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

Abstract

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 theirapproximate 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, 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, 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.