Abstract
A theory for the calculation of resonant and nonresonant hole-burning (HB) spectra of pigment–protein complexes is presented and applied to the water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The theory is based on a non-Markovian line shape theory (Renger and MarcusJ. Chem. Phys.2002, 116, 9997) and includes exciton delocalization, vibrational sidebands, and lifetime broadening. An earlier approach by Reppert (J. Phys. Chem. Lett.2011, 2, 2716) is found to describe nonresonant HB spectra only. Here we present a theory that can be used for a quantitative description of HB data for both nonresonant and resonant burning conditions. We find that it is important to take into account the excess energy of the excitation in the HB process. Whereas excitation of the zero-phonon transition of the lowest exciton state, that is, resonant burning allows the protein to access only its conformational substates in the neighborhood of the preburn state, any higher excitation gives the protein full access to all conformations present in the original inhomogeneous ensemble. Application of the theory to recombinant WSCP from cauliflower, reconstituted with chlorophyll a or chlorophyll b, gives excellent agreement with experimental data by Pieper et al. (J. Phys. Chem. B2011, 115, 405321417356) and allows us to obtain an upper bound of the lifetime of the upper exciton state directly from the HB experiments in agreement with lifetimes measured recently in time domain 2D experiments by Alster et al. (J. Phys. Chem. B2014, 118, 352424627983).
Highlights
Protein dynamics plays an essential role in many biophysical/ biochemical processes in living organisms, like excitation energy and electron transfer in photosynthesis[1] and in catalysis.[2]
J(ω) that was extracted by Pieper et al.[33] from experimental delta-fluorescence line narrowing (FLN) spectra of water-soluble chlorophyll-binding protein (WSCP),[33] using standard Kubo− Toyozawa−Lax (KTL) theory, as discussed in detail in the Supporting Information
For chlorophyll a (Chla)−WSCP, HB spectra at low-energy burning conditions are compared in Figure 4 with the of the low-energy exciton state, a sharp hole results at the burn wavelength that is accompanied by two sharp positive features at both sides of the sharp bleach
Summary
Protein dynamics plays an essential role in many biophysical/ biochemical processes in living organisms, like excitation energy and electron transfer in photosynthesis[1] and in catalysis.[2]. Since the parameters of the Hamiltonian of PPCs (site energies, excitonic coupling, spectral densities) in good approximation are temperature-independent, the temperature dependence of optical spectra can be well described by the line shape theory discussed below. In this way, it is possible to characterize a PPC by low-temperature spectroscopy and use the parameters to describe the biological function at physiological temperatures. In case of Chlb, we find that the earlier value of 72 cm−1 should be somewhat increased to 82 cm−1 for a better fit of τpd/fs
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