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

dSix low-energy structures of arginine were studied at the zero-point corre cted CCSD/6-31 ++G(d,p)+5(sp)//MP2/ 6-31++G(d,p)+5(sp) level. Two new non-i onic structures were identified, one of which is 1.75 kcal/mol lower than a ny previously reported structure. Two new zwitterion conformers are lower i n energy than any previously reported zwitterion. The lowest non-ionic stru cture 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 s tructure suggested to be the global minimum. Finally, we also examined, at Koopmans' theorem level, the electron binding energies of the six structure s. (C) 2001 Elsevier Science B.V. All rights reserved.