engleski

Neutral vs. Zwitterionic Form of Arginine-An Ab Initio Study

The problem of the intramolecular proton transfer tautomerism in arginine, leading to conventional neutral and zwitterionic forms of this compound, is addressed by the high level theoretical models. It is shown that the neutral isomer is slightly more stable than the zwitterion, which implies that the former should be prefered in the gas phase. Examination of atomic charges obtained by the electron density partitioning techniques reveals that the charge distributions of neutral and zwitterionic tautomers are not as widely different as expected. This finding is counterintuitive, since it contradicts the classical notion of chemical bonding and a conventional picture of zwitterions involving two local fragments possessing unit charges of opposite sign. The true distribution of the electron density is more uniform and quite similar to that of the neutral form. The proton affinity of arginine is estimated to be 249 kcal/mol. Hence, it follows that arginine is a very basic compound although it belongs to a family of 20 fundamental a-amino acids. A very high proton affinity is interpreted in terms of the resonance effect spurred by protonation in the guanidine moiety and by a strong hydrogen bonding taking place in the protonated form.

Arginine; Neutral form; Zwitter-ion; Proton affinity; Ab initio Calculations

nije evidentirano