Protein folding and functions depend extremely on the amino acid conformations. Thus, the knowledge of the conformations of amino acids is essential to understand the biochemical behaviors and physical properties of proteins that contain the building blocks called amino acids. Comprehensive computational study is focused to understand the conformational landscape of aromatic amino acids (AAAs) tryptophan (W), tyrosine (Y), and phenylalanine (F) in gaseous and aqueous phases. Full geometry optimizations of all the conformers generated using molecular mechanics method for these amino acids were performed at DFT methods that include B3LYP, M06-2X and ωB97X-D functionals using the basis sets 6-31G(d) and 6-31+G(d,p). The global minimum structures of neutral AAAs are stabilized by intramolecular hydrogen bond linking OH of the carboxylic acid to the amino group (O-H…N hydrogen bond). Our study indicates that DFT levels generally produce similar trends of relative energies of conformers for each of the amino acids. Overall, AAAs conformers that possess high intramolecular interactions are generally observed more stable than conformers having weak or low number of hydrogen bonds. Conformers with low relative energies exhibit high value of dipole moment in many cases. Several conformers were experimentally reported. The IR frequency values obtained using different levels of DFT are compared against the experimental results reported for different conformers of AAAs. The n(C=O) stretching frequencies at different DFT levels are in good agreement with the experimental results. ωB97X-D/6-31+G(d,p) level is less favorable than other DFT levels in case of O-H and N-H stretching frequencies for the conformers of AAAs.

Conformational exploration of aromatic amino acids: Assessment of DFT levels by comparison of vibrational frequencies with experimental data

2019 Fall National Meeting

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