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_2h symmetry, which makes transitions from their ground states to their S₁ (S₂) 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 S₁ 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 S₁ and S₂ regions, with the spectrum slightly red-shifted in the protein sample. The peridinin S₁ energy was found to be higher than that of typical light harvesting carotenoids, making its S₁ state very close in energy to its S₂ 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 S₀-S₂ transition.