P-700 a chlorophyll species

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]. 

Chi a is the C 10 epimer of Chi a at C 10 the two substituents (R, = H and R2 = COOCH3) are inverted with respect to the molecular plane. The same stoichiometry has been found in various PS I preparations and the spectroscopic properties of a Chi a dimer seem to resemble those of P-700. 

A dimeric nature of P-700 was first proposed to explain its long wavelength of absorption and its circular dichroism spectrum, which can be attributed to chlorophyll-chlorophyll exciton interaction [15]. When two chlorophylls are close together, excitation by light tends to promote electronic transitions which belong to the pair and not to the individual molecules. In particular for the Qy transition, instead of one transition at the frequency v, an excitonic interaction will create two transitions at v +dv and v-dv. The latter is considered to be the transition at 700 nm, whereas the former could be of low intensity. 

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