Theoretical investigations on the hydrolysis pathway of tin verdoheme complexes: elucidation of tin's ring opening inhibition role.

In order to obtain a better molecular understanding of inhibitory role of tin metal in the verdoheme ring opening process, hydrolysis of three possibly six, five, and four coordinate verdoheme complexes of tin(IV) and (II) have been studied using DFT method. The results of calculations indicate that, in excellent accord with experimental reports, hydrolysis of different possibly coordinated tin(IV) and (II) verdohemes does not lead to the opening of the macrocycle. Contrary to iron and zinc verdohemes, in five and four coordinate verdoheme complexes of tin(IV) and (II), formation of open ring helical complexes of tin are unfavorable both thermodynamically and kinetically. In these pathways, coordination of hydroxide nucleophile to tin metal due to the highly charged, exclusive oxophilicity nature of the Sn center, and high affinity of Sn to increase coordination state are proposed responsible as inhibiting roles of tin via the ring opening. While, in saturated six coordinate tin(IV) and (II) verdoheme complexes the ring opening of tin verdohemes is possible thermodynamically, but it is not predicted to occur from a kinetics point of view. In the six coordinate pathway, tin plays no coordination role and direct addition of hydroxide nucleophile to the positive oxo-carbon centers and formation of closed ring hydroxy compounds is proposed for preventing the verdoheme ring opening. These key points and findings have been corroborated by the results obtained from atomic charge analysis, geometrical parameters, and molecular orbital calculations. In addition, the results of inhibiting ring opening reaction of tin verdoheme complexes could support the great interest of tin porphyrin analogues as pharmacologic means of chemoprevention of neonatal jaundice by the competitive inhibitory action of tin porphyrins on heme oxygenase.