Probing the Fucoxanthin-Chlorophyll a/c-Binding Proteins (FCPs) of the Marine Diatom Fragilariopsis sp. by Resonance Raman Spectroscopy

Abstract

We report resonance Raman spectra of the light-harvesting fucoxanthin-chlorophyll a/c-binding proteins (FCPs) of marine diatom Fragilariopsis sp. The Raman shifts in the 15N-isotope-enriched diatom provide the first spectroscopic evidence for the characterization of the Ca-N marker bands and, thus, of the penta- and hexacoordinated states of chlorophylls a/c in the FCPs. Under 405 and 442 nm Raman excitations, all of the marker bands of Chl a/c are observed and the isotope-based assignments provide new information concerning the structure of Chls a/c in the FCPs and their interactions with the protein environment. Therefore, the Raman spectrum at 405 nm originates from the π-π* transitions of Chl a/c and not from a different, non π-π* electronic transition, as previously reported (BBA Bioenergetics, 2010, 1797, 1647-1656). Based on the 15N isotope shifts of the Ca-N and in conjunction with other marker bands, two distinct conformations of five- and six-coordinated Chl a and Chl c are observed. In addition, two keto carbonyls were observed at 1679 (strong H-bonded) and 1691 cm-1 (weak H-bonded) in both the 405 and 442 nm Raman spectra, respectively. Collectively, the results provide solid evidence of the nature of the vibrational modes of the active Chl a/c photosynthetic pigments in the FCPs.