Monomeric chlorophyll a enol : Evidence for its possible role as the primary electron donor in photosystem I of plant photosynthesis

Abstract

The chlorophyll a (ChI a) special-pair model of the primary donor of photosystem I (P700) does not account in a completely adequate fashion for the magnetic resonance properties observed for P700'. Moreover, P700 is at least 420 mV easier to oxidize than is Chi a in vitro. Neither Chl a dimer formation nor selective ligation ofChl a can account for this potential difference. Enolization of the ChI a ring V f3-keto ester results in a very different ir electronic structure. The Chl a enol can be trapped as a silyl enol ether. In addition, the enol analog 9-desoxo-9,10-dehydro-Chl a can be prepared. Both the trapped enol and its 9-H analog are -350 mV easier to oxidize than Chi a. The ESR spectrum of the cation radical consists of a single 6.1-G gaussian line that is line narrowed relative to that of Chi a+ in a manner similar to P700'. Electron-nuclear double resonance (ENDOR) spectroscopy resolves only a 3.5-MHz hyperfine splitting for the 3-methyl group. The remaining splittings are all less than 3.5 MHz. The second moment of the ESR line of fully '3C-enriched 9-desoxo-9,10-dehydro-Chl a' agrees with that of['3C]P700' to within 10%. Application of the special-pair model to the [13C]P700' secondmoment data yields a 100% error. Ab initio molecular orbital calculations on ethyl chlorophyllide a enol cation bear out the ESR and ENDOR data. We conclude that a monomeric Chl a enol model provides a better description of the magnetic resonance parameters and oxidation potential of P700 than a Chl a specialpair model.Evidence concerning the structural nature of the primary photochemical electron donors of both bacterial and green plant photosynthesis is derived principally from ESR and electronnuclear double resonance (ENDOR) spectroscopy of the oxidized donors. In purple photosynthetic bacteria such as Rhodopseudomonas sphaeroides, the oxidized primary donor P865+ exhibits a single gaussian ESR signal having a linewidth that is narrowed by a factor of 1/V relative to that of mono-

Evidence that a chlorophyll a' dimer constitutes the photochemical reaction centre 1 (P700) in photosynthetic apparatus

“…620 mV vs NHE (normal hydrogen electrode), is shifted substantially from the oxidation potential of monomeric Chl a' or Chl a (860 mV) [2] in the direction of the estimated P700 oxidation potential (370 to 520 mV) [2]. Though such a negative shift of the oxidation potential from the monomer potential is one of the criteria in ensuring the validity of a 'P700 model compound' [1,2], it cannot be a sufficient condition since redox potentials often shift greatly with a change in environment (e.g.…”

“…Of these two, P700 is generally supposed to be a Chl a dimer, on the grounds that the ESR signal assignable to photogenerated P700'+ exhibits a linewidth roughly I /v'%fold narrower than that for monomeric Chl a'+ in vitro [ I]. In contrast, Wasielewski et al [2] consider that a monomeric Chl a enol could be a more plausible model of P700.…”

Synthesis and characterization of chlorophyllaenol derivatives: Chlorophyllatert-butyldimethylsilyl-enol ether and 131-deoxo-131, 132-didehydro-chlorophylla

“…Our plan for the synthesis of the Chl a enol analog 4 was based mainly on the article by Wasielewski and Thompson [19] with the exception that we planned to start our synthesis from Chl a (1) instead of pheophytin (Pheo) a to enable us to study whether dehydration straight from 13 1 -hydroxy-Chl a is possible or not. As to the structural characterization of the synthesis products 3 and 4, we noticed that the previous NMR characterizations were incomplete [16,17,19]. The 1 H chemical shifts were reported only for the most important proton groups and the 13 C chemical shifts were totally missing.…”

“…For the synthesis of the enol derivative 3, we chose a route different from that used by Wasielewski et al [17], because the method used by them may result in a low yield or no yield at all [18] and because new efficient silylating reagents had been developed since 1981. From these, we chose tert-butyldimethylsilyltrifluoromethanesulfonate (TBDMS-OTf) for our synthesis.…”

Long-wavelength-absorbing forms of bacteriochlorophyll a in solutions of Triton X-100

“…The red shifts of these BCh1 a-protein complexes (as well as similar shifts in chlorophyll a-protein complexes) are as yet only partly understood and subject to much theoretical, spectroscopic, and synthetic work. They have been related to aggregation (5)(6)(7)(8)(9)(10)(11)(12)(13)(14) or hydration (or both) (5,6,(12)(13)(14)(15), enolization of the chlorophyll groups (16)(17)(18)(19), point charges in their environment (20), and charge-transfer complexes (21). In particular, the primary donor P870 in BChl a-containing reaction centers has been suggested to be a "special pair" of BChl a, with their exact relationship still under discussion (22)(23)(24)(25)(26)(27).…”