Two-Photon Excitation Study of Peridinin in Benzene and in the Peridinin Chlorophyll a-Protein (PCP)

Peridinin chlorophyll a- protein (PCP) is a unique light harvesting protein found in dinoflagellates, that contains a large amount of the carotenoid peridinin. Carotenoids have unusual spectroscopic properties due to their approximate C 2 h symmetry, which makes transitions from their ground states to their S 1 (S 2 ) states one-photon forbidden (allowed). To gain information about one-photon forbidden electronic states in peridinin, fluorescence excitation spectra were measured after two-photon excitation for peridinin in benzene and in the PCP. The samples were excited using 920−1320 nm light. Fluorescence of the isolated peridinin S 1 state was then measured at 750 nm. In PCP, the excited peridinin transfers energy to chlorophyll whose fluorescence was monitored at 670 nm. Surprisingly, two-photon absorption was observed in both the peridinin S 1 and S 2 regions, with the spectrum slightly red-shifted in the protein sample. The peridinin S 1 energy was found to be higher than that of typical light harvesting carotenoids, making its S 1 state very close in energy to its S 2 state. We suggest that peridinin's polar groups, symmetry breaking, and possible mixing of electronic states lead to two-photon character of the normally one-photon allowed S 0 −S 2 transition.