Vibrational circular dichroism of tartaric acid in water

Although tartaric acid (TA) and its esters have been among the first molecules to be characterized by means of VCD spectroscopy, the experimental spectra of TA recorded in water have not yet been reproduced using DFT-based spectral calculations. In the present study, we investigated the VCD spectra of TA in water and heavy water. We demonstrate that the commonly applied micro-solvation approach, i.e., the modelling of the effect of solvation by considering only a few solute-solvent interactions, is not sufficient to reproduce the experimentally observed VCD signatures. Following a recently introduced solvent-shell approach, we thus simulated the spectra based on the clusters of 30 water molecules (∼1.5 solvation shell), which were extracted from QM/MM molecular dynamics simulations at the AM1 and PM6 levels for the solute. An exceptionally good match with the experiment is achieved for both TA-(H₂O)₃₀ and TA-d₄-(D₂O)₃₀, which stressed the need to consider a significant number of solvent molecules in the spectral prediction. The same approach was applied to disodium tartrate (TA2-), for which the QM/MM MD simulations surprisingly revealed different conformational preferences depending on the chosen semi-empirical level. By comparison of the simulated VCD spectra obtained for the two conformational distributions, it is confirmed that the AM1-based MD simulations provide a better microscopic picture of TA2- in water.