The gas phase conformers and vibrational spectra of valine, leucine and isoleucine: An ab initio study

The present work reports the results of a conformational study performed on three essential non-polar amino acids: valine, leucine and isoleucine. Ab initio calculations were done at the MP2/6-311++G(2d,2p) level in the gas phase. The 6, 8 and 12 conformers served as a basis to represent the shapes of valine, leucine and isoleucine in the gas phase, respectively, with the relative energies (ΔETotal) below 1.25 kcal mol−1 compared to the most stable conformer of each amino acid. The ΔETotal values are reported at the MP2/6-311++G(2d,2p) level with corrections for zero-point vibrational energies. The lowest energy conformers of each amino acid contained the intramolecular hydrogen bond (H-bond) interactions between the NH2 and COOH groups which are the N-H…O = C, N-H…O-H and O-H…N-H H-bonds. It is obvious that the H-bond interaction plays an important role in the conformational energy and harmonic vibrational frequencies. From the theoretical vibrational spectra, the O-H out of plane and in-plane bending shift ranged from ∼1100 cm−1 to ∼1360 cm−1 and the O-H stretching shifts from ∼3600 cm−1 to 3400 cm−1 due to the O-H…N-H H-bond interaction. In addition, the side-chain orientations can also affect the conformational stability and diversity, resulting in a large number of low energy conformational minima of amino acid.