Ultrafast Two-Photon Spectroscopy of Carotenoids involved in the Regulation of Photosynthesis

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A quick balancing between utilization and dissipation of absorbed light energy is essential for the survival of natural and gene modified plants.[1-3] However, despite intensive investigations (see recent publications in Nature and Science)[1, 4-6] the biophysical mechanisms of these regulation processes are still poorly understood. The molecular gear shift model proposes that a reversible and step-wise enzymatic modification of the chemical structure of xanthophyll carotenoids enables a switching between carotenoid-to-chlorophyll lightharvesting and chlorophyll-to-carotenoid quenching.[7] Unfortunately, the involved firstexcited singlet state of the carotenoids, Car S1, is optically forbidden[8] which renders impossible a direct investigation of its role by conventional absorption or fluorescence spectroscopy. In the past we have shown that two-photon excitation (TPE) allows a selective population of the forbidden state because the optical selection rules are different for this nonlinear process.[9-15] Preliminary two-photon sensitized fluorescence experiments[9] indicated that the proposed change in carotenoid-chlorophyll energy flow indeed takes place in plants. We now plan to investigate isolated pigment-protein complexes as well as thylakoid and chloroplast preparations containing well defined amounts of high- or low-light xanthophylls. The aim of these two-photon experiments is to provide a clear prove or disprove for a major contribution of the proposed mechanism and to elucidate which of the pigment-protein complexes are involved in such a regulation.