Photophysics and optical switching in green fluorescent protein mutants

We demonstrate by using low-temperature high-resolution spectroscopy that red-shifted mutants of green fluorescent protein are photo- interconverted among three conformations and are, therefore, not photostable 'one-color' systems as previously believed. From our experiments we have further derived the energy-level schemes governing the interconversion among the three forms. These results have significant implications for the molecular and cell biological applications of the green fluorescent protein family; for example, in fluorescence resonant energy transfer experiments, a change in 'color' on irradiation may not necessarily be due to energy transfer but can also arise from a photo-induced conversion between conformers of the excited species.