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 (Delta E-Total) below 1,25 kcal mol(-1) compared to the most stable conformer of each amino acid. The Delta E-Total 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 center dot center dot center dot O = C, N-H center dot center dot center dot O-H and O-H center dot center dot center dot 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 similar to 1100 cm(-1) to similar to 1360 cm(-1) and the O-H stretching shifts from similar to 3600 cm(-1) to similar to 3400 cm(-1) due to the O-H center dot center dot center dot 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. (C) 2010 Elsevier B.V. All rights reserved.