Carboxylic acid dimers chain motifs

One of the most well-known hydrogen-bond motifs is the carboxylic acid dimer. This particular intermolecular interaction is not only present among simple organic acids (there are few examples of chain-like, catemeric interactions between carboxylic acids), but self-coraple-inentary head-to-head carboxylic acid dimers are also often observed in coordination compounds as well as in organometallic species.The availability of these robust and transferable intermolecular interactions (Scheme 2) allows for the assembly of discrete building blocks into extended networks of well-defined and predictable connectivities. 

Figure 4.18 Some common supramolecular synthons based on hydrogen bonds. Carboxylic acids can form (a) discrete dimers and (b) infinite chains, (c) Amides can form dimers in a similar manner to carboxylic acids, while (d) urea may form a tape motif. Alcohols may also form both (e) infinite and (f) discrete structures.

Figure 39-15. The leucine zipper motif. A shows a helical wheel analysis of a carboxyl terminal portion of the DNA binding protein C/EBP. The amino acid sequence is displayed end-to-end down the axis of a schematic a-helix. The helical wheel consists of seven spokes that correspond to the seven amino acids that comprise every two turns of the a-helix. Note that leucine residues (L) occur at every seventh position. Other proteins with "leucine zippers" have a similar helical wheel pattern. B is a schematic model of the DNA binding domain of C/EBP. Two identical C/EBP polypeptide chains are held in dimer formation by the leucine zipper domain of each polypeptide (denoted by the rectangles and attached ovals). This association is apparently required to hold the DNA binding domains of each polypeptide (the shaded rectangles) in the proper conformation for DNA binding. (Courtesy ofS McKnight)...

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