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.