Hydrogen bonding: Attractive arrays.

hydrogen bonding is considered by many to be the ‘master key’ of molecular recognition — owing to its relatively predictable strength and directionality — and plays a major role in the assembly of diverse non-covalent assemblies1. Consequently, the study and exploitation of hydrogen bonds has been central to the development of modern supramolecular chemistry. In particular, numerous applied settings — such as catalysis and materials assembly — have benefitted from a greater understanding of the properties and behaviour of hydrogen bonds. The early focus on DNA bases has generated significant interest in the study of synthetic linear arrays2 as model systems for understanding hydrogenbonding interactions. In more recent years, linear arrays have been used to assemble supramolecular polymers3 — stimuliresponsive materials constructed from small and easy-to-synthesize molecules. Now, writing in Nature Chemistry, David Leigh and co-workers report on the design and synthesis of an exceptionally stable quadruply hydrogen-bonded heterodimer in which all of the donor (D) atoms are in one monomer and all of the acceptor (A) atoms are in its complementary partner, namely an AAAA–DDDD array4. The affinity with which linear arrays of contiguous hydrogen-bonds interact with complementary partners is determined by a number of factors. These include the hydrogen-bonding propensity of individual donor and acceptor atoms, pre-organization, conformation and tautomerization (all of which can be influenced by intramolecular hydrogen-bonding interactions) and the arrangement of individual donor and acceptor atoms within each component2. Where supramolecular polymers are concerned, the affinity with which linear arrays interact is a critical factor in obtaining non-covalent assemblies with a sufficiently high degree of polymerization to exhibit materials’ properties — arrays comprising four or more hydrogen-bonds have in general been identified as ideally suited to such applications. Of the six possible quadruply hydrogen-bonded motifs (ADAD–DADA, AADD–AADD, ADDA–DAAD, AAAD–DDDA, ADAA–DADD, AAAA–DDDD), however, only three have previously been identified as motifs that can be readily synthesized and exhibit sufficiently high affinity to have practical applications in supramolecular polymerizations (Fig. 1a). Of particular significance is the limited availability of heterodimers such as ureidopyrimidinone (UPy)/diamidonaphthyridine (DAN) and ureidoguanosine (UG)/DAN, which are desirable for the assembly of more complicated supramolecular architectures comprising different building blocks hYDROGEN bONDING