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| - Internal Conversion in the Chromophore of the Green Fluorescent Protein: TemperatureDependence and Isoviscosity Analysis
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| - The temperature dependence of internal conversion in model compounds of the chromophore of the greenfluorescent protein and one of its mutants has been measured. The strong temperature dependence persists inall charge forms of the model compounds, in all solvents and in a polymer matrix. The ultrafast internalconversion mechanism is thus an intrinsic property of the chromophore skeleton, rather than one of a specificcharge or hydrogen-bonded form. An isoviscosity analysis shows that the coordinate which promotes internalconversion is essentially barrierless at room temperature. At reduced temperatures (or high viscosity) there isevidence for the formation of a small barrier. This may reflect a change in the nature of the microscopicsolvent dynamics close to the glass transition temperature. In all cases the viscosity dependence of the rateconstant for internal conversion is very weak, being approximately proportional to viscosity raised to thepower of 0.25 ± 0.06. This suggests weak coupling between the relevant coordinate and macroscopic solventviscosity. It is suggested that a potential candidate for the coordinate which promotes internal conversion isthe volume-conserving “hula twist”.
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