Neutral vs. zwitterionic form of arginine - an ab initio study

The problem of the intramolecular proton transfer isomerism in arginine, le ading to conventional neutral and zwitterionic forms of this compound, is a ddressed by high level theoretical models. It is shown that arginine has tw o neutral and two zwitterionic isomers implying that there exist two additi onal unconventional isomers, which have not been identified so far. It appe ars also that the most stable neutral isomer is energetically more favourab le than both zwitterions, which implies that the former should be preferred in the gas phase. Examination of atomic charges obtained by the electron d ensity partitioning techniques reveals that the charge distributions of neu tral and zwitterionic isomers are not as widely different as expected. This finding is counterintuitive, since it contradicts the classical notion of chemical bonding and a customary picture of zwitterions involving two local complementary fragments possessing unit charges of opposite sign. The true distribution of the electron density is more uniform and The proton affini ty of arginine is estimated to be 249 kcal mol(-1). Hence, it quite similar to that of the neutral form. follows that arginine is a very basic compoun d although it belongs to a family of 20 fundamental alpha-amino acids. A ve ry high proton affinity is interpreted in terms of the resonance effect spu rred by protonation in the guanidine moiety and by a strong hydrogen bondin g taking place in the protonated form.