An Anomalous Distance Dependence of Intra-Protein Chlorophyll-Carotenoid Triplet Energy Transfer

¹ Purdue University

^* To whom correspondence should be addressed. E-mail: sergei{at}physics.purdue.edu.

Submitted on June 28, 2005
Revised on July 18, 2005
Accepted on 20 July 2005

	Abstract

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 -carotene, as defined by X-ray structures from both a cyanobacterium and a green alga (1, 2). In spite of this separation, rapid (< 8 ns) long-range triplet energy transfer from the chlorophyll a to -carotene is documented in the present 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 intra-protein channel connecting the chlorophyll a and -carotene, can serve as a mediator in chlorophyll-carotenoid triplet energy transfer in the b₆f complex

Key Words: carotenoid, chlorophyll, cytochrome b6f, singlet oxygen protection, triplet energy transfer

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