Does the
Singlet Minus Triplet Spectrum with Major
Photobleaching Band Near 680–682 nm Represent an Intact Reaction
Center of Photosystem II?
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Abstract
We use both frequency- and time-domain
low-temperature (5–20
K) spectroscopies to further elucidate the shape and spectral position
of singlet minus triplet (triplet-bottleneck) spectra in the reaction
centers (RCs) of Photosystem II (PSII) isolated from wild-type Chlamydomonas reinhardtii and spinach. It is shown
that the shape of the nonresonant transient hole-burned spectrum in
destabilized RCs from C. reinhardtii is very similar to that typically observed for spinach. This suggests
that the previously observed difference in transient spectra between
RCs from C. reinhardtii and spinach
is not due to the sample origin but most likely due to a partial destabilization
of the D1 and D2 polypeptides. This supports our previous assignments
that destabilized RCs (referred to as RC680) (Acharya, K. et al. <i>J. Phys. Chem. B</i> <b>2012</b>, <i>116</i>, 4860–4870), with a major photobleaching band near 680–682
nm and the absence of a photobleaching band near 673 nm, do not represent
the intact RC residing within the PSII core complex. Time-resolved
absorption difference spectra obtained for partially destabilized
RCs of C. reinhardtii and for typical
spinach RCs support the above conclusions. The absence of clear photobleaching
bands near 673 and 684 nm (where the P<sub>D1</sub> chlorophyll and
the active pheophytin (Pheo<sub>D1</sub>) contribute, respectively)
in picosecond transient absorption spectra in both RCs studied in
this work indicates that the cation can move from the primary electron
donor (Chl<sub>D1</sub>) to P<sub>D1</sub> (i.e., P<sub>D1</sub>Chl<sub>D1</sub><sup>+</sup>Pheo<sub>D1</sub><sup>–</sup> →
P<sub>D1</sub><sup>+</sup>Chl<sub>D1</sub>Pheo<sub>D1</sub><sup>–</sup>). Therefore, we suggest that Chl<sub>D1</sub> is the major electron
donor in usually studied destabilized RCs (with a major photobleaching
near 680–682 nm), although the P<sub>D1</sub> path (where P<sub>D1</sub> serves as the primary electron donor) is likely present
in intact RCs, as discussed in Acharya, K. et al. <i>J. Phys.
Chem. B</i> <b>2012</b>, <i>116</i>, 4860–4870