Photophysics and optical switching in green fluorescent protein mutants

We demonstrate by using low-temperature high-resolution spectroscopy that r ed-shifted mutants of green fluorescent protein are photo-interconverted am ong three conformations and are, therefore, not photostable "one-color" sys tems as previously believed. From our experiments we have further derived t he energy-level schemes governing the interconversion among the three forms . These results have significant implications for the molecular and cell bi ological applications of the green fluorescent protein family; for example, in fluorescence resonant energy transfer experiments, a change in "color" on irradiation may not necessarily he due to energy transfer but can also a rise from a photo-induced conversion between conformers of the excited spec ies.