Design and synthesis of water soluble (metallo)porphyrins with pendant arms: studies of binding to xanthine oxidase

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The tethering of a substrate analogue to a covalently attached luminophore may give rise to a probe for reporting enzyme activity spectrofluorometrically. The overall design incorporates a water-soluble porphyrin luminophore, a substrate analogue and a planar conjugated bridge of the appropriate length designed to bind in the substrate-access channel of xanthine oxidase (XO). 5,10,15-Tris(N-methyl-4-pyridiniumyl)-20-phenyl porphyrins was functionalised at the 4-position of the phenyl group with amide-linked 2-methoxy-benzamide groups, [(2-methoxy-4-amino-phenylcarbonyl)-amino] or [(2-methoxy-4-[(pyridine-4-carbonyl)-amino]-phenylcarbonyl)-amino]. The products were isolated as free base or zinc porphyrins with chloride or hexafluorophosphate counterions and characterised by optical emission and absorption in addition to other spectroscopic methods. Binding studies of bovine XO to the zinc porphyrin trichloride derivatives with the above linkers were used to determine the IC50 of 81 and 113 μM and Ki values of 4.6 and 6.5 μM, respectively. Furthermore, addition of XO to an aqueous solution of the products caused quenching of the porphyrin emission and a red shift in the absorption spectrum.