Unusual Emitting States of the Kindling Fluorescent
Protein: Appearance of the Cationic Chromophore in the GFP Family
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Abstract
The
kindling fluorescent protein (KFP), the Ala143Gly variant of
the natural chromoprotein asFP595, is a prospective biomarker in live
cells. Following the results of QM/MM calculations, we predict that
excitation of the protein under certain conditions, favoring formation
of KFP fractions with the neutral chromophore, should result in fluorescence
from the cationic form of the chromophore which is unusual for the
members of the green fluorescent protein family. Occurrence of the
neutral form is due to a water wire connecting the chromophore with
the exterior of the protein. Occurrence of the cationic form is due
to the excited-state proton transfer from the conserved Glu215 to
the imidazolinone ring nitrogen of the chromophore. The emission band
from conformations with the trans cationic chromophore should be noticeably
shifted to the blue side around 520 nm compared to the well-known
red fluorescence around 600 nm arising from the cis anionic species
