Hydrogen-bonded sheets

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

Fig. 3.—Parallel packing arrangement of the 2-fold helices of cellulose I (1). (a) Stereo view of two unit cells approximately normal to the ac-plane. The two comer chains (open bonds) in the back, separated by a, form a hydrogen-bonded sheet. The center chain is drawn in filled bonds. All hydrogen bonds are drawn in dashed lines in this and the remaining diagrams, (b) Projection of the unit cell along the c-axis, with a down and b across the page. No hydrogen bonds are present between the comer and center chains.

Fig. 18. The hydrogen-bonded sheet structure of l,l,5,5-tetrakis(hydroxydimethylsi-loxy)-3,3,7,7-tetraphenylcyclotetrasiloxane. Note the crown-shaped eight-membered rings of oxygen atoms at the junctions where four of the molecules meet. All hydrogen and carbon atoms have been omitted for clarity. Drawn using coordinates taken from the Cambridge Crystallographic Database.

Addition of a second proton donor, as in p-nitroaniline (PNA), serves to link acentric chains together (8-10). These chains form an acentric hydrogen-bonded array, as shown. PNA is a centric crystal because the hydrogen-bonded sheets pack with an inversion center between them. There are no inversion centers within the layer. 

These qualitative assays show that one-armed cationic guanidiniocarbonyl pyrrole receptors can indeed effectively bind tetrapeptides even in water. Molecular modeling studies suggest a complex structure as shown for one specific example, the receptor Val-Val-Val-CBS, in Figure 2.3.11. Receptor and substrate form a hydrogen bonded //-sheet which is further stabilized by additional hydrophobic interactions between the apolar groups in the side-chains. Recognition of the tetrapep-tide thus seems to be controlled by a fine balanced interplay between electrostatic and hydrophobic interactions. 

Figure 30 Hydrogen-bonded sheet in crystal structure of [Zn(SC(NH2 )NHiVH2 )2 (OH2)2][l, 4-02CC6H4C02] 2H20 [73a], Cationic metal complexes [Zn(SC(NH2)NHAH2)2 (OH2)2]2+ (open circles) serve as hydrogen bond donors to anionic terephthalate bridges (shaded). Layers are linked via N-H O and O-H O hydrogen bonds to solvent water molecules (not shown).

Figure 37 Part of hydrogen-bonded sheet formed by [Fe2(ti5-C5H4CH2CH20H)2 (CO)2(p-CO)2] [89] showing R ( 12 ) O-H O hydrogen-bonded rings. Adapted with permission from Figure 11c in D. Braga, F. Grepioni, P. Sabatino and G. R. Desiraju, Organo metallics, 13, 3532 43 (1994). Copyright 1994 American Chemical Society.

Figure 4.2. Crystal structure of less soluble (R)-l (R)-l-phenylethylamine. (a) Hydrogen-bond sheet viewed down the b axis. (b) Edge-on view of the hydrogen-bond sheets and their packing.

Figure 4.5. Schematic representations of the crystal structures of the less and more soluble salts of enantiopure 1 with 1-arylethylamines in success, (a) Less soluble salts, which are stable from the viewpoint of hydrogen-bonding and van der Waals interactions. (,b) More soluble (R)-l (S)-l-(m-methoxyphenyl)ethylamine, in which a stable hydrogen-bond sheet is formed while the close packing of the sheets is not achieved, (c) More soluble (R)-l (i )-l-phenylethylamine, in which a stable hydrogen-bond sheet is not formed while the close packing of the sheets is achieved.

Figure 4.10. Edge-on views of the hydrogen-bond sheets in (a) more soluble (R)-5 (5)-2 and (,b) more soluble (R)-5 (S )-l-(/ -chlorophenyl)ethylamine.

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