Structural rigidity

The axial (EA), torsional (GJ) and flexural El) rigidities for tubular structure of uniform composition are  

---

Epoxides are found in thousands of biological molecules and constitute vital functional entities. They can impart localized structural rigidity, confer cytotoxicity through their role as alkylating agents, or act as reactive intermediates in complex synthetic sequences. The widespread occurrence of epoxides is contrasted by only a handful of aziridines that are known to date. In this chapter we would like to introduce the different mechanisms by which enzymes produce epoxides. 

In order to make polymers behave as liquid crystals it is necessary to introduce some structural rigidity. A typical polymer which has the required rigidity is poly(phenylenetetraphthalamide) (10.7). This material belongs to a class of polymer known as the aramids. Other liquid crystalline polymers are the thermotropic polyesters derived from /7-hydroxybenzoic acid, p, p -biphenol and terephthalic acid (10.8). 

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

Scheme 19. Example of synthesis of a structurally rigid phosphine ligand...

In the early 1990s, Moore et al. reported the syntheses of a tremendous variety of meffl-connected PAMs [5 b, 36]. They recognized that the structural rigidity of systems like 8 could be useful in supramolecular chemistry. The convergent, stepwise approach of linear oligomeric phenylacetylene sequences developed in the Moore laboratory permitted absolute control over chain length, order of... 

As distinct from the gelling properties of certain natural gums, usually exhibited by structural rigidity, viscosity is a thickening of the aqueous portion of a system by means of particle water absorption, and swelling of the colloid. From a practical angle, it manifests itself by the phenomena of thickening and reduced flow characteristics. 

The multiple conformational restriction of dermorphin-related tet-rapeptide analogues that was performed represents a rational design of opioid peptidomimetics characterized by a high degree of structural rigid-ification. This is indicated by the fact that the p-selective agonist H-Hat-D-Orn-Aic-Glu-NH2 contains only two freely rotatable bonds, whereas there are 14 freely rotatable bonds in [Leu5]enkephalin. 

Even larger, conjugated systems were synthesized by employing both difunctional isocyanides and difunctional alkynes. In all the cases, the components have structural rigidity, which lends rigid-rod character to the complexes prepared from them. Many of the complexes are associated via intermolecular aurophilic contacts, which appear to have a distinct influence on the photophysical behavior, but are as yet not a necessary condition for luminescence (Scheme 51).196... 

New chiral oxazaborolidines that have been prepared from both enantiomers of optically active inexpensive a-pinene have also given quite good results in the asymmetric borane reduction of prochiral ketones.92 Borane and aromatic ketone coordinate to this structurally rigid oxazaborolidine (+)- or (—)-94, forming a six-membered cyclic chair-like transition state (Scheme 6-41). Following the mechanism shown in Scheme 6-37, intramolecular hydride transfer occurs to yield the product with high enantioselectivity. With aliphatic ketones, poor ee is normally obtained (see Table 6-9). 

It was found that Cope rearrangement of the structurally rigid tetracyclic molecule 506 is remarkably accelerated by creating a remote (i.e. non-conjugated) carbenium ion center by an ionization of a ketal group (equation 191)249. The possibility of both classical and non-classical ion participation in this Cope rearrangement was revealed by using MNDO calculations. 

---