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Pigment-Pigment Interactions in PCP of Amphidinium carterae Investigated by Nonlinear Polarization Spectroscopy in the Frequency Domain

Maria Krikunova ^*, Heiko Lokstein , Dieter Leupold ^*, Roger G. Hiller  and Bernd Voigt ^*

^* Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Berlin, Germany; Institut für Biologie/Pflanzenphysiologie, Humboldt-Universität zu Berlin, Berlin, Germany; and Department of Biological Sciences, Macquarie University, Sydney, New South Wales, Australia

Correspondence: Address reprint requests to Bernd Voigt, E-mail: bevoigt{at}rz.uni-potsdam.de, or Heiko Lokstein, E-mail: lokstein{at}uni-potsdam.de.

Peridinin-chlorophyll a-protein (PCP) is a unique antenna complex in dinoflagellates that employs peridinin (a carotenoid) as its main light-harvesting pigment. Strong excitonic interactions between peridinins, as well as between peridinins and chlorophylls (Chls) a, can be expected from the short intermolecular distances revealed by the crystal structure. Different experimental approaches of nonlinear polarization spectroscopy in the frequency domain (NLPF) were used to investigate the various interactions between pigments in PCP of Amphidinium carterae at room temperature. Lineshapes of NLPF spectra indicate strong excitonic interactions between the peridinin's optically allowed S₂ (1Bu⁺) states. A comprehensive subband analysis of the distinct NLPF spectral substructure in the peridinin region allows us to assign peridinin subbands to the two Chls a in PCP having different S₁-state lifetimes. Peridinin subbands at 487, 501, and 535 nm were assigned to the longer-lived Chl, whereas a peridinin subband peaking at 515 nm was detected in both clusters. Certain peridinin(s), obviously corresponding to the subband centered at 487 nm, show(s) specific (possibly Coulombic?) interaction between the optically dark ) and/or intramolecular charge-transfer (ICT) state and S₁ of Chl a. The NLPF spectrum, hence, indicates that this peridinin state is approximately isoenergetic or slightly above S₁ of Chl a. A global subband analysis of absorption and NLPF spectra reveals that the Chl a Q_y-band consists of two subbands (peaking at 669 and 675 nm and having different lifetimes), confirmed by NLPF spectra recorded at high pump intensities. At the highest applied pump intensities an additional band centered at 660 nm appears, suggesting—together with the above results—an assignment to a low-dipole moment S₀ S₁/ICT transition of peridinin.

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