Effect of the axial ligands on the structure and reactivity of tin verdoheme in the ring opening process

Conversion of tin(IV) verdoheme complex to tin(IV) biliverdin complex in the presence of hydroxide ion as a nucleophile and various axial ligands has been investigated using the B3LYP method. Our calculations show that in the six coordinate tin(IV) verdoheme complex with imidazole and hydroxyl axial ligands, conversion of tin(IV) verdoheme to open chain macrocycle is favorable thermodynamically but not kinetically. It has been determined that ring opening is prevented through a ligand-centered mechanism. Whereas, in other six coordinate complexes, formation of open chain macrocycles is favorable from both thermodynamics and kinetics point of views. On the contrary, in the five and four coordinate verdoheme complexes of tin(IV) binding of hydroxide to tin terminates the reaction at verdoheme stage and the ring opening is blocked. In fact, in the two latter coordination states, metal-centered inhibition is the proposed mechanism for blocking the ring opening. NBO analysis has shown that in the six coordinate verdoheme complexes internal hydrogen bonding has a key role in inhibition or facilitation of ring opening. These key findings have been confirmed with the results obtained from MO analysis. The presented results could be a hint for the possible implications or coordination for the competitive inhibition of degradation of verdoheme to biliverdin by hydrolysis reaction.