Peridinin Chlorophyll a Protein: Relating Structure and Steady-State Spectroscopy

Abstract

Peridinin chlorophyll a protein (PCP) from Amphidinium carterae has been studied using absorbance (OD), linear dichroism (LD), circular dichroism (CD), fluorescence emission, fluorescence anisotropy, fluorescence line narrowing (FLN), and triplet-minus-singlet spectroscopy (T-S) at different temperatures (4−293 K). Monomeric PCP binds eight peridinins and two Chls a. The trimeric structure of PCP, resolved at 2 Å [Hofmann et al. (1996) Science 27, 1788−1791], allows modeling of the Chl a−protein and Chl a−Chl a interactions. The FLN spectrum shows that Chl a is not or is very weakly hydrogen-bonded and that the central magnesium of the emitting Chl a is monoligated. Simulation of the temperature dependence of the absorption spectra indicates that the Huang−Rhys factor, characterizing the electron−phonon coupling strength, has a value of ∼1. The width of the inhomogeneous distribution function is estimated to be 160 cm^-1. LD experiments show that the two Chls a in PCP are essentially isoenergetic at room temperature and that a substantial amount of PCP is in a trimeric form. From a comparison of the measured and simulated CD, it is concluded that the interaction energy between the two Chls a within one monomer is very weak, -¹. In contrast, the Chls a appear to be strongly coupled to the peridinins. The 65 cm^-1 band that is visible in the low-frequency region of the FLN spectrum might indicate a Chl a−peridinin vibrational mode. The efficiency of Chl a to peridinin triplet excitation energy transfer is ∼100%. On the basis of T-S, CD, LD, and OD spectra, a tentative assignment of the peridinin absorption bands has been made.