Relaxation Dynamics of Tryptophan in Water: A UV Fluorescence Up-Conversion and Molecular Dynamics Study

Ahmad Ajdar Zadeh Oskouei, Andrea Cannizzo, Majed Chergui, Frank van Mourik
American Chemical Society, 2010
Article

We report on an ultrafast experimental and simulations study of the early relaxation events of photoexcited tryptophan in water. Experimentally, we used fluorescence up-conversion in both polychromatic and single wavelength detection modes in the 300-480 nm range with polarization dependence. We report on the time evolution of the Stokes shift, bandwidth, and anisotropy from tens of femtoseconds to picoseconds. These observables contain signatures of the simultaneous occurrence of intramolecular and solvent-molecule interactions, which we disentangle with the help of nonequilibrium molecular dynamics simulations. We also observe a breakdown of the linear response approximation to describe our results.

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Ahmad Ajdar Zadeh Oskouei, Andrea Cannizzo, Majed Chergui, Frank van Mourik
American Chemical Society, 2010
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https://infoscience.epfl.ch/record/152567?ln=fr

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Probing the Transition from Hydrophilic to Hydrophobic Solvation with Atomic Scale Resolution

Frederico Alves Lima, Paul Beaud, Majed Chergui, Amal El Nahhas, Thomas James Penfold, Van Thai Pham, Ivano Tavernelli, Renske Marjan van der Veen

Picosecond and femtosecond X-ray absorption spectroscopy is used to probe the changes of the solvent shell structure upon electron abstraction of aqueous iodide using an ultrashort laser pulse. The transient L-1,L-3 edge EXAFS at 50 ps time delay points to the formation of an expanded water cavity around the iodine atom, in good agreement with classical and quantum mechanical/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. These also show that while the hydrogen atoms pointed toward iodide, they predominantly point toward the bulk solvent in the case of iodine, suggesting a hydrophobic behavior. This is further confirmed by quantum chemical (QC) calculations of I-/I-0(H2O)(n=1-4) clusters. The L-1 edge sub-picosecond spectra point to the existence of a transient species that is not present at SO ps. The QC calculations and the QM/MM MD simulations identify this transient species as an I-0(OH2) complex inside the cavity. The simulations show that upon electron abstraction most of the water molecules move away from iodine, while one comes closer to form the complex that lives for 3-4 Ps. This time is governed by the reorganization of the main solvation shell, basically the time it takes for the water molecules to reform an H-bond network Only then is the interaction with the solvation shell strong enough to pull the water molecule of the complex toward the bulk solvent. Overall, much of the behavior at early times is determined by the reorientational dynamics of water molecules and the formation of a complete network of hydrogen bonded molecules in the first solvation shell.

2011

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Fluorescence

La fluorescence est une émission lumineuse provoquée par l'excitation des électrons d'une molécule (ou atome), généralement par absorption d'un photon immédiatement suivie d'une émission spontanée.

Dynamique moléculaire

La dynamique moléculaire est une technique de simulation numérique permettant de modéliser l'évolution d'un système de particules au cours du temps. Elle est particulièrement utilisée en sciences de

Solvatation

vignette|Un ion sodium solvaté par des molécules d'eau. La solvatation est le phénomène physico-chimique observé lors de la dissolution d'une substance chimique dans un solvant. Lors de l'introductio