Molecular Complex of Lumiflavin and 2-Aminobenzoic Acid : Crystal Structure, Crystal Spectra, and Solution Properties

Abstract

The molecular complex lumiflavin-2-aminobenzoic acid monohydrate (C13H12N402●C7H7N02●H2O)crystallizes from aqueous solution as red triclinic prisms. The space group is P1 with cell dimensions a = 9.660 A, b = 14.866 Å, c = 7.045 Å, α = 95.44°, β = 95.86°, and γ = 105.66°. The crystal structure was solved by direct methods and refined by block-diagonal least-squares procedures to an R value of 0.050 on the basis of 1338 observed reflections. The structure is composed of stacks of alternating lumiflavin and un-ionized (neutral) 2-aminobenzoic acid molecules. Two different modes of stacking interaction are observed. In one, 2-aminobenzoic acid overlaps all three of the isoalloxazine rings, at a mean distance of 3.36 Å; in the other, 2-aminobenzoic acid interacts with the pyrazine and dimethylbenzene moieties, at a distance of 3.42 Å. Perpendicular to the stacking direction, the molecules form a continuous sheet. Each flavin is hydrogen bonded via O(2) and NH(3) to two symmetrically related aminobenzoates; the water of crystallization forms three hydrogen bonds, bridging two flavins, via O(4) and N(5), and one aminobenzoic acid. The red color of the crystals results from a charge-transfer transition involving stacked flavin and 2-aminobenzoic acid molecules. Measurements of the polarized optical absorption spectra of crystals show that the transition moment direction for the long wavelength absorbance (beyond 530 nm) contains an out-of-plane component which can only arise from a charge-transfer interaction. Since the amino N does not make exceptionally close interactions with isoalloxazine atoms in either stacking mode (minimum interatomic distance 3.52 Å), the charge transfer is presumed to involve Pi orbitals of the 2-aminobenzoic acid donor.