PNNL

Abstract

The properties of six carefully selected structures of arginine have been studied using electronic structure methods. The relative energies were determined at several levels of theory including the CCSD/6-31++G(d,p)+5(sp)//MP2/6-31++G(d,p)+5(sp) level and corrected for zero-point vibrational contribution determined at the B3LYP level. Two new non-ionic structures were identified, one of which is lower (by 1.75 kcal/mol) than any previously reported structure. Two new conformers of the zwitterion were identified and are lower in energy than any previously reported zwitterion. The lowest non-ionic structure is lower in energy than the lowest zwitterion by 2.8 kcal/mol at our highest level of theory. The energy orderings of these structures vary somewhat with the basis set and level of theory, but for no basis or theory level is a zwitterion structure suggested to be the global minimum. Somewhat surprisingly, dipole moments of ca. 9 D are displayed by the lowest energy zwitterion and one of the non-ionic structures, but the Koopmans? vertical electron attachment energy is 2.4 times larger for the former than for the latter.