Critical assessment of the emission spectra of various photosystem II core complexes

Abstract

We evaluate low-temperature (low-T) emission spectra of photosystem II core complexes (PSII-cc) previously reported in the literature, which are compared with emission spectra of PSII-cc obtained in this work from spinach and for dissolved PSII crystals from Thermosynechococcus (T.) elongatus. This new spectral dataset is used to interpret data published on membrane PSII (PSII-m) fragments from spinach and Chlamydomonas reinhardtii, as well as PSII-cc from T. vulcanus and intentionally damaged PSII-cc from spinach. This study offers new insight into the assignment of emission spectra reported on PSII-cc from different organisms. Previously reported spectra are also compared with data obtained at different saturation levels of the lowest energy state(s) of spinach and T. elongatus PSII-cc via hole burning in order to provide more insight into emission from bleached and/or photodamaged complexes. We show that typical low-T emission spectra of PSII-cc (with closed RCs), in addition to the 695 nm fluorescence band assigned to the intact CP47 complex (Reppert et al. J Phys Chem B 114:11884-11898, 2010), can be contributed to by several emission bands, depending on sample quality. Possible contributions include (i) a band near 690-691 nm that is largely reversible upon temperature annealing, proving that the band originates from CP47 with a bleached low-energy state near 693 nm (Neupane et al. J Am Chem Soc 132:4214-4229, 2010; Reppert et al. J Phys Chem B 114:11884-11898, 2010); (ii) CP43 emission at 683.3 nm (not at 685 nm, i.e., the F685 band, as reported in the literature) (Dang et al. J Phys Chem B 112:9921-9933, 2008; Reppert et al. J Phys Chem B 112:9934-9947, 2008); (iii) trap emission from destabilized CP47 complexes near 691 nm (FT1) and 685 nm (FT2) (Neupane et al. J Am Chem Soc 132:4214-4229, 2010); and (iv) emission from the RC pigments near 686-687 nm. We suggest that recently reported emission of single PSII-cc complexes from T. elongatus may not represent intact complexes, while those obtained for T. elongatus presented in this work most likely represent intact PSII-cc, since they are nearly indistinguishable from emission spectra obtained for various PSII-m fragments.

On the Conflicting Estimations of Pigment Site Energies in Photosynthetic Complexes: A Case Study of the CP47 Complex

“…These data clearly indicate that absorption of the CP47 complex has experienced nonphotochemical HB at 5 K and that the 695 nm emission of CP47 is consistent with that observed for intact PSII-cc samples (for more details, see Ref. 39 ). Only data in Figure 2B are consistent with Figure 3 .…”

“…We emphasize again that only the redshifted emission near 695 nm (see Fig. 2B ) is clearly observed in PSII membrane fragments 41 , 45 , 46 and PSII-cc from Thermosynechococcus elongatus , 39 which are considered to be intact preparations. Figure 3 shows 5 K PSII-cc emission spectra before and after HB (curves a and b, respectively), while the inset shows 77 K spectra of intact PSII-cc and CP47 (red curve).…”

“…This antenna facilitates excitation energy transfer to the PSII RC. 11 , 25 , 39 Even though CP47 has been extensively studied over the years, the shape of various optical spectra of intact complexes (eg, absorption and emission spectra), extracted site energies (vide infra), and the resulting excitonic structure are still under debate. Typically, low-temperature steady-state absorption, emission, nonresonant HB, and room temperature (RT) CD and LD spectra of CP47 are used in modeling studies utilizing simple excitonic calculations 7 and Redfield simulations, taking lifetime broadening effects into account.…”

Frequently asked questions about chlorophyll fluorescence, the sequel

“…Chen et al (2015) made an extensive comparison between published results and new fluorescence emission spectra recorded on isolated PSII cores of spinach and from re-dissolved PSII core crystals of the cyanobacterium Thermosynechococcus elongatus . The authors showed that isolation procedures that preserve the intactness of the PSII core result in nearly overlapping 77 K emission spectra of cyanobacteria, the green alga Chlamydomonas reinhardtii and spinach.…”

Structural basis of light‐harvesting in the photosystem II core complex

“…To understand the significance of PsbH in the context of light‐harvesting, we have to consider low‐energy states that have been discovered in many photosynthetic systems as reviewed by Reimers et al These states become manifest in optical absorptions energetically well below that of the RC, that is, in the case of PSII at wavelengths larger than about 680 nm (see also the discussion of the RC absorption spectrum below). A distinction has to be made between extremely red‐shifted states of yet unknown origin and more moderately red‐shifted states giving rise to absorption at 694 nm as well as corresponding emission signals . We are only concerned with the latter type of states here.…”