Theoretical investigations of the reactivity of verdoheme analogues: Opening of the planar macrocycle by amide, dimethyl amide, and hydroxide nucleophiles to form helical biliverdin type complexes

Abstract

Nucleophilic addition reactions of NH 2 - , NMe 2 - and OH − to a zinc(II) verdoheme complex have been investigated using B3LYP method. Results show that presence of zinc(II) ion in the center of macrocycle leads to an increase of positive charge on the carbon atoms adjacent to the oxygen in the zinc(II) verdoheme complex relative to the free 5-oxaporphyrin macrocycle. It has been determined that an intermediate is initially formed by nucleophilic attack to one of aforementioned carbon atoms. This intermediate is then directly converted to helical open-ring complex [Zn II(OEBNü)] or [Zn II(BNü)] by passing through a transition state. Even though the most positive center for the nucleophile to attack is the zinc ion of zinc(II) verdoheme, it has been shown that such addition does not lead to a stable intermediate. Thus the zinc atom has no coordination role in transferring the nucleophiles to the oxo-carbon, but it just has the effect of activating oxo-carbon for nucleophile addition. The following order of nucleophile strength has been obtained: NH 2 - > NMe 2 - > OH - NBO analysis has shown that interaction of nucleophile with the zinc ion of zinc(II) verdoheme complex decreases charge transfer of porphyrin ring to the zinc. This can be translated as an effective perturbation in the complex planar structure and thus an unstable intermediate. Even though the NBO analysis has demonstrated that bond strength of the oxo-carbon with the oxygen atom in the zinc(II) verdoheme is diminished when nucleophile has connected to the oxo-carbon, a relatively more stable intermediate is formed. Besides, it has been illustrated that molecular orbital calculations satisfy the NBO findings.