Cooperative Dynamics in Duplexes of Stacked Hydrogen-Bonded Moieties

The self-assembly of two bis-2-ureido-4[1H]-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-4[1H]-pyrimidinone. These experiments allowed us
to determine the cooperativity in conformational behavior and tautomerization of stable dimers.