Gaseous arginine conformers and their unique intramolecular interactions

Extensive ab initio calculations were employed to characterize stable conformers of gaseous arginine, both the canonical and zwitterionic tautomers. Step-by-step geometry optimizations of possible single-bond rotamers at the B3LYP/6-31G(d), B3LYP/6-31++G(d,p), and MP2/6-31++G(d,p) levels yield numerous structures that are more stable than any known ones. The final electronic energies of the conformers were determined at the CCSD/6-31++G(d,p) level. The lowest energies of the canonical and zwitterionic structures are lower than the existing values by 2.0 and 2.3 kcal/mol, respectively. The relative energies, rotational constants, dipole moments, and harmonic frequencies of the stable conformers remain for future experimental verification. The conformational distributions at various temperatures, estimated according to thermodynamic principles, consist almost exclusively of the newly found structures. One striking feature is the occurrence of blue-shifting hydrogen bonds in all six of the most stable conformers. A unique feature of important conformations is the coexistence of dihydrogen and blue- and red-shifting hydrogen bonds. In addition to the hydrogen bonds, the stereoelectronic effects were also found to be important stabilization factors. The calculated and measured proton affinities agree within the theoretical and experimental uncertainties, affirming the high quality of our conformational search. The theoretical gas-phase basicity of 245.9 kcal/mol is also in good agreement with the experimental value of 240.6 kcal/mol. The extensive searches establish firmly that gaseous arginine exists primarily in the canonical and not the zwitterionic form.