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.

Persistent spectral hole burning at 4.5 K has been used to investigate the excitonic energy level structure and the excited state dynamics of the recombinant class-IIa water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The hole-burned spectra are composed of four main features: (i) a narrow zero-phonon hole (ZPH) at the burn wavelength, (ii) a number of vibrational ZPHs, (iii) a broad low-energy hole at ~665 and ~683 nm for chlorophyll b- and chlorophyll a-WSCP, respectively, and (iv) a second satellite hole at ~658 and ~673 nm for chlorophyll b- and chlorophyll a-WSCP, respectively. The doublet of broad satellite holes is assigned to an excitonically coupled chlorophyll dimer. The lower-energy holes at ~665 and ~683 nm for chlorophyll b- and chlorophyll a-WSCP, respectively, represent the lower exciton states. Taking into account the parameters of electron-phonon coupling, the lower exciton state can be assigned as the fluorescence origin. The lower exciton state is populated by two processes: (i) exciton relaxation from the higher exciton state and (ii) vibrational relaxation within the lower exciton state. Assuming identical site energies for the two excitonically coupled chlorophyll molecules, the dipole-dipole interaction energy J is directly determined to be 85 and 100 cm(-1) for chlorophyll b- and chlorophyll a-WSCP, respectively, based on the positions of the satellite holes. The Gaussian low-energy absorption band identified by constant fluence hole burning at 4.5 K has a width of ~150 cm(-1) and peaks at 664.9 and 682.7 nm for chlorophyll b- and chlorophyll a-WSCP, respectively. The action spectrum is broader and blue-shifted compared to the fluorescent lower exciton state. This finding can be explained by a slow protein relaxation between energetically inequivalent conformational substates within the lowest exciton state in agreement with the results of Schmitt et al. (J. Phys. Chem. B2008, 112, 13951).

Plants contain water-soluble chlorophyll-binding proteins (WSCPs) that function neither as antennas nor as components of light-induced electron transfer of photosynthesis but are likely constituents of regulatory protective pathways in particular under stress conditions. This study presents results on the spectroscopic properties of recombinant WSCP from cauliflower reconstituted with chlorophyll b (Chl b) alone or with mixtures of Chl a and Chl b. Two types of experiments were performed: (a) measurements of stationary absorption spectra at 77 and 298 K and CD spectra at 298 K and (b) monitoring of laser flash-induced transient absorption changes with a resolution of 200 fs in the time domain of up to 100 ps. On the basis of a theoretical analysis outlined by Renger et al. (J. Phys. Chem. B 2007, 111, 10487) the data obtained in part (a) are interpreted within a model where tetrameric WSCP binds predominantly two Chl molecules in the form of an excitonically coupled "open sandwich" dimer with a tilt angle of about 30 degrees between the chlorin planes. The time-resolved measurements on Chl a/Chl b heterodimers are described by two exponential kinetics with time constants of 400 fs and 7 ps. These kinetics are assumed to reflect a heterogeneous population of WSCPs with Chl dimers either in excitonic coupled "open sandwich" or weakly coupled geometric arrays. The 400 fs component is assigned to excited-state relaxations from the upper to the lower excitonic level of the strongly coupled "open sandwich" dimer, while the 7-8 ps component probably indicates excitation energy transfer from 1Chl b* to Chl a in a dimer array with weak coupling due to significantly longer mutual distances between the chlorin rings.

The crystal structure of the class IIb water-soluble chlorophyll binding protein (WSCP) from Lepidium virginicum is used to model linear absorption and circular dichroism spectra as well as excited state decay times of class IIa WSCP from cauliflower reconstituted with chlorophyll (Chl) a and Chl b. The close agreement between theory and experiment suggests that both types of WSCP share a common Chl binding motif, where the opening angle between pigment planes in class IIa WSCP should not differ by more than 10 degrees from that in class IIb. The experimentally observed (Schmitt et al. J. Phys. Chem. B 2008, 112, 13951) decrease in excited state lifetime of Chl a homodimers with increasing temperature is fully explained by thermally activated superradiance via the upper exciton state of the dimer. Whereas a temperature-independent intersystem crossing (ISC) rate is inferred for WSCP containing Chl a homodimers, that of WSCP with Chl b homodimers is found to increase above 100 K. Our quantum chemical/electrostatic calculations suggest that a thermally activated ISC via an excited triplet state T4 is responsible for the latter temperature dependence.