Spatial arrangement of pigments and their interaction in the fucoxanthin-chlorophyll [formula omitted] protein assembly (FCPA) isolated from the brown alga Dictyota dichotoma. Analysis by means of polarized spectroscopy

Abstract

Molecular interaction and energy transfer between pigments in the fucoxanthin-chlorophyll (Chl) a c protein assembly (FCPA) isolated from the brown alga Dictyota dichotoma was investigated mainly by polarized spectroscopy. FCPA consists of 7 identical units of 54 kDa apoprotein, each of them containing 13 Chl a, 3 Chl c, 10 fucoxanthin and 1 violaxanthin. Spectral heterogeneity was found in the component pigments; two types of Chl a (Chl a 673 and Chl a 669), two types of Chl c (Chl c l, long-wavelength form of Chl c and Chl c s, short-wavelength form). In fucoxanthin, two functionally active and one inactive species were found. Energy flow in the FCPA is attained by a direct coupling of donor-acceptor pair and those are classified into four types: from fucoxanthin to Chl a 669, from fucoxanthin to Chl a 673, from Chl c l to Chl a 669 and from Chl c s to Chl a 669. The number of those four pathways was estimated to be 7, 2, 2 and 1, respectively, per unit peptide. Energy migration in the Chl a molecules is always functioning. Dissociation of FCPA into unit peptides induces the uncoupling of energy transfer between the respective donor and acceptor Chl a. The spatial orientation of individual pigments, investigated by linear dichroism and polarized fluorescence spectroscopy, was shown to be favorable for an efficient energy transfer. Based on the results of polarized spectroscopy, a spatial orientation of individual chromophores in the peptide was proposed.