EPR, ENDOR, and TRIPLE resonance studies of modified bacteriochlorophyll cation radicals

Abstract

A series of substituted bacteriochlorophyll molecules, all used in reconstitution experiments of reaction centers of Rhodobacter sphaeroides (Struck et al. Biochim. Biophys. Acta 1991,1060,262-270), were characterized by EPR, electron-nuclear double (ENDOR), and electron-nuclear-nuclear triple (TRIPLE) resonance spectroscopy in their monomeric radical cation states. Effects of different substituents at position 3 in the porphyrin macrocycle were considered, especially for two "crosslinks" between plant and bacterial chlorophylls. These are 3-vinylbacteriochlorophyll where the "bacteria" acetyl group at position 3 was substituted by vinyl and 3-acetylchlorophyll where the "plant" vinyl group was substituted by acetyl. In addition, effects of substitutions at position 132 were studied. All major hyperfine coupling constants of proton and nitrogen nuclei were elucidated from the spectra and assigned to molecular positions by comparison with the parent radicals. The data were compared with those calculated by an INDO-type program, showing that INDO essentially models the effect of the different substituents correctly.

EPR and ENDOR investigation of the primary electron acceptor radical anion QA.bul.- in iron-depleted photosystem II membrane fragments.

“…Recently two different Chl a cation radicals in PS I1 have been characterized together with P680*+ by ENDOR (Rigby et al, 1994). Our hfc values obtained for this species by ENDOR (MacMillan, 1993) are given in Table 3 (caption) and agree well with those from Chl a*+ (I) in Rigby et al (1994) and with those of Chl a*+ in organic solvents (Kass et al, 1994).…”

“…For QA*-in Rhodobacter sphaeroides recently a value of 0.71 has been determined with high accuracy from ENDOR experiments on reaction center single crystals (Isaacson et al, 1995a). Furthermore, it has been found that the sum of the x-spin densities at positions 2, 3, 5, and 6 (and therefore also the Lubitz, 1991. f Methyl hf tensor components and assignments (positions in brackets) for Chl a'+: 3.0 MHz ( 2 ) , 3.6 MHz (7), and 7.2 MHz (12); see Kass et al (1994) for numbering. (Table 4) are numbered in the spectra (B, C).…”

ENDOR and Special Triple Resonance Studies of Chlorophyll Cation Radicals in Photosystem 2

“…Such weak couplings have been previously assigned to the methine protons of Chla'+ in vitro (Scheer et al, 1977;O'Malley & Babcock, 1984), although here, because of the possibility of coupling to protons from the surrounding matrix, this assignment must be considered tentative. The largest proton hyperfine couplings observed for Chla'+ in vitro arise from the protons at positions 7 and 8 on ring IV (Scheer et al, 1977;Huber et al, 1986;Kass et al, 1994). Although these protons are B to the ?r system and give rise to sharp resonances in liquid solution, in frozen solution their inequivalence and immobility lead to resonance broadening [for an example of this effect, see Figure 4 in Rautter et al ( 1 992)].…”

“…The ENDOR spectrum of Chll'+ is shown in panels a and b of Figure 3. The ENDOR resonances may be assigned by comparison with previously published spectra of Chl a cation radicals (Chla*+) in frozen and liquid solutions (Scheer et al, 1977;O'Malley & Babcock, 1984;Kass et al, 1994).…”

Radicals and Radical Pairs in Photosynthesis

“…While direct exchange of parts or all of the primary donor have not yet been achieved, it was proven that selective exchange of the two accessory BChl monomers on either side (A or B) is possible (105). The EPR, ENDOR and TRIPLE resonance spectra where taken of in vitro-generated cations of (3-vinyl)-1 32-hydroxy-BChl a and other substituents that fit into the monomer-binding pockets, and hyperfine coupling (hfc) constants assigned to their protons and nitrogens (106). The RC containing these modified pigments showed no differences in their Psf5 ENDOR and TRIPLE spectra as compared to native RC, which proved that the exchange was limited to the monomer sites (106).…”