The recently developed red fluorescent protein Keima exhibits the largest Stokes shift (180 nm) observed to date. Combining X-ray crystallography with (in crystallo) UV-visible absorption, fluorescence, and Raman spectroscopy, we have investigated molecular determinants of this peculiar property. The results demonstrate a pH-dependent "reverse chromophore protonation" triggered by the key residue Asp157 and which couples to cis/trans isomerization of the chromophore. These data provided guidelines to rationally design a useful Keima variant.
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Violot et al. (2009) studied this question.
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