Characterization of tetrapyridylporphyrinatozinc(II)—apomyoglobin complexes as a potential photosynthetic model

Abstract

The interaction between chlorophylls and the protein on which they are bound, is thought to play an important role in photosynthesis. When the haem group of myoglobin is replaced by tetrapyridylporphyrinatozinc(II)(ZnTPyP) it provides a model system for this phenomenon. Time-resolved photophysical measurements were used to characterise ZnTPyP in this environment. The fluorescence anisotropy of ZnTPyP decays by two pathways when ZnTPyP is bound to the protein: a slow route which reflects the rotation of the whole protein and a faster mechanism which involves rotation of ZnTPyP within a cone. The longer lifetime agrees with the theoretical value for rotation of this protein and shows that ZnTPyP forms a 1 : 1 complex with the protein and that the complex exists as a monomer in solution. The shorter lifetime is similar to that of a water-soluble zinc porphyrin and is insensitive to changes in the protein structure, which suggests the ZnTPyP is free to rotate within the binding site. There is a red shift in the ground-state and triplet-state absorption spectra of ZnTPyP when it is bound to the protein. Two fluorescence lifetimes are observed for the complex, and the shorter lifetime is more significant when the tertiary structure of the protein is destroyed. The lifetime of the triplet excited state is five times longer for the ZnTPyP complex than for ZnTPyP in pyridine of aqueous micelles.