Non-ionic and zwitterionic forms of neutral arginine – an ab initio study

Six low-energy structures of arginine were studied at the zero-point corrected CCSD/6-31++G(d,p)+5(sp)//MP2/6-31++G(d,p)+5(sp) level. Two new non-ionic structures were identified, one of which is 1.75 kcal/mol lower than any previously reported structure. Two new zwitterion conformers 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, and for no basis or theory level is a zwitterion structure suggested to be the global minimum. Finally, we also examined, at Koopmansʹ theorem level, the electron binding energies of the six structures.