The primary donor of far-red Photosystem II: Chl_D1or P_D2?

Abstract

Far-red light (FRL) Photosystem II (PSII) isolated from Chroococcidiopsis thermalis is studied using parallel analyses of low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies in conjunction with fluorescence measurements. This extends earlier studies Science 360 (2018) 1210-1213. We confirm that the chlorophyll absorbing at 726 nm is the primary electron donor. At 1.8 K efficient photochemistry occurs when exciting at 726 nm and shorter wavelengths;but not at wavelengths longer than 726 nm. The 726 nm absorption peak exhibits a 21 ± 4 cm -1 electrochromic shift due to formation of the semiquinone anion, QA •-. Modelling indicates that no other FRL pigment is located among the 6 central reaction center chlorins: PD1, PD2 ChlD1, ChlD2, PheoD1 and PheoD2. Two of these chlorins, ChlD1 and PD2, are located at a distance and orientation relative to QA •so as to account for the observed electrochromic shift. Previously, ChlD1 was taken as the most likely candidate for the primary donor based on spectroscopy, sequence analysis and mechanistic arguments. Here, a more detailed comparison of the spectroscopic data with exciton modelling of the electrochromic pattern indicates that PD2 is at least as likely as ChlD1 to be responsible for the 726 nm absorption. The correspondence in sign and magnitude of the CD observed at 726 nm with that predicted from modelling favors PD2 as the primary donor. The pros and cons of PD2 vs ChlD1 as the location of the FRL-primary donor are discussed.

Structure of the far-red light utilizing photosystem I of Acaryochloris marina

“…High-resolution 24 and ultrafast 25 spectroscopic studies support this. Still, PSII in some cyanobacteria may use Chl f (and/or Chl d) for electron transfer when grown under far-red light conditions 21,26 . In contrast to Chl f-carrying PSIs 19,20,22,23,25,27 , Chl d in A. marina PSI is always induced even under natural white light and does, in fact, take the place of the special pair Chls in the photochemical reaction chain.…”

Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [PD1PD2]+

“…The PSII samples used in this study were purified from i ) Thermosynechoccocus elongatus and ii ) Chroococcidiopsis thermalis PCC7203 grown under far-red light (750 nm). PSII from C. thermalis has 4 Chl- f and 1 Chl- d replacing 5 of the 35 Chl- a (Nürnberg et al 2018, Judd et al 2020) see also (Chen et al 2010, Gan et 2014, Gisriel et al 2022b).…”

The natural design for harvesting far-red light: the antenna increases both absorption and quantum efficiency of Photosystem II

“…Indeed, the excitations formed on FRL-BCs would only equilibrate with the Chls of CP43 without reaching those of CP47, and vice versa. Notably, a recent work has left space for another RC chlorophyll, PD2, to function as the red-shifted primary donor instead of ChlD1 35 , though this possibility seems less likely due to mechanistic and structural considerations. However, even when the primary donor is assumed to be located on PD2, Chl 505 of CP43 remains the most probable candidate for connecting the FRL-BCs to FRL-PSII RCs (see Table S1 for details).…”