Spectroscopic Properties of the Main-Form and High-Salt Peridinin−Chlorophyll a Proteins from Amphidinium carterae

The main-form (MFPCP) and high-salt (HSPCP) peridinin−chlorophyll a proteins from the dinoflagellate Amphidinium carterae were investigated using absorption, fluorescence, fluorescence excitation, two-photon, and fast-transient optical spectroscopy. Pigment analysis has demonstrated previously that MFPCP contains eight peridinins and two chlorophyll (Chl) a molecules, whereas HSPCP has six peridinins and two Chl a molecules [Sharples, F. P., et al. (1996) Biochim. Biophys. Acta 1276, 117−123]. Absorption spectra of the complexes were recorded at 10 K and analyzed in the 400−600 nm region by summing the individual 10 K spectra of Chl a and peridinin recorded in 2-MTHF. The absorption spectral profiles of the complexes in the Q y region between 650 and 700 nm were fit using Gaussian functions. The absorption and fluorescence spectra from both complexes exhibit several distinguishing features that become evident only at cryogenic temperatures. In particular, at low temperatures the Q y transitions of the Chls bound in the HSPCP complex are split into two well-resolved bands. Fluorescence excitation spectroscopy has revealed that the peridinin-to-Chl a energy transfer efficiency is high (>95%). Transient absorption spectroscopy has been used to measure the rate of energy transfer between the two bound Chls which is a factor of 2.9 slower in HSPCP than in MFPCP. The kinetic data are interpreted in terms of the Förster mechanism describing energy transfer between weakly coupled, spatially fixed, donor−acceptor Chl a molecules. The study provides insight into the molecular factors that control energy transfer in this class of light-harvesting pigment−protein complexes.