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* Department of Biophysics and Physics of Complex Systems, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, Amsterdam, The Netherlands; and 
Laboratory for Microbiology, Swammerdam Institute for Life Sciences, Universiteit van Amsterdam, Amsterdam, The Netherlands
Correspondence: Address reprint requests to L. L. Premvardhan, Faculty of Sciences, Div. of Physics and Astronomy, Dept. of Biophysics, Vrije Universiteit, de Boelelaan, 1081, 1081 HV Amsterdam, The Netherlands. Tel.: 31-20-444-7932; Fax: 31-20-444-7999; E-mail: lp2f{at}nat.vu.nl.
The change in the electrostatic properties on excitation of the cofactor of wild-type photoactive yellow protein (WT-PYP) have been directly determined using Stark-effect spectroscopy. We find that, instantaneously on photon absorption, there is a large change in the permanent dipole moment, 
(26 Debye) and in the polarizability, 
(1000 Å3). We expect such a large degree of charge motion to have a significant impact on the photocycle that is associated with the important blue-light negative phototactic response of Halorhodospira halophila. Furthermore, changing E46 to Q in WT-PYP does not significantly alter its electrostatic properties, whereas, altering the chromophore to prevent it from undergoing trans-cis isomerization results in a significant diminution of 
and 
We propose that the enormous charge motion that occurs on excitation of 4-hydroxycinnamyl thioester, the chromophore in WT-PYP, plays a crucial role in initiating the photocycle by translocation of the negative charge, localized on the phenolate oxygen in the ground state, across the chromophore. We hypothesize that this charge motion would consequently increase the flexibility of the thioester tail thereby decreasing the activation barrier for the rotation of this moiety in the excited state.
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