Structure ribbon-like

Nylon crystallites consist of sheets of chains that are hydrogen-bonded to their neighbors. On a supermolecular scale, crystallites have a lamellar structure, that is they are many times longer and broader than they are thick. When nylon crystallizes from an isotropic molten state, it generally forms spherulites, which consist of ribbon-like lamellae radiating in all directions... 

X-ray structural analysis of the charge-transfer complex between 2,4,7-trinitrofluore-none (80) and 2,6-dimethylnaphthalene shows the presence of C—H- O hydrogen bonding196 involving both nitro and carbonyl groups of 80. These hydrogen bonds importantly influence the molecular arrangement in a nearly coplanar ribbon-like structure also for other aromatic and aliphatic nitro derivatives. 

Although the existence of the 30nm fiber is widely accepted, several models have been proposed for its structure (Felsenfeld and McGhee, 1986). Thoma and colleagues proposed a solenoid model, in which the nucleosomes are ordered in a spiral manner (Thoma et al, 1979). Woodcock and colleagues postulated a helical ribbon model, in which the nucleosomes are arranged in a zig-zag manner and the sheet of the zig-zag nucleosomes winds up helically to form a ribbon-like stmcture (Woodcock ef a/., 1984). 

These results clearly show that our solvent displacement process leads to the formation of AmB-lipid structures that are different are from the ribbon-like ones described by Janoff et al. (11,21) for the same composition. It was therefore interesting to investigate the toxicity and efficacy of this formulation. 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

Figure 7 The ribbon-like structure of SnFCl (reproduced by permission from Acta CrystaUogr., Sect. B, 1976, 32,...

An alternative proposal might be that bile salts elongate in ribbonlike structures with increased counterion concentration. These micelles would be very asymmetric. However, in all previous studies where the shape of the bile salt micelles has been studied (11) it appeared to be almost spherical. This tends to rule out an elongated ribbon-like structure. 

A—Pyrolytic carbon showing ribbon-like structure in vitrinoid bands. B—Faint gray lines define compression cracks in a bright micrinoid particle. C—Pyrrhotite (white) formed by the thermal decomposition of pyrite impregnating semifusinoids (gray). D—Bright coke particles in a baked-bone coal layer... 

Fig. 47. X-Ray structure of the hydrogen-bonded, ribbon-like species formed by self-assembly of the self-complementary component 168. In the schematic representation (right) the two DMF molecules contained in each cavity between the octyl chains are omitted [9.122].

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