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Electrostatic control of photoisomerization pathways in proteins

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Science  03 Jan 2020:
Vol. 367, Issue 6473, pp. 76-79
DOI: 10.1126/science.aax1898

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Electrostatics guide chromophore twist

Photoisomerization—the twisting of bonds in a molecule in response to absorption of light—is exploited in biology to sense light and can influence the photophysical properties of fluorescent proteins used in imaging applications. Romei et al. studied this behavior by introducing unnatural amino acids into the photoswitchable green fluorescent protein Dronpa2, thus systematically altering the electronic properties of the chromophore (see the Perspective by Hu et al.). Crystal structures and spectroscopic analyses of a series of these variants support a model in which the electrostatic interactions between the chromophore and its environment influence the barrier heights for twisting around different bonds during photoisomerization. These insights may guide future design of photoswitchable proteins with desired properties.

Science, this issue p. 76; see also p. 26

Abstract

Rotation around a specific bond after photoexcitation is central to vision and emerging opportunities in optogenetics, super-resolution microscopy, and photoactive molecular devices. Competing roles for steric and electrostatic effects that govern bond-specific photoisomerization have been widely discussed, the latter originating from chromophore charge transfer upon excitation. We systematically altered the electrostatic properties of the green fluorescent protein chromophore in a photoswitchable variant, Dronpa2, using amber suppression to introduce electron-donating and electron-withdrawing groups to the phenolate ring. Through analysis of the absorption (color), fluorescence quantum yield, and energy barriers to ground- and excited-state isomerization, we evaluate the contributions of sterics and electrostatics quantitatively and demonstrate how electrostatic effects bias the pathway of chromophore photoisomerization, leading to a generalized framework to guide protein design.

  • These authors contributed equally to this work.

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