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How Light-Induced Charge Transfer Accelerates the Receptor-State Recovery of Photoactive Yellow Protein from its Signaling State

L. Premvardhan ^*, M. A. van der Horst , K. J. Hellingwerf  ^* and R. van Grondelle ^*

^* Department of Biophysics and Physics of Complex Systems, Faculty of Sciences, Vrije Universiteit, Amsterdam, The Netherlands; and Department of Microbiology, Swammerdam Institute for Life Sciences, Universiteit van Amsterdam, Amsterdam, The Netherlands

Correspondence: Address reprint requests and inquiries to L. Premvardhan, E-mail: lp2f{at}few.vu.nl or premvard{at}gmail.com.

Stark (electroabsorption) spectra of the M100A mutant of photoactive yellow protein reveal that the neutral, cis cofactor of the pB intermediate undergoes a strikingly large change in the static dipole moment ( = 19 Debye) on photon absorption. The formation of this charge-separated species, in the excited state, precedes the cis trans isomerization of the pB cofactor and the regeneration of pG. The large , reminiscent of that produced on the excitation of pG, we propose, induces twisting of the cis cofactor as a result of translocation of negative charge, from the hydroxyl oxygen, O1, toward the C7-C8 double bond. The biological significance of this photoinduced charge transfer reaction underlies the significantly faster regeneration of pG from pB in vitro, on the absorption of blue light.