Amide Hydrogen Bond Chains

The liquid structure of the fibres can be solidified by linear hydrogen bond chains which may run parallel or perpendicular to the fibre axis. The strongest and most prominent type of chain-like bonding interaction is provided by the amide group, which has been extensively studied in helical peptides and proteins. Here, a few characteristics of interest are summarized before turning to solid fibres. The assumption is that other hydrogen bond chains between COOH OOC and OPOsH - O3PO—, for example, are of a similar character. 

In protein crystal structures, ordered water molecules were frequently observed at instances where a-helices bend or fold. Molecular dynamic simu- 

Pimentel, A.L. McClellan, The Hydrogen Bond, Freeman, London, 1960 

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Figure 2.6.7 Model of a flat self-assembled monolayer with three amide hydrogen-bond chains.

Crystalline SAMs with three amide hydrogen bond chains have been produced on gold (Fig. 2.6.7). Infrared spectra show a narrow amide II band at 1563 cm supporting the assumption of uniform hydrogen bonding (Clegg et al., 1998). 

Fig. 10 A possible molecular packing and hydrogen bond scheme for (a) the heteroassembly formed from an equimolar mixture of 14a and 15a and (b) the homoassembly from 16a. (a, b) Top view of a layered structure composed of linear polymolecular arrays ( reversed Hoogsteen base pair configuration is employed here for the thymine-adenine heteroassociation), (c) Front view showing 2-D complementary and 1-D amide hydrogen bond network, (d) Side view of the polymolecular arrays. In (d), the one-dimensional amide hydrogen bond chain contributes to the stabilization of the base stacking and the formation of complementary hydrogen bonds. Reprinted with permission from J Am Chem Soc 2001, 123, 5947...

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]. 

The presence of. vyn-postioned hydroxyl groups on the C3 and C5 of the N-octyl-D-gulon-(35a), altron-(37a), allon-(36a), and idon-(38a) amides, makes these compounds water-soluble and therefore does not allow the formation of aggregates. And induced a bent. This bent does not allow the formation of any regular chain amide hydrogen bonds due to the excessive hydration. The crystal structures of D-Gul-8 35a164 and D-Tal-8 34a165 have been reported and shown to contain tail-to-tail bilayers. 

Hypothetical model of an internally hydrogen-bonded chain, with the simplification that adjacent amide groups are connected by hydrogen bonds. Note the distinction between the amino acid residue and the amide group the latter is the more convenient unit of structure for the present discussion. 

Spherical micelles and vesicles are formed from amphiphiles and bolaamphi-philes by the solvophobic effect and are protected against crystallization by head group repulsion. What happens if the head groups carry secondary amide groups which have an inborn and irresistible drive to form linear hydrogen bond chains in polar and apolar environments (see section 5.4) Chains will be formed, of course. The usual result will be vesicular tubules, as in the case of amphiphiles with a low cmc (typically < 10 M) and thinner micellar rods in the case of amphiphiles with a relatively high cmc (typically 10 -10" M). 

Figure 7.2 Palmitoyl-R-lysine monolayers on water are not only stabilized by binding interactions between the hydrocarbon chains and amino acid head groups, but linear amide hydrogen bonds between the e-amide groups are also formed. This requires a 30° tilt.

Poly(p-phenylene terephthalamide) 345 In-chain stiffening from p-phenylene groups together with amide hydrogen bonding... 

Through systematic modification of the polymer backbone, the effects of chemical structure upon the oxygen permeation properties of aliphatic-aromatic amorphous polyamides were determined. In this class of polymers, the greatest effects were obtained by alteration of the chain length and disruption of the amide hydrogen bonding by N-alkylation. It is remarkable that reversal of the amide linkage has no effect whatsoever on the permeation properties of the examples studied. 

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