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Contrasting the Excited-State Dynamics of the Photoactive Yellow Protein Chromophore: Protein versus Solvent Environments

Mikas Vengris ^*, Michael A. van der Horst , Goran Zgrabli , Ivo H. M. van Stokkum ^*, Stefan Haacke , Majed Chergui , Klaas J. Hellingwerf , Rienk van Grondelle ^* and Delmar S. Larsen ^*

^* Faculty of Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands; Department of Microbiology, Swammerdam Institute for Life Sciences, University of Amsterdam, Amsterdam, The Netherlands; and Laboratoire de Spectroscopie Ultrarapide, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

Correspondence: Address reprint requests to Delmar S. Larsen, Fax: 31-0-20-444-7999; E-mail: dslarsen{at}nat.vu.nl.

Wavelength- and time-resolved fluorescence experiments have been performed on the photoactive yellow protein, the E46Q mutant, the hybrids of these proteins containing a nonisomerizing "locked" chromophore, and the native and locked chromophores in aqueous solution. The ultrafast dynamics of these six systems is compared and spectral signatures of isomerization and solvation are discussed. We find that the ultrafast red-shifting of fluorescence is associated mostly with solvation dynamics, whereas isomerization manifests itself as quenching of fluorescence. The observed multiexponential quenching of the protein samples differs from the single-exponential lifetimes of the chromophores in solution. The locked chromophore in the protein environment decays faster than in solution. This is due to additional channels of excited-state energy dissipation via the covalent and hydrogen bonds with the protein environment. The observed large dispersion of quenching timescales observed in the protein samples that contain the native pigment favors both an inhomogeneous model and an excited-state barrier for isomerization.

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