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Biophysical Journal 85:1128-1134 (2003)
© 2003 The Biophysical Society

Direct Measurement of the Photoelectric Response Time of Bacteriorhodopsin via Electro-Optic Sampling

J. Xu *, A. B. Stickrath *, P. Bhattacharya *, J. Nees *, G. Váró {dagger}, J. R. Hillebrecht {ddagger}, L. Ren {ddagger} and R. R. Birge {ddagger}

* Solid State Electronics Laboratory and Center for Ultrafast Optical Science, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, USA; {dagger} Biological Research Center of the Hungarian Academy of Sciences, Szeged, Hungary; and {ddagger} Departments of Chemistry and of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut, USA

Correspondence: Address reprint requests to Pallab Bhattacharya, Tel.: 734-763-6678; Fax: 734-763-9324; E-mail: pkb{at}eecs.umich.edu; or Robert R. Birge, Tel.: 860-486-6720; Fax: 860-486-2981; E-mail: rbirge{at}uconn.edu.

The photovoltaic signal associated with the primary photochemical event in an oriented bacteriorhodopsin film is measured by directly probing the electric field in the bacteriorhodopsin film using an ultrafast electro-optic sampling technique. The inherent response time is limited only by the laser pulse width of 500 fs, and permits a measurement of the photovoltage with a bandwidth of better than 350 GHz. All previous published studies have been carried out with bandwidths of 50 GHz or lower. We observe a charge buildup with an exponential formation time of 1.68 ± 0.05 ps and an initial decay time of 31.7 ps. Deconvolution with a 500-fs Gaussian excitation pulse reduces the exponential formation time to 1.61 ± 0.04 ps. The photovoltaic signal continues to rise for 4.5 ps after excitation, and the voltage profile corresponds well with the population dynamics of the K state. The origin of the fast photovoltage is assigned to the partial isomerization of the chromophore and the coupled motion of the Arg-82 residue during the primary event.




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