Low-energy tautomers and conformers of neutral and protonated arginine

The relative stabilities of zwitterionic and canonical forms of neutral arg inine and of its protonated derivative were studied by using ab initio elec tronic structure methods. Trial structures were first identified at the PM3 level of theory with use of a genetic algorithm to systematically vary geo metrical parameters. Further geometry optimizations of these structures wer e performed at the MP2 and B3LYP levels of theory with basis sets of the 6- 31++G** quality. The final energies were determined at the CCSD/6-31++G** l evel and corrected for thermal effects determined at the B3LYP level. Two n ew nonzwitterionic structures of the neutral were identified, and one of th em is the lowest energy structure found so far. The five lowest energy stru ctures of neutral arginine are all nonzwitterionic in nature and are cluste red within a narrow energy range of 2.3 kcal/mol. The lowest energy zwitter ion structure is less stable than the lowest nonzwitterion structure by 4.0 kcal/mol. For no level of theory is a zwitterion structure suggested to be the global minimum. The calculated proton affinity of 256.3 kcal/mol and g as-phase basicity of 247.8 kcal/mol of arginine are in reasonable agreement with the measured values of 251.2 and 240.6 kcal/mol, respectively. The ca lculated vibrational characteristics of the low-energy structures of neutra l arginine provide an alternative interpretation of the IR-CRLAS spectrum.