Refinement of a Structural Model of a Pigment−Protein Complex by Accurate Optical Line Shape Theory and Experiments

Abstract

Time-local and time-nonlocal theories are used in combination with optical spectroscopy to characterize the water-soluble chlorophyll binding protein complex (WSCP) from cauliflower. The recombinant cauliflower WSCP complexes reconstituted with either chlorophyll b (Chl b) or Chl a/Chl b mixtures are characterized by absorption spectroscopy at 77 and 298 K and circular dichroism at 298 K. On the basis of the analysis of these spectra and spectra reported for recombinant WSCP reconstituted with Chl a only (Hughes, J. L.; Razeghifard, R.; Logue, M.; Oakley, A.; Wydrzynski, T.; Krausz, E. J. Am. Chem. Soc. U.S.A. 2006, 128, 3649), the "open-sandwich" model proposed for the structure of the pigment dimer is refined. Our calculations show that, for a reasonable description of the data, a reduction of the angle between pigment planes from 60 degrees of the original model to about 30 degrees is required when exciton relaxation-induced lifetime broadening is included in the analysis of optical spectra. The temperature dependence of the absorption spectrum is found to provide a unique test for the two non-Markovian theories of optical spectra. Based on our data and the 1.7 K spectra of Hughes et al. (2006), the time-local partial ordering prescription theory is shown to describe the experimental results over the whole temperature range between 1.7 K and room temperature, whereas the alternative time-nonlocal chronological ordering prescription theory fails at high temperatures. Modified-Redfield theory predicts sub-100 fs exciton relaxation times for the homodimers and a 450 fs time constant in the heterodimers. Whereas the simpler Redfield theory gives a similar time constant for the homodimers, the one for the heterodimers deviates strongly in the two theories. The difference is explained by multivibrational quanta transitions in the protein which are neglected in Redfield theory.

Water soluble chlorophyll binding protein of higher plants: A most suitable model system for basic analyses of pigment–pigment and pigment–protein interactions in chlorophyll protein complexes

“…This striking feature markedly contrasts the corresponding properties of the exciton levels in strongly coupled pigments of light harvesting complexes and RCs where the oscillator strength of the transition into the lower exciton state is much larger than that of excitation into the upper exciton state (for reviews, see Green and Parson, 2003;Renger, 2008). This peculiar- Renger et al (2007) (panel a) and strengths and transition wavelengths between the ground state |0> and the one exciton states |+> and |−> as well as between the latter and the two exciton states |2> for the Chl a/b hetero-(left) and Chl a/a homo-(right) dimer (panel b) For further details, see Renger et al (2007) and Theiss et al (2007). ity of the Chl dimers in reconstituted recombinant class IIa WSCP is of great advantage for a model system of strong pigment-pigment interaction because it opens the way for systematic studies on the properties of the upper exciton level and its relaxation to the lower exciton state.…”

Theory of excitation energy transfer: from structure to function

“…11 (Adolphs and Renger 2006). An important difference between the two theories is that in Redfield theory only one-vibrational quantum transitions in the protein are included in the description of dissipation, whereas modified Redfield theory takes into account multivibrational-quanta processes (Yang and Fleming 2002;Renger et al 2007). The experimentally observed exciton relaxation time constant in WSCP-complexes containing heterodimers of Chla and Chlb Theiss et al (2007) could only be described by modified Redfield theory, since multivibrational-quanta transitions are needed to bridge the large gap between the exciton states in the heterodimer, whereas in the homodimer case both, Redfield and modified Redfield theory, gave very similar results (Renger et al 2007).…”

Exciton Interactions

“…Treatments using spectral density functions (Chap. 10) provide more practical ways of incorporating these states, and have been used successfully for a protein complex with four molecules of chlorophyll [25][26][27].…”