The primary donor of far-red photosystem II: Chl or P?

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