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An Anomalous Distance Dependence of Intraprotein Chlorophyll-Carotenoid Triplet Energy Transfer

Hanyoup Kim ^*, Naranbaatar Dashdorj ^*, Huamin Zhang , Jiusheng Yan , William A. Cramer  and Sergei Savikhin ^*

^* Department of Physics and Department of Biological Sciences, Purdue University, West Lafayette, IN 47907

Correspondence: Address reprint requests and inquiries to Sergei Savikhin, E-mail: sergei{at}physics.purdue.edu.

In the light-harvesting chlorophyll pigment-proteins of photosynthesis, a carotenoid is typically positioned within a distance of 4 Å of individual chlorophylls or antenna arrays, allowing rapid triplet energy transfer from chlorophyll to the carotenoid. This triplet energy transfer prevents the formation of toxic singlet oxygen. In the cytochrome b₆f complex of oxygenic photosynthesis that contains a single chlorophyll a molecule, this chlorophyll is distant (14 Å) from the single ß-carotene, as defined by x-ray structures from both a cyanobacterium and a green alga. Despite this separation, rapid (<8 ns) long-range triplet energy transfer from the chlorophyll a to ß-carotene is documented in this study, in seeming violation of the existing theory for the distance dependence of such transfer. We infer that a third molecule, possibly oxygen trapped in an intraprotein channel connecting the chlorophyll a and ß-carotene, can serve as a mediator in chlorophyll-carotenoid triplet energy transfer in the b₆f complex.

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