Conformational behavior and potential energy profile of gaseous histidine

Histidine as a natural amino acid is found to be biologically important and is known to function as a nucleophile or enzyme co-factor, or in proton transfer process. The properties of gaseous aromatic amino acid histidine depend on the structural forms it may take in gas-phase. Ab initio method has been used to characterize the gas-phase conformer/tautomers of histidine. Wide range of possible structures for histidine was surveyed at the MM level, and then the geometries of the unique conformers were refined at the B3LYP/6-311++G (d,p) levels.

At this theoretical level, 25 conformers were located for both tautomers of histidine i.e., His [N^πH] and His [N^τH]. The MM level provides a poor description of the relative energies. Calculations at the B3LYP/6-311++G (d,p) level represent a significant improvement. Ab initio dipole moments are reported for all the conformers/tautomers. The results are compared to previous studies of amino acids and are analyzed in terms of intramolecular hydrogen-bonding interactions and Newman projections. In addition, we calculated the infrared frequencies and intensities of the most stable structures in order to assist in the assignment of hydrogen-bonding. Furthermore, Ramachandran backbone potential energy surfaces (PES) of 25 conformers of His [N^πH] and His [N^τH] tautomers of histidine were considered.

The stationary points characterizing the potential energy profile of the various conformers/tautomers of histidine were investigated by the same density functional theory B3LYP/6-311++G (d,p). Minima and transition states characterizing the energetic paths for the interconversion of various structures of histidine were explored in detail.