The primary donor of far-red photosystem II: ChlD1 or PD2?

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 (Nurnberg a al 2018 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, Q(A).(-). Modelling indicates that no other FRL pigment is located among the 6 central reaction center chlorins: P-D1, P-D2 Chl(D1), Chl(D2), Pheo(D1) and Pheo(D2). Two of these chlorins, Chl(D1) and P-D2, are located at a distance and orientation relative to Q(A).(-) so as to account for the observed electrochromic shift. Previously, Chl(D1) 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 P-D2 is at least as likely as Chl(D1) 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 P-D2 as the primary donor. The pros and cons of P(D2 )vs Chl(D1) as the location of the FRL-primary donor are discussed.