How Well Does the Hole-Burning Action Spectrum Represent the Site-Distribution Function of the Lowest-Energy State in Photosynthetic Pigment–Protein Complexes?

Abstract

For the first time, we combined Monte Carlo and nonphotochemical hole burning (NPHB) master equation approaches to allow for ultrahigh-resolution (<0.005 cm–1, smaller than the typical homogeneous line widths at 5 K) simulations of the NPHB spectra of dimers and trimers of interacting pigments. These simulations reveal significant differences between the zero-phonon hole (ZPH) action spectrum and the site-distribution function (SDF) of the lowest-energy state. The NPHB of the lowest-energy pigment, following the excitation energy transfer (EET) from the higher-energy pigments which are excited directly, results in the shifts of all excited states. These shifts affect the ZPH action spectra and EET times derived from the widths of the spectral holes burned in the donor-dominated regions. The effect is present for a broad variety of realistic antihole functions, and it is maximal at relatively low values of interpigment coupling (V ≤ 5 cm–1) where the use of the Förster approximation is justified. These findings need to be considered in interpreting various optical spectra of photosynthetic pigment–protein complexes for which SDFs (describing the inhomogeneous broadening) are often obtained directly from the ZPH action spectra. Water-soluble chlorophyll-binding protein (WSCP) was considered as an example.

Frequency-Domain Spectroscopic Study of the Photosystem I Supercomplexes, Isolated IsiA Monomers, and the Intact IsiA Ring

“…The above calculations were performed for all individual IsiA monomers and all three IsiA 6 hexamers to consider EET processes along the ring, as EET times between monomers are not the same. The EET rates for Chls belonging to different IsiA monomers (see Section ) were calculated in Förster approximation; for details see ref Briefly, ZPL-based site energies of the relevant pigments were obtained by a Monte Carlo approach, and the dependence of spectral overlap on donor–acceptor ZPL-ZPL energy gap was used to produce a distribution of EET rates (or times). , In the current work, single chromophore Förster resonance energy transfer (FRET) approach is used; in the future, particularly when higher resolution structure becomes available, one could apply the multichromophore FRET approach. − The latter, however, may ultimately be more relevant to properly describe EET from the ring to the core, but not so much within the ring, as discussed below.…”

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes

“…For details and application to various photosynthetic systems, see [34,94,104] and the references therein. Recently, a combined Monte-Carlo/NPHB master equation approach has been introduced, but due to the high computational cost, it has so far been limited to a small number (2-3) of interacting molecules per complex [105].…”