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

• Published in: The journal of physical chemistry. B 2009-07, Vol.113 (29), p.9948-9957
• Main Authors: Renger, T, Madjet, M. E, Müh, F, Trostmann, I, Schmitt, F.-J, Theiss, C, Paulsen, H, Eichler, H. J, Knorr, A, Renger, G
• Format: Article
• Language: English
• Subjects: B: Biophysical Chemistry; Circular Dichroism; Crystallography, X-Ray; Lepidium - chemistry; Light-Harvesting Protein Complexes - chemistry; Models, Molecular; Quantum Theory; Solubility; Static Electricity; Temperature; Water - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp901886w

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° 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.