Molecular structure and spectroscopy of a bacteriopurpurin. A new class ofbacteriochlorin photosensitizers

Bacteriochlorins mediate energy and electron transfer in vivo and serve as biomimetic models of bacterial photosynthesis. Recently, the same physicoch emical properties of the chromophores have made them attractive as photosen sitizers for photodynamic therapy, a medical treatment that combines light, porphyrins, and oxygen to attack and destroy malignant tissue. Despite thi s intense biophysical, chemical, and medicinal interest in bacteriochlorins , very few high precision structures of the chromophores exist to provide t he basis for theoretical calculations that seek to predict their extensivel y studied physicochemical properties. We report hen the synthesis, spectral characterization, and structural determination of a metal-free bacteriopur purin 1, a paradigm for a new class of photosensitizers comprised of porphy rins at the saturation level of bacteriochlorins flanked by two annelated, exocyclic cyclopentenyl rings. The molecule exhibits a very red-shifted Qy transition at 843 nm and is readily oxidized (at 0.39V vs SCE) to a pi cati on radical with a distinctive EPR signal. The crystallographic results for 1 unambiguously identify the bacteriopurpurin, provide the first stereochem ical parameters for this new class of expanded bacteriochlorins, extend the high resolution structural database for bacteriochlorins, and form the bas is for theoretical calculations which correctly describe the optical featur es and electron density profile of 1 and its cation radical.