Unusual spin density localization in a porphyrin {pi} radical. The {pi} anion radical of octaethylisobacteriochlorin

Journal Article · Thu Nov 26 04:00:00 EST 1992 · Journal of Physical Chemistry

DOI:https://doi.org/10.1021/j100203a006· OSTI ID:554936

Renner, M W; Fujita, E; Fujita, I; Procyk, A D; Bocian, D F; Fajer, J ^[1]

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1. Brookhaven National Lab., Upton, NY (United States)

Isobacteriochlorins (iBCs) are naturally occurring isomers of bacteriochlorins in which two adjacent rather than two opposite pyrrole rings have lost a {beta}-{beta} 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. They report here optical, EPR, and ENDOR results for the {pi} 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 {pi} anion radical that results from one-electron reduction of the iBC macrocycle is highly unusual. 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 {pi} 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. 26 refs., 4 figs.

• Research Organization: Brookhaven National Laboratory (BNL), Upton, NY
• DOE Contract Number: AC02-76CH00016
• OSTI ID: 554936
• Journal Information: Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 24 Vol. 96; ISSN JPCHAX; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
BINDING ENERGY
ELECTRON TRANSFER
ELECTRONS
MOLECULAR ORBITAL METHOD
PHOTOCHEMISTRY
PORPHYRINS
RADICALS
VALENCE