Spectroscopic Studies of Isolated PSII

The development of methods for efficiently isolating highly active PSII preparations from spinach, by Yocum and colleagues (the BBY preparation) led to an increase in the output of important papers on the OEC and water splitting

a

Mn(il)acac -

/ / !

Mn(lll)acac —

Mn(IV)salicyl ■

OEC {S, State) —

6540 6550 6560 6570

6540 6550 6560 6570

X-ray Energy (eV)

Figure 5.3 XAS spectra of PSII and Mn model compounds. (a): XANES spectra of dark adapted (S,) PSII and representative Mn(II), Mn(III) and Mn(IV) models. Spectra were obtained with permission form Yano,

Yachandra and colleagues. (b): uncorrected, Fourier transformed EXAFS data of dark adapted PSII with the corresponding distances indicated. Spectra collected as part of the first author's PhD research.

Distance (A)

Yachandra and colleagues. (b): uncorrected, Fourier transformed EXAFS data of dark adapted PSII with the corresponding distances indicated. Spectra collected as part of the first author's PhD research.

[30, 31]. In particular, the new preparations enabled Klein and coworkers to obtain much improved XAS results. EXAFS spectra of isolated PSII showed three distinct peaks of electron as shown in Figure 5.3b [32].

While XAS is a highly useful spectroscopic technique, in that it is capable of isolating the spectra of a particular element from the rest of the elements within the protein, the interpretation of spectra from multinuclear species, such as the OEC, is complicated because one records an average spectrum for all of the selected atoms. Specifically, the Mn EXAFS spectrum from the OEC is an average of all four individual Mn environments within the cluster. Nevertheless, three well resolved peaks were observed that are most consistent with Mn_O ligation for the 1.8À peak, two Mn_Mn distances for the 2.7À interaction, and one Mn_Mn distance with the possibility of a contribution from a Mn_Ca distance for the 3.3 À interaction [32, 33]. The first two shells are very similar to that observed for Mn(III)(IV) dimers, while the third shell provided additional support for additional Mn or Ca interactions. While these results represented a great improvement over those obtained with chloroplast membranes and very precise distance measurements of the various interactions within the cluster were obtained, no specific structures were initially proposed.

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