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

Abstract

We focus on problems with elucidation of site energies (E0n) for photosynthetic complexes (PSCs) in order to raise some genuine concern regarding the conflicting estimations propagating in the literature. As an example, we provide a stern assessment of the site energies extracted from fits to optical spectra of the widely studied CP47 antenna complex of photosystem II from spinach, though many general comments apply to other PSCs as well. Correct values of E0n for chlorophyll (Chl) a in CP47 are essential for understanding its excitonic structure, population dynamics, and excitation energy pathway(s). To demonstrate this, we present a case study where simultaneous fits of multiple spectra (absorption, emission, circular dichroism, and nonresonant hole-burned spectra) show that several sets of parameters can fit the spectra very well. Importantly, we show that variable emission maxima (690–695 nm) and sample-dependent bleaching in nonresonant hole-burning spectra reported in literature could be explained, assuming that many previously studied CP47 samples were a mixture of intact and destabilized proteins. It appears that the destabilized subpopulation of CP47 complexes could feature a weakened hydrogen bond between the 131-keto group of Chl29 and the PsbH protein subunit, though other possibilities cannot be entirely excluded, as discussed in this work. Possible implications of our findings are briefly discussed.

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

“…In the case of CP43, a satisfactory agreement between simulated and measured optical spectra of various types (i.e., absorption, fluorescence, linear dichroism (LD), circular dichroism (CD), circularly polarized luminescence (CPL)) could be achieved after adding to the PBQC results obtained with cyanobacterial structures a slight readjustment of site energies to fit the experimental data of cyanobacterial and plant material . The situation is less pleasant in the case of CP47, where the PBQC computations are in an intermediate stage, and the site energy assignment made is more controversial . The discussion of this problem is continued below in the context of “red chlorophylls.”…”

“…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.…”

“…A problem is that disintegration of the core complex may damage CP47. Indeed, Jankowiak and coworkers argue on the basis of data from hole‐burning spectroscopy that the spinach samples are heterogeneous and possibly not intact . Thus, it remains unclear at present, to what extent the differences between cyanobacteria and plants are due to PsbH, species‐specific differences in PsbB or artifacts of biochemical preparation procedures.…”

“…In the case of LHCII, evidence was found that a pigment might have a different orientation in the crystal structure than in typical samples used for optical spectroscopy . Jankowiak and coworkers collected plenty of data indicating sample heterogeneity and a possible lack of intactness …”

“…In recent simulations of CP47, five exciton domains are distinguished. According to Hall et al, the domains containing more than one pigment are labeled R (containing Chls B1 to B3, B5, and B6), G (B4 and B7) as well as C (B8 to B14).…”