<p>Double-bond photoisomerization in
molecules such as the green fluorescent protein (GFP) chromophore can occur
either via a volume-demanding one-bond-flip pathway or via a volume-conserving
hula-twist pathway. Understanding the factors that determine the pathway of photoisomerization
would inform the rational design of photoswitchable GFPs as improved tools for
super-resolution microscopy. In this communication, we reveal the
photoisomerization pathway of a photoswitchable GFP, rsEGFP2, by solving
crystal structures of <i>cis</i> and <i>trans</i> rsEGFP2 containing a
monochlorinated chromophore. The position of the chlorine substituent in the <i>trans</i> state breaks the symmetry of the
phenolate ring of the chromophore and allows us to distinguish the two
pathways. Surprisingly, we find that the pathway depends on the arrangement of
protein monomers within the crystal lattice: in a looser packing, the
one-bond-flip occurs, whereas in a tighter packing (7% smaller unit cell size),
the hula-twist occurs.</p><p>
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Super-Resolution Microscopy: From Single Molecules to Supramolecular Assemblies