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J. Biol. Chem., Vol. 281, Issue 35, 25381-25387, September 1, 2006

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Stepwise Two-photon Excited Fluorescence from Higher Excited States of Chlorophylls in Photosynthetic Antenna Complexes^*

Dieter Leupold, Klaus Teuchner, Jürgen Ehlert^¶, Klaus-Dieter Irrgang^||, Gernot Renger^||, and Heiko Lokstein^**1

From the Institut für Physik/Photonik, Universität Potsdam, Postfach 601553, D-14415 Potsdam, Gesellschaft zur Förderung Angewandter Optik, Optoelektronik, Quantenelektronik und Spektroskopie e. V., Rudower Chaussee 29, D-12489 Berlin, ^¶Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2a, D-12489 Berlin, ^||Max-Volmer-Laboratorium für Biophysikalische Chemie, Technische Universität Berlin, Strasse des 17. Juni 135, D-10623 Berlin, and ^**Institut für Biochemie und Biologie/Pflanzenphysiologie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, Haus 20, D-14476 Golm, Germany

Stepwise two-photon excited fluorescence (TPEF) spectra of the photosynthetic antenna complexes PCP, CP47, CP29, and light-harvesting complex II (LHC II) were measured. TPEF emitted from higher excited states of chlorophyll (Chl) a and b was elicited via consecutive absorption of two photons in the Chl a/b Q_y range induced by tunable 100-fs laser pulses. Global analyses of the TPEF line shapes with a model function for monomeric Chl a in a proteinaceous environment allow distinction between contributions from monomeric Chls a and b, strongly excitonically coupled Chls a, and Chl a/b heterodimers/-oligomers. The analyses indicate that the longest wavelength-absorbing Chl species in the Q_y region of LHC II is a Chl a homodimer with additional contributions from adjacent Chl b. Likewise, in CP47 a spectral form at 680 nm (that is, however, not the red-most species) is also due to strongly coupled Chls a. In contrast to LHC II, the red-most Chl subband of CP29 is due to a monomeric Chl a. The two Chls b in CP29 exhibit marked differences: a Chl b absorbing at 650 nm is not excitonically coupled to other Chls. Based on this finding, the refractive index of its microenvironment can be determined to be 1.48. The second Chl b in CP29 (absorbing at 640 nm) is strongly coupled to Chl a. Implications of the findings with respect to excitation energy transfer pathways and rates are discussed. Moreover, the results will be related to most recent structural analyses.

Received for publication, January 4, 2006 , and in revised form, June 2, 2006.

^* This work was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 429, TPs A2, A3). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. We dedicate this paper to Professor Dr. Thomas Börner on the occasion of his 60th birthday.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3, Table S1, and references.

¹ To whom correspondence should be addressed. Tel.: 49-331-977-2648; Fax: 49-331-977-2512; E-mail: lokstein{at}uni-potsdam.de.

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