Histamine-bound magnesium porphyrins: diverse coordination modes, inhibitory role in photodegradation of chlorophyll a and antioxidant activity

Abstract

The neurotransmitter histamine exists in two isomeric forms and could be an interesting ligand due to three nitrogen atoms with the possibility of binding to metals in different ways besides its crucial role in biological systems. However, no metal–histamine interaction is known in the literature. Therefore, two histamine-bound magnesium porphyrins [MgT(4-Cl)PP(hist)₂] 1 and [MgT(4-Br)PP(hist)] 2 have been synthesized and structurally characterized. Interestingly, 1 is a hexa-coordinated magnesium porphyrin due to the axial coordination of two histamine molecules via the nitrogen of the aliphatic amino group with the Mg–N_histamine distance of 2.300 Å, while 2 is penta-coordinated due to the axial coordination of one histamine molecule through the imidazole nitrogen atom with the Mg–N_histamine distance of 2.145 Å. The diverse coordination modes of this unique ligand are explored for the first time. Theoretical studies at the level of DFT supported the binding of histamine via imidazole nitrogen atoms for complex 2. Histamine-bound magnesium porphyrins are found to be stable against the photodegradation of magnesium porphyrin in the presence of light and oxygen. Freshly isolated chlorophyll a from spinach showed similar resistivity against photodegradation. Moreover, the histamine-bound complexes showed higher antioxidant activity for 1 (92.45%) compared to the free base porphyrin (73.11%) and MgT(4-Cl)PP (75.89%).