Orientations between Red Antenna States of Photosystem I Monomers from Thermosynechococcus elongatus Revealed by Single-Molecule Spectroscopy

Abstract

Single-molecule spectroscopy at low temperature was used to study the spectral properties, heterogeneities, and spectral dynamics of the chlorophyll a (Chl a) molecules responsible for the fluorescence emission of photosystem I monomers (PS I-M) from the cyanobacterium Thermosynechococcus elongatus. The fluorescence spectra of single PS I-M are dominated by several red-shifted chlorophyll a molecules named C708 and C719. The emission spectra show broad spectral distributions and several zero-phonon lines (ZPLs). Compared with the spectra of the single PS I trimers, some contributions are missing due to the lower number of C719 Chl's in monomers. Polarization-dependent measurements show an almost perpendicular orientation between the emitters corresponding to C708 and C719. These contributions can be assigned to chlorophyll dimers B18B19, B31B32, and B32B33.

Cryo-EM structure of a functional monomeric Photosystem I from Thermosynechococcus elongatus reveals red chlorophyll cluster

“…This remarkably loose coordination and spring-like architecture makes B1219 an excellent candidate for a red chlorophyll. This is in line with the suggestion that the chlorophyll pair B1218/B1219 is part of the red chlorophylls based on spectroscopy studies of monomeric PSI from T. elongatus 35,69 . Likewise, the outermost staircase chlorophyll B1233 is also weakly coordinated by water on the protein side and is directly exposed to the membrane, ideally situated to receive thermal energy from bulk lipids.…”

“…This puts us now in a good position to discuss the possible locations of the C719 red chlorophylls of the T. elongatus PSI trimer. Previous proposals for the molecular identity of red chlorophylls were made on the basis of their relative distance to each other, alignment of Q Y to the membrane plane, the possible excitonic coupling of their chlorine rings, and site energies 8,35,37,[59][60][61][65][66][67][68][69][70][71] . The observation that ΔpsaL mutants lead to both monomerization and the loss of red chlorophylls suggested the presence of C719 chlorophylls in the vicinity of the trimerization domain such as B1237/B1238 and A1132/B1207 [59][60][61]66,69,70 .…”

“…Interestingly, the triple 'staircase' chlorophylls B1231, B1232 and B1233 have been previously proposed as red chlorophylls on the basis of their spatial arrangement 8,35,61,66,69,74 that is thought to be ideal for excitonic coupling. Also, as mentioned above, spectroscopy of T. elongatus PSI performed at 5K suggested that the C719 red chlorophylls are localized in a single cluster of the peripheral antenna.…”

“…Red chlorophylls allow uphill excitation energy transfer to P700 for its oxidation via red light beyond 700 nm by using thermal energy supplied by the vibrations of the surrounding phonon bath, effectively expanding the light spectrum available for electron pumping [32][33][34] . Thus far spectroscopy performed at room temperature identified three distinct absorption bands for long-wavelength chlorophylls at 708 nm, 715 nm, and 719 nm [35][36][37] . In some cyanobacteria, e.g.…”

Single Molecule Fluorescence Spectroscopy of PSI Trimers from Arthrospira platensis: A Computational Approach

“…SMS measurements at cryogenic temperatures give more information about the pure physical aspects: the LHC exciton manifold properties and exciton-phonon interaction, while measurements at physiological temperatures allow observing an optical response that is much closer to the natural biological processes in the LHC [29,30,31,32]. Concerning studies of LWCs, SMS applications have provided insight into the polarization of the red emitters [33,34]. To simulate the SMS signal from a PSI complex of any species and at any temperature, a detailed exciton model of the energy transfer and charge separation has to be developed.…”