Photoreactions of macrocyclic dyes bound to human serum albumin

The photophysical properties of tetrakis(4-sulfonatophenyl)porphyrin (H2TSPP), its tin (IV) complex (SnTSPP), aluminium(III) trisulfonatophthalocyanine (AIPCS), and the corresponding zinc(II) complex (ZnPCS), have been measured in H2O, D2O, and upon binding to human serum albumin (HSA). The triplet excited states of the various macrocyclic dyes generate singlet molecular oxygen, O2(1 delta g) in high quantum yield upon illumination in O2-saturated solution, even in the presence of HSA. The triplet states also abstract an electron from 4-aminophenol, forming the radical anion of the macrocycle. Quenching rate constants and quantum yields have been measured for the various processes in the presence and absence of HSA. It is found that HSA binds all the dyes at nonspecific sites close to the interface in such a manner that the dyes remain accessible to species residing in the solution phase. Dyes that do not possess axial ligands complexed to the central cation (e.g. H2TSPP, ZnPCS) are able to bind also at a deeper, more specific site on the protein where they are protected from species in solution. Under such conditions, triplet quenching by 4-aminophenol is restricted to long-distance electron tunnelling, for which the rate is relatively slow.