Does the Singlet Minus Triplet Spectrum with Major Photobleaching Band Near 680–682 nm Represent an Intact Reaction Center of Photosystem II?

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. J. Phys. Chem. B 2012, 116, 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 PD₁ chlorophyll and the active pheophytin (PheoD₁) 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 (ChlD₁) to PD₁ (i.e., PD₁ChlD₁⁺PheoD₁– → PD₁⁺ChlD₁PheoD₁–). Therefore, we suggest that ChlD₁ is the major electron donor in usually studied destabilized RCs (with a major photobleaching near 680–682 nm), although the PD₁ path (where PD₁ serves as the primary electron donor) is likely present in intact RCs, as discussed in Acharya, K. et al. J. Phys. Chem. B 2012, 116, 4860–4870.