Conformational preferences of taurine in the gas phase and in water

All feasible local minima of the neutral taurine (2-aminoethanesulfonic acid) in the gas phase and its zwitterionic form in water are explored using a density functional (M06-2X) that provides an accurate treatment of medium-range correlation energy and the recently improved solvation model SMD with the larger basis set (cc-pVTZ). In the gas phase, the conformer gtGg (II1) with the gauche− conformation around the NCH2CH2S sequence and the hydrogen bond between the sulfonic OH and the amine NH2 is found to be most preferred, which can be ascribed to the favored hyperconjugative n → σ* interaction between the lone electron pair of the amine nitrogen and the antibonding OH orbital of the sulfonic moiety. The calculated rotational constants of the conformer gtGg are consistent with the values obtained from LA-MB-FTMW experiments. In addition, the feasible structures of taurine± · (H2O)n complexes with a small number of water molecules (n = 0, 1, 2, and 4) were investigated in the implicit SMD aqueous medium. The calculated results of taurine± with four water molecules support that taurine exists as a mixture of gauche and trans conformers in aqueous solution. Its gauche population is calculated to be 71%, which is in good agreement with the observed value of 67% from 1H NMR experiments in D2O. It is known that the increased population of the trans conformer for the taurine± · (H2O)4 complex is ascribed to the preferred solvation by the interactions between the solute and the discrete water molecules in the complex and by the polarization of the continuum water due to the complex.