Molecular Structure and Spectroscopy of a Bacteriopurpurin. A New Class of Bacteriochlorin Photosensitizers

Bacteriochlorins mediate energy and electron transfer in vivo and serve as biomimetic models of bacterial photosynthesis. Recently, the same physicochemical properties of the chromophores have made them attractive as photosensitizers for photodynamic therapy, a medical treatment that combines light, porphyrins, and oxygen to attack and destroy malignant tissue. Despite this intense biophysical, chemical, and medicinal interest in bacteriochlorins, very few high precision structures of the chromophores exist to provide the basis for theoretical calculations that seek to predict their extensively studied physicochemical properties. We report here the synthesis, spectral characterization, and structural determination of a metal-free bacteriopurpurin 1, a paradigm for a new class of photosensitizers comprised of porphyrins 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 π cation radical with a distinctive EPR signal. The crystallographic results for 1 unambiguously identify the bacteriopurpurin, provide the first stereochemical parameters for this new class of expanded bacteriochlorins, extend the high resolution structural database for bacteriochlorins, and form the basis for theoretical calculations which correctly describe the optical features and electron density profile of 1 and its cation radical.