Biopolymers and cell. 1994. Volume 10. № 6. 52 - 60

/. V. Kondratyuk, D. M. GovOran,• N. V. Zheltovsky

PROTOTROPIC MOLECULAR-ZWITTERION TAUTOMERISM OF XANTHINE: AMI CALCULATION

Summary

    The prototropic molecular-zwitierion tautomerism of xanthine (Xan) was investigated by means of semiempirical quantum mechanical AMI method with full parameters optimization in vacuum. It was established that molecular-ZAvitterkm family of Xan occupies the energetical range about 82 ccal/mol and it consists of 32 structural isomers—24 molecular and 8 zwitlerion; most of them are planar dipole-stable structures. The diketo-configuration N7H is the main and practically the only tautomeric form of Xan in gase phase; there is zero concentration of the main z-witterion (with protons localization at Nl, N7 and N9) at the same conditions. Transition to solvent with universal solvatation mechanism does not violate the character of equilibrium, that is evidenced from guantitative estimation in the frame of Onzager's classical model. We construct rows of donor and acceptor properties for two energetically prevalent molecular tautomers of N7H(I) ans N9H(II) and the main tautomer-zwitterion (Z) based on their calculated fundamental vibrational frequencies: N7(Z) >N9H(Z);>N1H(Z); N9H(II) >N7H(I) > >N3H(II)>N3H(I) > N1H(I) > NIH(II); C60(Z) >C20(Z); C60(I) >C60(II)> >C20(I)>C20(I1); C8H(I)>C8H(II). The ilid mechanism of hydrogen-deuterium-tritium exchange of C8H group of Xan in water at the acidic and neutral pH is confirmed from the analysis of calculated in va¬cuum physico-chemical properties (the heat of formation, proton affinity and creative energy) of the main Xan's ilid form (with iminoprotons localization at Nl, N3, N7, N9 and without proton at C8).