules embedded in interstellar ice

“…These trends in isomer conformerisations are consistent with the fact that cc -N-Gly and tt -N-Gly are the most energetically stable forms of neutral glycine in the gas phase and in water. 34 In addition, rotations around the C–C bond ( ct -N-Gly → tt -N-Gly and tc -N-Gly → cc -N-Gly) appear to be to be more favourable than those around the C–O bond ( ct -N-Gly → cc -N-Gly and tc -N-Gly → tt -N-Gly), which is consistent with the higher rotational barriers around the C–O bond than those around the C–C bond, computed at the MP2 34 and CPCM-CCSDT 66 levels of theory.…”

Study of the Stepwise Deprotonation Reactions of Glyphosate and the Corresponding pKa Values in Aqueous Solution

“…In the literature, 41 it is commonly thought that a zwitterionic structure would be more stable in aqueous solution, perhaps due to observations analogous to glycine tautomerization. 50 It is important to mention that theoretical studies on glycine tautomerization demonstrate that it is necessary to consider a cavity model with explicit hydrogen atoms (Bondi or Pauling) to have the zwitterionic species be more stable than the uncharged species. The solvation model we have used satisfies this requirement, yet even when the hydrogen atoms of glyphosate are described by their own cavities, the final standard Gibbs free energy for the uncharged species is smaller than that for the zwitterions in the PCM (see Figure 5).…”

“…From Figure , it is clear that, among the species in the FC = 0 group, N1 is the most stable; i.e., the structure with no local charge is preferred relative to the zwitterionic structures. In the literature, it is commonly thought that a zwitterionic structure would be more stable in aqueous solution, perhaps due to observations analogous to glycine tautomerization . It is important to mention that theoretical studies on glycine tautomerization demonstrate that it is necessary to consider a cavity model with explicit hydrogen atoms (Bondi or Pauling) to have the zwitterionic species be more stable than the uncharged species.…”

Crystallization: From the Conformer to the Crystal

“…This has been done theoretically by performing full geometry optimizations using the CPCM continuum solvation model and the B3LYP/aug-cc-pVDZ model chemistry, both implemented in the Gaussian 03 program. The cavity was built with the Bondii radii which have been found to yield accurate results for the hydration of similar molecules in the zwitterionic state, such for example glycine [80]. Nine starting geometries were built by assuming the three standard orientations (g, g' and t) for each one of the C-C-C-C dihedrals.…”