First-principle studies of gaseous aromatic amino acid histidine

Abstract

The properties of gaseous aromatic amino acid l-histidine depend on the structural forms it may take in gas phase. Systematic ab initio calculations were employed to characterize the conformational topology of the gaseous histidine. A total of 42 unique local minima conformers were located at the B3LYP/6-311G ∗ level of theory after geometry optimization of all possible single-bond rotamers. Single-point energy calculations of all conformers were performed at the levels of MP2/6-311++G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311G(2df,p). The equilibrium distributions of the gaseous histidine conformers at various temperatures were obtained based on the principle of thermodynamics. Due to the big energy gap between the lowest energy conformer and the other conformers, the most stable conformer of histidine is dominant in the gas phase, a unique feature that is characteristically different from that of the other three aromatic amino acids, tryptophan, tyrosine and phenylalanine. The measurable quantities such as the rotational constants, dipole moments, vibrational spectra and ionization energies were also given for comparison with future experiments. In analyzing the structural characteristics, three types of H-bond (OH⋯N 3, OH⋯N 4 and N 3H⋯N 4) were identified and characterized in detail by the atoms in molecules (AIM) theory based upon the B3LYP/6-311++G(d,p) electron density ρ( r ). The H-bonding features of the two most stable conformers are similar for all the aromatic amino acids. The values of vertical ionization energies for all the conformers suggest that the ionization depends strongly on the type of intramolecular interaction in the neutral conformers. The average adiabatic ionization energy for the six lowest energy conformers is lower than the corresponding average vertical ionization energy by an amount of 0.45 eV due to the structural relaxations.