posted on 1999-08-10, 00:00authored byByron C. Robinson, Kathleen M. Barkigia, Mark W. Renner, Jack Fajer
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.