Fast and Efficient Red-Absorbing Photoswitching Proteins Based on Flavin–Ligand Charge Transfer Complexes

Recently a novel class of reversible protein photoswitches has been discovered that is based on a charge transfer (CT) complex composed of the flavin cofactor and a substrate-analogue inhibitor molecule in the family of sarcosine oxidase flavoproteins. Here, excitation of the CT band results in barrierless dissociation of the CT complex on the femtosecond timescale followed by its thermally activated reformation, on the timescale of a few nanoseconds at ambient temperature. The photoreaction is thought to involve a well-defined isomerization of the inhibitor without its dissociation from the protein. This reaction occurs with an unusually high quantum yield (~80 %), is initiated by absorption in the red part of the visible absorption spectrum, and leads to a photoproduct absorbing in the blue spectral region (negative photochromism). Therefore, this class of photoswitches can be considered a promising template for developing a new class of fast negative photochromic compounds for Life Science applications provided the lifetime of the photoproducts (‘light state’) can be prolonged. Potential future developments will be discussed.