X, an intermediate acceptor

At room temperature, flash absorption studies revealed that an electron acceptor designated Aj was functioning under conditions where F and Fg were presumably reduced [37]. The state (P-700, A2 ) is formed upon flash excitation and recombines with tiu — 250 jUS. The difference spectrum due to its formation was analysed into contributions of P-700 and A2. The latter includes mainly a small and broad bleaching around 430 nm, and perhaps some absorption shifts in the red. These absorption properties, together with the disappearance of the A2 absorption signal when iron-sulfur proteins are denatured [38,39], indicate that Aj may be an iron-sulfur centre. 

In early work, it was proposed that A2 and Fx were the same chemical species detected under different experimental conditions [37]. This proposal fits most of the present data, although Fx has also been proposed to be identified with P-430. In recent work [40,41] it was shown that a mild treatment with lithium dodecyl sulfate denatures F and Fb, but not Fx- In such preparations, a flash-induced charge separation at room temperature decays with t 2 = 1.2 ms. This could be equivalent to the 250 /as decay observed earlier which occurs when F and Fg are chemically reduced, the kinetic difference originating possibly in an electrostatic effect of Fa and Fb on the lifetime of (P-700 , Fx ). An important difference between the behaviour of A2 and Fx resides in the efficiency of their light-induced reduction a saturating flash reduces A2 in all the reaction centers at room temperature [37], whereas Fx is reduced in only 10-15% of the centres at low temperature [31]. A good correlation has been established between Fx and A2 by studying the effect of time in darkness after illumination at 3°C, in PS I particles [42]. 

In the present state of our knowledge two electron acceptors are implicated between P-700 and the bound iron-sulfur centres. They are named Aq and Aj because they have not been chemically identified. The first evidence for the existence of these acceptors came from kinetic absorption studies, under conditions where Fx (or A2) was reduced or inactive [37,39]. An important development resulted from the discovery of a spin-polarized triplet state [21,22] and the hypothesis of its formation as a result of a back-reaction between P-700 and a reduced primary acceptor. The analysis of the triplet signal versus the extent of reduction of the acceptors [43,44] led to the present hypothesis of two acceptors, Aq and Aj, arranged sequentially. 

The species Aq is assumed to be the direct partner of P-700 in the primary photochemistry P-700. ..Aq - (P-700 , Aq ). The study of Aq has been performed using three different methods. 

Ki) bound to purified PS I particles [60], Recent work by flash absorption [61] strongly supports the hypothesis that Aj is a vitamin K[. Aj must reduce the bound iron-sulfur centres and thus have a very low reduction potential. The rn for fho one-electron reduction of quinones can be rather low in purely apolar media [62]. These values are perhaps compatible with the functional properties of Aj, which is probably located in a hydrophobic protein and must have an of about -0.9 V. 

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