Time Dependent Density Functional Theory Modeling of Chiroptical Properties of Small Amino Acids in Solution

Time dependent density functional theory (TDDFT) and the conductor-like screening model (COSMO) of solvation were used to model the specific rotation and optical rotatory dispersion (ORD) of alanine, proline and serine solutions. Zwitterionic, cationic and anionic forms of amino acids were investigated and the results compared with experimental literature data obtained in neutral, acidic and basic conditions, respectively. It was found that TDDFT consistently underestimated the electronic excitation energies of the molecules, leading to calculated optical rotations that are of the correct sign but somewhat larger in magnitude than those of experiment. An additional challenge was encountered in the modeling of serine, an amino acid with a strong tendency to form intramolecular hydrogen bonds. The model used overestimated the extent of such hydrogen bonding for the zwitterions while possibly underestimating such bonding for the cationic form. This effect on the calculated mole fractions of the different conformers had an impact on the specific rotation.