Chlorophyll redox properties

The primary donor in Photosystem I P700 is thought to be a special pair of chlorophyll a molecules. Katz and Hindman (18) have reviewed a number of systems designed to mimic the properties of P700 ranging from chlorophyll a in certain solvents under special conditions where dimers form spontaneously (19) to covalently linked chlorophylls (20). Using these models it has been possible to mimic many of the optical, EPR and redox properties of the in vivo P700 entity. 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

The chemical nature of P-700 is difficult to establish. The absorption bleachings correspond approximately to the peaks of Chi a. which appears to be virtually the only tetrapyrrolic pigment in purified PS I particles. It has thus been assumed that P-700 is Chi a bound to a protein. A few recent results, however, may require this hypothesis to be refined. An examination of the spectroscopic and redox properties of P-700 led Wasielewski et al. [13] to propose that P-700 could be the enol form of Chi a where enolization was of the keto ester on ring V. This has not been confirmed by chemical extraction. Extraction experiments, however, have evidenced two other chlorophyll derivatives. A species named Chl-RC I has been isolated from PS I, at a nearly 1/1 molar ratio with P-700 and its structure shown to be a chlorinated derivative. It is not yet clear whether Chl-RC I is a native constituent of PS I or an extraction artefact. Chl-RC I has not been obtained in a recent chemical analysis by HPLC, which instead revealed two Chi a per P-700 [14]. 

Covalently linked dimers of both chlorophyll a and pyrochlorophyll a have been prepared, which mimic the spectroscopic and redox properties of P700. The two chlorophylls are joined in each case at their propionic acid side chains via an ethylene glycol diester linkage. The orientation of the chlorophyll macrocycles with respect to one another (Figs. 11 and 12) and consequently their electronic properties depend strongly on the solvent. The structure of most interest is the folded one (Fig. 11) because of its similarity to the photoactive dimer in photosystem I. 

The electron transfer proceeded with the participation of the triplet excited state of Chi. The discovery of the reversible reaction of chlorophyll photoreduction served as a stimulus for starting systematic research on photochemical redox reactions of chlorophyll and its synthetic analogs, i.e, various metalloporphyrins (MP). For a review of the physical and chemical properties of MP, see, for example, Ref. [62]. 

This composite satellite image displays areas on the surface of the Earth where chlorophyll-bearing plants are located. Chlorophyll, which is one of nature s most important biomolecules, is a member of a class of compounds called porphyrins. This Glass also includes hemoglobin and cytochrome c, which is discussed in Feature 19-1. Many analytical techniques have been used to measure the chemical and physical properties of chlorophyll to explore Its role in photosynthesis. The redox titration of chlorophyll with other standard redox couples reveals the oxidation/ reduction properties of the molecule that help explain the photophysics of the complex process that green plants use to oxidize water to molecular oxygen. 

Very recently the synthesis of a covalently bound chlorophyll-a-dimer analogue has been reported703). It was shown that bis-(chlorophyllide-a)ethylene glycol diester with porphyrin rings held together via nucleophilic hydrogen bridging closely resembles in its spectral properties, photochemical activity and redox potential to Chl-aj and therefore can serve as an in vitro model for the native Chl-ai-dimer-protein complex. 

Publications of Mees, Homer and Tomlinson in the 1960s on general herbicidal properties indicated that the phytotoxic action is connected with chlorophyll and light. The authors presumed, on the basis of the relationships between the reducibility of the single compounds and the phytotoxic action, that a reduction to a stable free radical occurs in the plant and that this free radical is responsible for phytotoxic action. The redox potentials of -0.446 and 0.346 mV of paraquat and diquat, respectively, are ensured by the reduction potential of light reaction I of photosynthesis (Calderbank, 1968). 

Photosystem II (PS II) of higher plant possess unique properties with respect to function, organization and protein turnover. It is a multisubunit protein complex which Is composed of at least 20 different polypeptides (1). The two reaction center polypeptides, designated D1 and D2, appear to carry all the redox components necessary for the primary photochemistry of PS II (2) and possibly also the Mn (3). The great majority of the PS II units is located in the appres-sed thylakold regions in association with its chlorophyll a/b antenna (4). PS II has a central catalytic role, but It also plays a central role In the long and short term acclimation of the photosynthetic apparatus. It Is also the target for the photoinhibition process which leads to Impaired electron transport capacity and the subsequent breakdown of the two reaction center subunits. In particular the Dl-protein. 

From these results, it is clear that chlorophyll adsorbed at an interface can catalyze redox reactions between solutes in the two liquid phases, and these reactions are accompanied by injection of negative charges into the low dielectric membrane interior. Metallo-complexes of porphyrins also have similar catalytic properties at the oil/water interface without illumination [13,14,43]. An advantage of porphyrins as catalysts in such experiments is their chemical stability in different media. 

---