Crystal and molecular structure of a donor–acceptor complex involving protonated riboflavin, quinol, and bromide ions

Black crystals of stoicheiometry riboflavin:quinol:hydrogen bromide:water 1:1:2:2 have been grown. The crystals are monoclinic, a= 20·55(5), b= 13·69(4), c= 10·18(4)A, β= 91·2(1)°, with four ‘molecules’ of this composition in a cell of space group P21. The structure was solved from photographic data by Patterson and Fourier methods, and refined by least squares to R 0·145 for 2358 independent reflections. There are charge-transfer interactions between riboflavin molecules (as acceptors) and quinol molecules and bromide ions (as donors). The riboflavin molecules are probably in the semiquinone state and their interaction with the donors is not confined to a specific region of the isoalloxazine nuclei.