Synthetic models of cytochrome P450 and photosynthesis
The work presented in this dissertation describes the prepartion and characterisation of various strapped porphyrin compounds which are designed to model cytochrome P450 and the charge separation in photosynthesis. The major part of this work is concerned with models of cytochrome P450. After a brief introduction to the philosophy of modelling, there follows a review of work on the enzyme and various compounds which are designed to reproduce the enzymic characteristics. The synthesis of two singly bridged porphyrins is reported. These incorporate a pendent methyl ester in the centre of the strap and aim to mimic the proposed acylation step in the catalytic cycle. Evaluation as models was performed by a series of experiments involving the addition of potassium superoxide to the manganese complexes of these compounds. Characterisation of the mode of reactivity required the use of many physical techniques and necessitated the synthesis of a radio-labelled sample of one of the porphyrins. The results obtained suggest that superoxide binds preferentially to the bridge-free face of the macrocycle. Doubly bridged analogues of the above models were prepared which force the two faces of the porphyrin to be equivalent. Superoxide binding studies indicated a different mode of reactivity to the singly bridged models, for one of the compounds, and experiments to distinguish between possible interpretations of the results are suggested. A crown ether thiolate doubly bridged porphyrin was prepared as a model for the carbon monoxide complex of the enzyme. This was characterised by ultraviolet spectroscopy and attempts to produce a stable crystalline adduct are described. The remaining part of this work concerns a model for the charge separation process in photosynthesis. A discussion of natural systems and previous models is followed by a description of a tetraene pyromellitimide doubly bridged porphyrin, which shows significant quenching of the porphyrin fluorescence.