This proposal concerns the preparation and study of synthetic models of heme- and chlorophyll- containing proteins. In particular, we plan to investigate the role of specifically ordered aggregates to tetrapyrroles in such key biological processes as photosynthesis and electron transfer. We have developed a new strategy for the construction of covalently-linked multiple tetrapyrroles with different distances and orientations between the rings. In this application we propose to extend this approach to model more closely the structure of multiple tetrapyrrole sites in biology. Our primary targets are tetrameric and hexameric analogs of the known structure of bacterial photosynthetic reaction centers. These models will, respectively, contain the dimer-bis(monomer) and dimer-bis(monomer)-bis(non-metallated monomer) structure of the reaction centers. By varying the synthesis used we will be able to generate a series of compounds with different although related structures. Thorough physical investigation of these derivatives, using electrochemical, fluoresence and picosecond optical techniques, will provide details of the photoinduced electron transfer and charge separation processes occurring between the pigments. This will allow us, for the first time to probe the effect of chromophore distance and orientation on key aspects of the photosynthetic process. We will also construct simple linear arrays of photoactive redox partners as comparisons to the biomimetic models. In later model compounds we will introduce chlorin and bacteriochlorin macrocycles in place of the porphyrin building blocks. Model studies of this type will lead to an understanding of protein control of reactivity that is crucial for solving the medical problems associated with tetrapyrrole biochemistry.
