A conserved interaction with the chromophore of fluorescent proteins

The chromophore of fluorescent proteins, including the green fluorescent protein (GFP), contains a highly conjugated imidazolidinone ring. In many fluorescent proteins, the carbonyl group of the imidazolidinone ring engages in a hydrogen bond with the side chain of an arginine residue. Prior studies have indicated that such an electrophilic carbonyl group in a protein often accepts electron density from a main‐chain oxygen. A survey of high‐resolution structures of fluorescent proteins indicates that electron lone pairs of a main‐chain oxygen—Thr62 in GFP—donate electron density into an antibonding orbital of the imidazolidinone carbonyl group. This n→π* electron delocalization prevents structural distortion during chromophore excitation that could otherwise lead to fluorescence quenching. In addition, this interaction is present in on‐pathway intermediates leading to the chromophore, and thus could direct its biogenesis. Accordingly, this n→π* interaction merits inclusion in computational and photophysical analyses of the chromophore, and in speculations about the molecular evolution of fluorescent proteins.