Photophysics of the blue fluorescent protein

The blue fluorescent protein (BFP) is a mutant of the green fluorescent protein, where the phenolic ring of the chromophore has been replaced by imidazole cycle of histidine residue. The usability of BFP as a fluorescent marker is hampered by its low fluorescence quantum yield at room temperature. The intensity of fluorescence increases by a factor of 4.5 when the temperature is decreased from 320 K down to 225 K. The fluorescence is also enhanced by hydrostatic pressure. Both effects have been explained by shift of the equilibrium between hydrogen nonbonded and hydrogen-bonded chromophores. Our semi-empirical quantum chemical calculations show that the fluorescence quantum yield of the BFP chromophore is low due to isomerization in the electronically excited state -twisting of the bridging bond by 90°. At this twisted geometry the potential energy surfaces of ground and excited states are situated close to each other facilitating efficient nonradiative decay.