Carbon‐13 NMR spectroscopy of the biological pigments luteoskyrin and rugulosin and some polyhydroxyanthraquinone analogues

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Abstract
The bianthraquinonic biological pigments luteoskyrin and rugulosin and five polyhydroxyanthraquinone derivatives are studied by carbon‐13 NMR in DMSO solution. Peak assignment for the fourteen carbon atoms of these compounds is achieved by proton spin decoupling and by investigating the effect of ionisation of the hydroxyl groups upon the carbon chemical shifts. Carbon chemical shifts in the planar hydroxyanthraquinones can be rationalised in terms of conjugation and intramolecular hydrogen bonding. The latter is responsible for the relative acidity of the hydroxyl groups in the analogues, and for the different conformations proposed for luteoskyrin and rugulosin. Tautomeric equilibria occur in DMSO and water–DMSO solutions for the anionic species [LS]2− and [RG]2−. This can account for the binding of luteoskyrin and rugulosin to nucleic acids.