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The self-assembly of two bis-2-ureido-41H-pyrimidinones by eight intermolecular hydrogen bonds results in extremely stable dimers. These dimers exist both in solution and in the solid state as two different isomers. By single-crystal X-ray analyses the structures of both the syn-isomer and the anti-isomer have been determined. In addition, with ROESY experiments the structure of a third isomer is revealed. By a combination of NMR techniques the mutual rate of exchange has been determined. Interestingly, these values are significantly lower than the comparable processes of a single 2-ureido-41H-pyrimidinone. These experiments allowed us to determine the cooperativity in conformational behavior and tautomerization of stable dimers.
Folmer et al. (1999) studied this question.