SPECTRAL DIFFUSION AND N-14 QUADRUPOLE SPLITTINGS IN ABSORPTION DETECTED MAGNETIC-RESONANCE HOLE-BURNING SPECTRA OF PHOTOSYNTHETIC REACTIONCENTERS

Zero field absorption detected magnetic resonance hole burning measure ments were performed on photosynthetic reaction centers of the bacteri a Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis. Extrapolat ion to zero microwave power yielded pseudohomogeneous linewidths of 2. 0 MHz for Rhodopseudomonas viridis, 1.0 and 0.9 MHz for the protonated forms of Rhodobacter sphaeroides R26 with and without monomer bacteri ochlorophyll exchanged, and 0.25 MHz as an upper limit for fully deute rated reaction centers of Rhodobacter sphaeroides R26. The measured li newidths were interpreted as being due to unresolved hyperfine interac tion between the nuclear spins and the triplet electron spin, the line shape being determined by spectral diffusion among the nuclei. The di fference in linewidths between Rhodobacter sphaeroides R26 and Rhodops eudomonas viridis is then explained by triplet delocalization on the s pecial pair in the former, and localization on one dimer half on the l atter. In the fully deuterated sample, four quadrupole satellites were observed in the hole spectra arising from the eight N-14 nitrogens in the special pair. The quadrupole parameters seem to be very similar f or all nitrogens and were determined to kappa=1.25+/-0.1 MHz and eta=0 .9+/-0.1 MHz.