Photoinduced formation of flavin radicals in BLUF domains lacking the central glutamine

Blue light receptors using FAD (BLUFs) facilitate blue light‐induced signal transduction via light‐induced rearrangement of hydrogen bonds between the flavin chromophore and a conserved glutamine side chain. Here, we investigated the photochemistry of the BLUF domain Slr1694 from Synechocystis sp. in which the glutamine side chain was removed. Without the glutamine, no red‐shifted signaling state is formed, but light‐induced proton‐coupled electron transfer between protein and flavin takes place similarly as for the wild‐type protein. However, the lifetime of the neutral flavin semiquinone–tyrosyl radical pair is greatly prolonged from