Hydrogen-bonded chain motifs

While the polymorphic forms of carbamazepine all exhibit an anticarboxamide hydrogen-bond dimer motif, the related compounds oxcarbazepine (10-oxo-10,l l-dihydro-5H-dibenzo[fr,/]azepine-5-carboxamide) and dihy-drocarbamazepine (10,ll-dihydro-5H-dibenzo[fc/]azepine-5-carboxamide) adopt hydrogen-bond chain motifs in their crystal structures [47]. The structures of several cocrystals of the structurally related compound cyten amide (5H-dibenzo[fl,d][7]annulene-5-carboxamide) have been reported, with the details of the hydrogen-bonding patterns being discussed [48-51]. 

A ferris wheel assembly involving a 1 1 complex of 19 and metallated [18]crown-6 is found in the cationic supermolecule [La(H20)3([ 18]crown-6)] (19+2H) + [48]. The lanthanum ion is coordinated by one calixarene sulfonate group, the [18] crown-6 and three aquo ligands, and the metallated crown sits inside the calixarene cavity. A helical hydrogen bonded chain structure is formed between the cationic assembly, water and chloride ions. The ferris wheel structural motif is also found in Ce3+ complex which simultaneously contains a Russian Doll assembly [44]. 

Benzimidazole (BIm) crystallizes in hydrogen-bonded chains with nearly linear N-H---N donor-acceptor contacts.185 Viewed down the chain axis, the benzene rings form a sawtooth alternation. This motif can accommodate substitution in the 2-position and on the benzene ring, while still retaining the basic chain structure, as seen in the large majority of structures from the CSD. Lahti recently summarized186 magnetostructural studies carried out on radicals linked to neutral aza-arenes. This chapter will focus only on BIm-functionalized radicals. 

Fig. 29 Hydrogen-bonded chains that organize radicals into FM exchange-coupled piled stack motif among nitronylnitroxide radicals in BlmNN. Inset (right) shows the piled stack spin orbit N-O/C contact that gives rise to FM exchange.

Fig. 30 Comparison of hydrogen-bonded chain formation in F4BIm (flat ribbons) and F4BImNN (piled stack motif essentially the same as that in Fig. 29).

An example of side-chain SLCPs whose mesogen consists of part of the main-chain as well as the side chain (Fig. 3h) has been reported by Kato et al. [107]. In this case the hydrogen bond mesogenic motif utilized was a benzoic acid (as part of the small molecule side chain 42) with a 2,6-diamino-... 

A) The schematic representation of p-substituted diarylthioureas. (B) PXRD analysis of a series of aromatic thioureas. (C) A characteristic self-assembly motif of diarylthioureas in the soiid state by the bifurcated N—H- -S hydrogen bonds (head-to-head or parallel molecular dipoles arrangement is shown). (D) Arrangement of molecules in structural families I and II viewed along the N—H - -S hydrogen-bonded chains. 

Fig. 5. Protein folding. The unfolded polypeptide chain coUapses and assembles to form simple stmctural motifs such as -sheets and a-hehces by nucleation-condensation mechanisms involving the formation of hydrogen bonds and van der Waal s interactions. Small proteins (eg, chymotrypsin inhibitor 2) attain their final (tertiary) stmcture in this way. Larger proteins and multiple protein assembhes aggregate by recognition and docking of multiple domains (eg, -barrels, a-helix bundles), often displaying positive cooperativity. Many noncovalent interactions, including hydrogen bonding, van der Waal s and electrostatic interactions, and the hydrophobic effect are exploited to create the final, compact protein assembly. Further stmctural...

---