On the mechanism of photoinduced dimer dissociation in the plant UVR8 photoreceptor

Significance
In contrast to other chromophore/protein systems in photobiology, the UV-B photoreceptor UV resistance locus 8 (UVR8) utilizes tryptophan residues as light absorbers. The initial step of a signaling pathway that ultimately leads to UV-B protection and damage repair in plants is the breaking of salt bridges that stabilize the UVR8 protein consisting of two identical subunits. On the basis of the known X-ray structure and high-level quantum-chemical computations, we propose a mechanism that implies excitation energy transfer to the “special tryptophan” W233 near the monomer interface, followed by the formation of a charge-transfer state involving W233 and W285. In the calculations, the large dipole moment of W233(−)-W285(+) facilitates the breaking of arginine–aspartate salt bridges between the two subunits.