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

• Published in: Biochimica et biophysica acta. Bioenergetics Vol. 1861; no. 10; p. 148248
• Main Authors: Judd, Martyna, Morton, Jennifer, Nürnberg, Dennis, Fantuzzi, Andrea, Rutherford, A. William, Purchase, Robin, Cox, Nicholas, Krausz, Elmars
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2020
• Subjects: Circular dichroism; Electrochromic shift; Exciton coupling; Fluorescence; Magnetic circular dichroism; Photochemical charge separation; Photochemistry; CD; chl-f; chl-a-PS II; Fluorescence; MCD; Circular dichroism; SI; FRL-PS II; chl-a; Magnetic circular dichroism; Photochemical charge separation; FRL-chl; chl-d; Exciton coupling; Electrochromic shift; Photochemistry
• ISSN: 0005-2728; 1879-2650
• DOI: 10.1016/j.bbabio.2020.148248

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−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. [Display omitted] •Primary donor confirmed at 726 nm.•Determination of far-red chl pigment Qy excitation positions, widths, CD and MCD amplitudes•Quantification of electrochromic shifts and QA− photoconversion yield•Electrochromic shift consistent with primary donor at either ChlD1 or PD2•The CD amplitude favors the primary donor at PD2.