Peripheral metal complexes: chlorophyll "isomers" with magnesium bound to the ring E .beta.-keto ester system

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-

“…A similar effect has been observed in the 1H NMR spectra of the magnesium enolate of methyl pheophorbide a (21) and Eschenmoser's cyclochlorophyllide a enols (19,20).…”

supporting

confidence: 73%

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

Wasielewski

Norris

Shipman

et al. 1981

“…S2). This observation suggests very rapid and nonradiative quenching of the photoexcited singlet state of cPPB-aE, which is ascribed to internal conversion(s) to the ground singlet state (12) and/or intersystem crossing to the excited triplet state. Considering the biochemical significance of producing cPPB-aE by some protists, the latter possibility seems unreasonable owing to the highly toxic singlet oxygen that is readily produced from the triplet state ( Fig.…”

Section: Resultsmentioning

confidence: 95%

Ubiquity and quantitative significance of detoxification catabolism of chlorophyll associated with protistan herbivory

Kashiyama

Yokoyama²,

Kinoshita

et al. 2012

Chlorophylls are essential components of the photosynthetic apparati that sustain all of the life forms that ultimately depend on solar energy. However, a drawback of the extraordinary photosensitizing efficiency of certain chlorophyll species is their ability to generate harmful singlet oxygen. Recent studies have clarified the catabolic processes involved in the detoxification of chlorophylls in land plants, but little is understood about these strategies in aquatic ecosystem. Here, we report that a variety of heterotrophic protists accumulate the chlorophyll a catabolite 13 2 ,17 3 -cyclopheophorbide a enol (cPPB-aE) after their ingestion of algae. This chlorophyll derivative is nonfluorescent in solution, and its inability to generate singlet oxygen in vitro qualifies it as a detoxified catabolite of chlorophyll a. Using a modified analytical method, we show that cPPBaE is ubiquitous in aquatic environments, and it is often the major chlorophyll a derivative. Our findings suggest that cPPB-aE metabolism is one of the most important, widely distributed processes in aquatic ecosystems. Therefore, the herbivorous protists that convert chlorophyll a to cPPB-aE are suggested to play more significant roles in the modern oceanic carbon flux than was previously recognized, critically linking microscopic primary producers to the macroscopic food web and carbon sequestration in the ocean. phototoxicity of chlorophylls | microbial herbivory | phagocytosis | biodiversity of eukaryotes | microbial loop C hlorophylls are crucial to sustaining most life forms on Earth, the majority of which ultimately depend on solar energy. Photoexcitation of chlorophylls initiates various photosynthetic reactions that convert the energy of photons into chemical potentials, which in turn, drive the full range of metabolic reactions throughout the global ecosystem. Chlorophylls play a central role in the photosynthetic apparatus by absorbing light and transferring the excitation energy to the reaction centers of photosystems before photosynthetic electron transport. However, without measures to contain the excited energy, chlorophylls can harm organisms because of their high photosensitizing potential.

“…We conclude therefore that the slower The fraction containing [3-vinyl]-132-OH-BChl-a at site BA is called [3-vinyl]RCs. (27)(28)(29) and its potential functional importance in photosynthesis (30)(31)(32)(33) [3-vinyl]BChl-a at the monomeric site BA while -35% of the RCs have BChl-a at this position.tt The fraction containing [3-vinyl]BChl-a at site BA exhibits a slower decay of the excited electronic level P* (X' 32 ps). The additional observation of a long-lasting bleaching of the Q.…”

Section: P+b P+h-p+q- This Model Ismentioning

confidence: 99%

Primary electron transfer kinetics in bacterial reaction centers with modified bacteriochlorophylls at the monomeric sites BA,B.

Finkele

Lauterwasser

Struck

et al. 1992

The primary electron transfer has been investigated by femtosecond time-resolved absorption spectroscopy in two chemically modified reaction centers (RC) of Rhodobacter sphaeroides, in which the monomeric bacteriochlorophylls BA and BB have both been exchanged by 132-hydroxybacteriochlorophyll a or [3-vinylJ-132-hydroxybac-teriochlorophyll a. The kinetics of the primary electron transfer are not influenced by the 132-hydroxy modification. In RCs containing [3-vinyl]-132-hydroxybacteriochlorophyll a the primary rate is reduced by a factor of 10. HThe three known x-ray structures of bacterial reaction centers (RCs) show basically the same arrangement of the cofactors (see refs. 1-4). They form two branches [A (active) and B (inactive)], which are arranged in an approximate twofold symmetry. Surprisingly, the electron transfer (ET) has been found to proceed exclusively along the A branch (5-7). From the primary donor, P870 or P960, an electron is transferred to the acceptor quinone (QA) The structural arrangement of the chromophores suggests an ET via the two intervening pigments-i.e., the monomeric bacteriochlorophyll (BChl) BA and the bacteriopheophytin (BPhe) HAWhile there is general agreement that the BPhe HA is an intermediate electron acceptor (5-9), the role of the BChl BA located between the BChl dimer P and the BPhe HA is still controversially discussed. First studies with sufficient time resolution and an appropriate excitation wavelength led to the conclusion that the state P+B-(P, primary donor) is not a real intermediate in the ET (5,10,11). However, a direct ET from P* to HA over an edge-to-edge distance of '10 A can be accounted for only by a "superexchange" transfer via a virtual intermediate P+B-(12-15 Kinetic measurements were performed at T 298 K in cuvettes of 1-mm path length under stirring. The sample volume was 0.2-0.3 ml. The transmission was between T 10%o and 50% at A = 860 nm (80-25 ,uM The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.