Molecular complex of lumiflavin and 2-aminobenzoic acid: crystal structure, crystal spectra, and solution properties

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Abstract
The molecular complex lumiflavine 2-aminobenzoic acid monohydrate (C13H12N4O2 .cntdot. C7H7NO2 .cntdot. H2O) [a model for the charge-transfer complexes of flavoproteins] crystallizes from aqueous solution as red triclinic prisms. The space group is P.hivin.1 with cell dimensions a = 9.660 .ANG., b = 14.866 .ANG., c = 7.045 .ANG., .alpha. = 95.44.degree., .beta. = 95.86.degree. and .gamma. = 105.66.degree.. 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 lumiflavine and un-ionized (neutral) 2-aminobenzoic acid molecules. Two different modes of stacking interaction are observed. In one, 2-aminobenzoic acid overlaps all 3 of the isoalloxazine rings, at a mean distance of 3.36 .ANG.; in the other, 2-aminobenzoic acid interacts with the pyrazine and dimethylbenzene moieties, at a distance of 3.42 .ANG.. Perpendicular to the stacking direction, the molecules form a continuous sheet. Each flavine is H-bonded via O(2) and NH(3) to 2 symmetrically related aminobenzoates; the water of crystallization forms 3 H-bonds, bridging 2 flavine, via O(4) and N(5), and 1 aminobenzoic acid. The red color of the crystals results from a charge-transfer transition involving stacked flavine 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 .ANG.), the charge transfer is presumed to involve .pi. orbitals of the 2-aminobenzoic acid donor.