Crystal structure of a complex between lumiflavin and 2,6-diamino-9-ethylpurine: a flavin adenine dinucleotide model exhibiting charge-transfer interactions.

Abstract

The x-ray structure of the deep red crystalline complex lumiflavin*2,6-diamino-9-ethylpurine has been determined. The flavin and adenine derivatives form hydrogenbonded base pairs of the Watson-Crick type. The molecules in the crystal also associate via extensively overlapped flavin/ adenine and flavin/flavin stacking interactions in which there are several contacts that are closer than van der Waals distances. This, together with the red color of the crystals, is indicative of the formation of a charge-transfer complex.An intriguing problem of flavin biochemistry is the function of the adenine moiety of FAD. A study of the activity of the flavoenzyme D-amino acid oxidase upon binding adenosylmodified FAD analogues has shown that this enzyme exhibits considerable specificity with regard to the adenine portion of FAD (1). Yet it is clear that the adenine residue in FAD does not directly affect the oxidation potential of the flavin moiety. It has therefore been suggested that the role of the adenine group in FAD may involve its participation in an intramolecular complex with the flavin moiety (2). This report is concerned with the elucidation of a structural model for such a complex.The component molecules of Watson-Crick base pairs exhibit a selective affinity in hydrogen bonding that has been observed using infrared studies (3). Thus, uracil derivatives have a higher association constant for binding adenine derivatives than for derivatives of guanine or cytosine (4). The close resemblance of the pyrimidinoid ring of isoalloxazine to uracil suggests that FAD may form a strongly hydrogen-bonded intramolecular complex. This has been confirmed in nonaqueous solutions by infrared studies (5) and in aqueous solutions by nuclear magnetic resonance studies (6). In the solid state, flavin-adenine base pairing of the Hoogsteen variety has been directly observed by x-ray studies of the crystalline complexes of riboflavin with 5'-bromo-5'-deoxyadenosine (2, 7) and with adenine (8).In aqueous solutions the majority of FAD molecules assume a folded, internally stacked conformation in much the same manner as do oligonucleotides (9). This has been shown by a variety of techniques including fluorescence measurements, both at normal (10) and elevated pressures (11), and by nuclear magnetic resonance measurements (12). The adenine and isoalloxazine derivatives in the above-mentioned structural studies associate by normal stacking interactions (2, 7, 8). There have been occasional reports of charge-transfer bands in the spectra of flavin-purine complexes in solution but it appears that donor-acceptor interactions are unimportant in the stabilization of such complexes (13). Slifkin (14) has reported that evaporating aqueous solutions of purines and riboflavin to dryness yields strongly colored residues that are apparently composed of charge-transfer complexes.The complexing properties of reduced flavins are less familiar than those of oxidized flavins. Hence in an effort to elucidate the manner in which reduced flavins assoc...

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

“…A similar angle has been found in lumiflavinium bromide-sesqui(naphthalene-2,7-diol) monohydrate (Langhoff & Fritchie, 1970), lumiflavinium chloride-hydroquinone (Karlsson, 1972), and lumiflavinium bromide-hydroquinone . On the other hand, isoalloxazine and ligand nets are almost exactly parallel in lumiflavin-diaminopurine (Scarbrough et al, 1976) and 10-propylisoalloxazine-bis(naphthalene-2,3-diol) (Kuo et al, 1974). These variable geometries are consistent with the usual assumption that charge-transfer forces are weak and do not contribute to a major extent to the ground-state interactions (Mayoh & Pront, 1972).…”

“…The dihedral angle between the pyrimidinoid and benzenoid rings of the lumiflavin molecule in the present complex is only 0.7O. The mean planes of the stacked 2-aminobenzoic acid and lumiflavin rings in the complex structure are inclined at an angle of only 0.7' whereas the angles between donor and acceptor rings in other flavin complexes are 2.3' (Kuo et al, 1974), 3.6' (Tillberg & Norrestam, 1972, and 4.1-7.7' (Scarbrough et al, 1976).…”

The X-ray crystal structure of the molecular complex bis(lumiflavin–2,6-diamino-9-ethylpurine)–ethanol–water

“…The geometry of the foregoing stacks has been discussed in detail elsewhere (Scarbrough, Shieh & Voet, 1976). The extent and the regular geometry of the stacks together with their numerous close interatomic contacts as well as the red coloration of the crystals lead to the conclusion that the crystalline complex exhibits flavin/flavin as well as flavin/adenine chargetransfer interactions.…”

“…The extent and the regular geometry of the stacks together with their numerous close interatomic contacts as well as the red coloration of the crystals lead to the conclusion that the crystalline complex exhibits flavin/flavin as well as flavin/adenine chargetransfer interactions. The possibility that the red coloration of the crystals is due to the presence of diradicals, a hypothesis that is suggested by the failure of the red crystalline complex to form in solutions containing lumiflavin, must be rejected on the grounds that EPR measurements, both at liquid-helium and at room temperatures, proved the crystals to be diamagnetic (Scarbrough, Shieh & Voet, 1976). Fig.…”

“…A discussion of the biological implications of the structure has appeared elsewhere (Scarbrough, Shieh & Voet, 1976). The present report is concerned with a discussion of the procedures used to solve the structure and a presentation of its molecular parameters.…”