Unusual emitting states of the kindling fluorescent protein: Appearance of the cationic chromophore in the GFP family

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.