Theoretical Study of Glycine Conformers

Abstract

Glycine conformers were investigated with three density functional theory (DFT) methods (B3LYP, PBE1PBE, X3LYP), and the second order Moller-Plesset perturbation theory (MP2) combined with basis sets of 6-31+ G*, aug-cc-pVDZ, and aug-cc-pVTZ. Solvation effects were considered by using polarizable continuum model. Results from B3LYP and X3LYP were in generally good agreement with those of MP2, while PBE1PBE was shown to be inferior for the description of conformational potential energy surfaces. Conformers Ip, IIp, IIn, IIIp, IIIn, and IVn were all found to be low-lying states within 2.0 kcal/mol, with Ip being the global minimum in gas phase. Solvation effects can significantly change the nature of the conformational surfaces of glycine. A proper description of conformational equilibrium demands for a good treatment of both longrange and short-range solute-solvent interactions.National Natural Science Foundation of China [20525311, 20533030, 10774126, 20433030]; Ministry of Science and Technology [2004CB719902, 2007CB815206]; Research Grants Council of Hong Kong SAR Governmen

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

“…Therefore the theoretical studies of glycine in aqueous solution have been mainly carried out by three different approaches; (i) calculations of the solute surrounded by some discrete water molecules [10][11][12][13][14][15][16][17][18][19][20][21] (ii) calculations by statistical approaches such as Monte Carlo (MC) 25 or molecular dynamics (MD), 14,19,[22][23][24] and (iii) calculations by use of a dielectric continuum methodology. [26][27][28][29][30][31][32][33] Nevertheless, the experimental findings 1-9 could not be successfully reproduced by most of these approaches, although some qualitative characteristics were consistent with the experimental findings. 2,5 For example, it has been shown that the ZW of glycine exists as an energy minimum when at least two water molecules are included.…”

“…[1][2][3][4][5][6][7][8][9] High-level ab initio studies demonstrated that the neutral form (NE) of glycine exists in the gas phase but the zwitterionic form (ZW) is unstable in vacuo. [11][12][13][14][15][16][17][18][19][20]27,[30][31][32][33] However, as is well known, the ZW of glycine predominates in crystalline or in aqueous solution, 2 and hence the solvent (or environmental) effects should be included in the theoretical studies to explore the glycine chemistry correctly. Therefore the theoretical studies of glycine in aqueous solution have been mainly carried out by three different approaches; (i) calculations of the solute surrounded by some discrete water molecules [10][11][12][13][14][15][16][17][18][19][20][21] (ii) calculations by statistical approaches such as Monte Carlo (MC) 25 or molecular dynamics (MD), 14,19,[22][23][24] and (iii) calculations by use of a dielectric continuum methodology.…”

“…However, the continuum models could not reproduce accurately the characteristics of glycine such as the experimental energy difference between the NE and ZW species. 27,[30][31][32][33] Moreover, the main drawback of the continuum model could be the lack of microscopic information such as specific interactions between solute and solvent molecules. 69 Recently, Bandyopadhyay and Gordon…”

“…In earlier works, 11,17,19,20,27,[31][32][33]38,39 various conformers for the NE of glycine shown in Scheme 1 were considered in the gas phase, and the NE3 structure was found to be the global minimum. In the NE2, NE3, and NE4 structures, the NH 2 group bisected the molecular plane, denoted hereafter as Cs-like conformers, while the NH 2 group in the NE2-1, NE3-1, and NE4-1 structures positioned in the same plane, labeled as C 1 conformers.…”

“…The last five NE conformers, NE5~NE9, were not considered because they were clearly unstable due to repulsive interaction between lone pairs on nitrogen and oxygen or between hydrogens facing each other. [31][32][33] For the calculations in aqueous solution, all the structures have examined by use of the conductor-like polarizable continuum model (CPCM) with various cavity models and by use of a discrete/continuum solvation model with eight water clusters. In the continuum model, the choice of a cavity model is very important because the free energy of solvation, G s , largely depends on the choice of cavity model.…”