Isobacteriochlorins (iBCs) are naturally occurring isomers of bacteriochlorins in which two adjacent rather than two opposite pyrrole rings have lost a β-β double bond. Like the photosynthetic chromophores, biological iBCs transfer electrons, and their iron complexes, sirohemes, mediate the six-electron reductions of nitrite to ammonia and of sulfite to hydrogen sulfide. Intriguingly, the metal-free derivatives, sirohydrochlorins, have also been found to be active in sulfite reduction. The mechanisms of electron transfer by sirohemes or sirohydrochlorins are not established. We have thus considered the possibility that the sirohydrochlorin might act as an electron acceptor, as do the equivalent metal-free pheophytins and bacteriopheophytins in photosynthesis. We report here optical, EPR, and ENDOR results for the π anion radical of a synthetic model, 2,3,7,8,12,13,17,18-octaethyl-iBC, that provide spectral signatures for the putative reduced transient in vivo. The calculated spin profile for the π anion radical that results from one-electron reduction of the iBC macrocycle is highly unusual. Two different computations, iterative extended Hückel (IEH) and self-consistent-field Pariser-Parr-Pople (PPP) calculations predict that a single meso carbon atom (C15) would bear from 0.2 (IEH) to 0.4 (PPP) electrons. Experimental EPR and ENDOR results for the anion radical of octaethyl-iBC fall gratifyingly within the two theoretical values: the observed hyperfine coupling constant of 7.7 G for the proton at C15 corresponds to an unpaired spin density of 0.3 electrons for C15, a uniquely high value for a π anion radical of a porphyrin derivative. (Optical spectra and additional ENDOR results for the radical support the π anion formulation.) These results suggest the following: (a) If sirohydrochlorin acts as an electron acceptor in vivo, its anion radical should exhibit a diagnostic EPR signal similar to the one reported here for the synthetic iBC. (b) The high spin density localization at a single carbon renders that position in the radical particularly susceptible to electrophilic attack and opens a novel synthetic avenue for regiospecific chemistry of iBCs via their anion radicals. (c) A considerable effort is now devoted to covalently linked donor-acceptor porphyrin complexes as models of photosynthetic and catalytic electron transfer. Because the unpaired spin density in π radicals extends onto peripheral substituents, the unusually high spin density at C15 in iBC anion radicals should significantly enhance electronic coupling in donor-acceptor pairs incorporating iBCs covalently linked at that position.
|Number of pages||4|
|Journal||Journal of Physical Chemistry|
|Publication status||Published - 1992|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry