A photochromic and thermochromic fluorescent protein

Photochromism is a change of colour induced by irradiation with light, while thermochromism is a change of colour due to a change of temperature. Both photochromism and thermochromism have been observed in inorganic and organic compounds, however thermochromism is much less common in biological systems. One mechanism by which molecular photochromism can occur is a photo-induced isomerization to a second stable state with a different absorption wavelength. For such molecules, the rate of conversion is accelerated with increased temperature, and therefore the photochromic and thermochromic properties are intimately coupled. Only a handful of classes of naturally occurring photochromic proteins have been identied. Representative classes of photochromic proteins include bacteriophytochromes, rhodopsins, and uorescent proteins (FPs). The rst photochromic FP to be described was the reversible ‘kindling’ protein, asFP595, from the sea anemone Anemonia sulcata. Reports of a number of other reversibly photochromic FPs followed, including many with faster switching properties or red-shied emission colour, including: Dronpa, rsFastlime, mTFP0.7, rsCherry, and rsTagRFP. Mechanistic investigations of photochromic proteins have revealed that the photoswitching mechanism is typically based on coupled protonation changes and E–Z isomerizations of the protein chromophores.