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Peridinin-chlorophyll a protein (PCP) is a unique water soluble antenna complex that employs the carotenoid peridinin as the main light-harvesting pigment. In the present study the near edge X-ray absorption fine structure (NEXAFS) spectrum of PCP was recorded at the carbon K-edge. Additionally, the NEXAFS spectra of the constituent pigments, chlorophyll a and peridinin, were measured. The energies of the lowest unoccupied molecular levels of these pigments appearing in the carbon NEXAFS spectrum were resolved. Individual contributions of the pigments and the protein to the measured NEXAFS spectrum of PCP were determined using a "building block" approach combining NEXAFS spectra of the pigments and the amino acids constituting the PCP apoprotein. The results suggest that absorption changes of the pigments in the carbon near K-edge region can be resolved following excitation using a suitable visible pump laser pulse. Consequently, it may be possible to study excitation energy transfer processes involving "optically dark" states of carotenoids in pigment-protein complexes by soft X-ray probe optical pump double resonance spectroscopy (XODR).

An improved method for spectral subband decomposition based on simultaneous fitting of the initial spectrum and a set of its derivatives is introduced. Additionally, it procedure for finding an optimal smoothing filter to obtain undistorted derivatives IS Suggested. The proposed method is demonstrated with a model spectrum as well its with experimental absorption spectra of the photosynthetic antenna complexes, peridinin-chlorophyll a-protein (PCP) and the main light-harvesting complex of higher plants (LHC II). (c) 2005 Elsevier B.V. All rights reserved

A novel soft X-ray and optical short-pulse double resonance spectroscopic technique tailor-made to elucidate processes involving the optically forbidden S 1 (2 1 A g ) state of carotenoids in native biological samples (e.g., photosynthetic antenna complexes) is described. The principle relies on probing the near carbon K-edge absorption of the optically excited sample with soft X-rays generated by a laser-induced plasma. A first application concerns location of the 2 1 A g state of β-carotene in vitro. Further applications are proposed.