Figure 1.

The structures of the 2-(2′-hydroxyphenyl)-4-methyloxazole ligand in the planar (Upper) and twisted (Lower) geometries. The red atoms are oxygens, the black carbon, the blue nitrogen. A photoexcited proton transfer occurs from the OH to the heterocyclic N. The twisting degree of freedom is strongly impeded by the protein environment, whereas the stretching that facilitates proton transfer is not.