Thermally Activated Superradiance and Intersystem Crossing in the Water-Soluble Chlorophyll Binding Protein

Abstract

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.

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

“…For Chl a these values differ by 0.09 eV and for Chl b by 0.28 eV. [88] Thus, the effect is more pronounced for Chl b. It even changes the ordering of the Q y excitation energies for these pigments.…”

“…It even changes the ordering of the Q y excitation energies for these pigments. [88,89] CIS calculations predict the correct relative order of Q y -excitation energies of Chl a and b, but if the doubly excited configurations are perturbatively taken into account in CIS(D), this trend is no longer visible. [89] Vertical excitation energies from different electronic-structure methods have also been compared for D 4h -symmetric metalloporphyrins in terms of the Gouterman model.…”

Probing environment fluctuations by two-dimensional electronic spectroscopy of molecular systems at temperatures below 5 K

“…The parameters of our model are inspired by the HB and FLN spectroscopies of a recombinant water-soluble chlorophyll protein (WSCP). 36,37,41 It has been shown recently that both WSCP from Lepidium virginicum (LvWSCP) and cauliflower (Brassica oleracea L. var. botrys -CaWSCP) have four chlorophylls in a tetramer, i.e., two weakly coupled dimers 42 in which uncorrelated EET was observed.…”

Identification of genes encoding photoconvertible (Class I) water-soluble chlorophyll-binding proteins from Chenopodium ficifolium

“…[18][19][20][21][22][23] Furthermore, various molecular genetic analyses on recombinant Class II WSCPs have been performed. 18,[20][21][22][23][24][25][26][27][28][29][30][31][32] On the other hand, CaWSCP is the only gene encoding a Class I WSCP identified to date and few studies on the molecular genetics of the protein have been performed thus far. 33,34) Identification of novel genes encoding Class I WSCPs is essential for understanding the molecular diversity of the protein.…”