Model for triplet-triplet energy transfer in natural clusters of peridininmolecules contained in Dinoflagellate's outer antenna proteins

Triplet-triplet energy transfer among peridinin molecules in peridinin-chlo rophyll-protein (PCP) complexes from two different dinoflagellate species i s quantitatively evaluated by the use of the optically detected magnetic re sonance technique supplemented by triplet-state decay rate measurements. Th e two complexes are related by a dimer-monomer relationship in the sense th at the four peridinins-one chlorophyll cluster of the Heterocapsa pygmeae P CP (H-PCP) is a copy of the two clusters contained in the larger protein Am phidinium carterae PCP (A-PCP), which are related by a local 2-fold symmetr y axis. The system is conveniently discussed as a multistate Frenkel excito n system performing stochastic jumps from one exciton state to another. The rate of triplet-triplet energy exchange among peridinin molecules is obtai ned through the simulation of the spectral line shapes of optically detecte d magnetic resonance in zero field. Measurements were collected in a range of temperatures between 2 and 40 K and supplemented by the determination of the decay rate of the triplet to ground state in the same temperature rang e. The latter is of the order of 0.005 MHz and is relevant in determining t he line shapes, especially at the lowest temperatures where the exchange ra tes are low. Exchange rates within the four-member peridinin cluster of H-P CP exactly match those within each of the two clusters of A-PCP and span a range from 0 to 20 MHz for the uphill rate and from 10 to 300 MHz for the d ownhill rate, corresponding to an energy difference of 41 cm(-1). The inter cluster exchange rate in A-PCP goes from 0 to 12 MHz uphill and from 0.05 t o 30 MHz downhill, with an energy difference of only 12.5 cm(-1). The kinet ics conform to phonon-stimulated radiationless transition theory, and coupl ing parameters and reorganization energies are evaluated and discussed.