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Low-Intensity Pump-Probe Measurements on the B800 Band of Rhodospirillum molischianum

Markus Wendling^*, Frank van Mourik, Ivo H. M. van Stokkum^*, Jante M. Salverda^*, Hartmut Michel and Rienk van Grondelle^*

^* Department of Biophysics and Physics of Complex Systems, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands; Institut de Physique de la Matière Condensée, Faculté des Sciences, Université de Lausanne, BSP, 1015 Lausanne, Switzerland; and Max-Planck-Institut für Biophysik, 60528 Frankfurt am Main, Germany

Correspondence: Address reprint requests to Markus Wendling, Dept. of Biophysics and Physics of Complex Systems, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands. Tel.: +31-20-4447932; Fax: +31-20-4447999; E-mail: markus{at}nat.vu.nl.

We have measured low-intensity, polarized one-color pump-probe traces in the B800 band of the light-harvesting complex LH2 of Rhodospirillum molischianum at 77 K. The excitation/detection wavelength was tuned through the B800 band. A single-wavelength and a global target analysis of the data were performed with a model that accounts for excitation energy transfer among the B800 molecules and from B800 to B850. By including the anisotropy of the signals into the fitting procedure, both transfer processes could be separated. It was estimated in the global target analysis that the intra-B800 energy transfer, i.e., the hopping of the excitation from one B800 to another B800 molecule, takes 0.5 ps at 77 K. This transfer time increases with the excitation/detection wavelength from 0.3 ps on the blue side of the B800 band to 0.8 ps on the red side. The residual B800 anisotropy shows a wavelength dependence as expected for energy transfer within an inhomogeneously broadened cluster of weakly coupled pigments. In the global target analysis, the transfer time from B800 to B850 was determined to be 1.7 ps at 77 K. In the single-wavelength analysis, a speeding-up of the B800 B850 energy transfer rate toward the blue edge of the B800 band was found. This nicely correlates with the proposed position of the suggested high-exciton component of the B850 band acting as an additional decay channel for B800 excitations.