Role of tryptophanyl and tyrosyl residues of flavoproteins in binding with flavin coenzymes. X-ray structural studies using model complexes.

The crystal structures of 7,8-dimethylisoalloxazine-10-acetic acid-tryptamine (1:1) tetrahydrate and 7,8-dimethylisoalloxazine-10-acetic acid-tyramine (1:1) tetrahydrate complexes were determined by the X-ray diffraction method, as models for flavin-tryptophan and flavin-tyrosine interactions in flavoproteins. The observed parallel stackings and the intermolecular spacing distances, which were less than the normal van der Waals separation between the isoalloxazine and indole rings and between the isoalloxazine and phenol rings, suggest the existence of charge-transfer interactions in their ground states. The indole and phenol rings interact with the pyrimindinoid and pyrazinoid portions of the isoalloxazine ring and have short contacts, less than 3.4 A, with the reduction site (N1 and N5 atoms) of this ring. This suggests that the reduction of oxidized flavin to the semiquinone state may be facilitated by charge transfer from the former rings to the N1 and N5 atoms. Absorption difference spectra showed that both complexes associate with equimolar ratios in solution as well as in the crystalline state and that they have the same charge-transfer bands and association constants as flavin mononucleotide (FMN)-Trp and FMN-Tyr complexes, respectively. On the other hand, proton magnetic resonance spectra suggested that in solution, the stacking modes of the indole and phenol rings to the isoalloxazine ring are different from those observed in the crystal structures and both aromatic rings are stacked over the whole of the isoalloxazine ring.