Against the NEER principle: The third type of photochromism for GFP chromophore derivatives

Photochromism is the heart of photochromic fluorescent proteins. Excited-state proton transfer (ESPT) is the major cause of photochromism for the green fluorescent protein (GFP) andZ-Ephotoisomerization through τ-torsion is the major cause of photochromism for Dronpa (a GFP mutant). In this article,s-E-1opens a third type of photochromism for GFP chromophore derivatives, which involves light-driven φ-torsion with no τ-torsion, followed by excited-state intramolecular proton transfer (ESIPT), and is gated by environmental polarity. Sinces-E-1does not followZ-Ephotoisomerization through τ-torsion but undergoes light-driven φ-torsion, which involves equilibration of the excited-state rotamers, it is clearly against the NEER (Non-Equilibration of Excited-state Rotamers) principle.