Compression Conformational transition

This way it has been possible to observe the anticipated conformational transition by experiment. Cylindrical brushes of polybutylacrylate-graft polymethacrylate were adsorbed on the surface of water from a CHCI3 solution on a Langmuir trough and studied by recording the tt-A (surface pressure vs molecular area) isotherm. At different stages of compression, the monolayer was... 

Conformational transitions and electron redistribntion on pressure results in changes of the ion-radical properties and this is essential for optoelectronic devices. Pressure is the thrust distributed over a surface. The most important results of compression are reduction of molecular volumes and conformational changes of organic componnds. Spectral changes of solids are also termed piezo-chromism. Let ns consider several relevant examples of piezochromism. 

An elongational or extensional viscosity (%) develops as a result of a conformational transition when disperse systems are forced through constrictions, or compressed or stretched (Kulicke and Haas, 1984 Rinaudo, 1988 Barnes et al., 1989 Odell et al., 1989 Clark, 1992). The intuitive logic is that the random coils resist the initial distortion. % is believed to elicit the human sensation of stringiness (Clark, 1995). If shear viscosity is denoted iq, rheologists define a Trouton ratio as %/ti, wherein % > T) by a factor approximating 3 for uniaxial extension and 6 for biaxial extension. Alternatively stated, the Newtonian ly calculates to one-third to one-sixth % (Steffe, 1992). 

The exact mechanism of the action by chorismate mutase is stUl not clear, in spite of extensive experimental and theoretical investigations. Several suggestions have been proposed concerning the origin of the catalysis. They include (a) the stabilization of transition state by the enzyme, presumably through electrostatic interactions from the active site residues (b) the promotion of substrate conformational transition to generate the reactive CHAIR conformer at the active site (see Scheme 1.2) (c) the increase of populations of near attack conformers (NACs) and (d) strain effects and conformational compression. These proposals will be discussed below. 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

The thermodynamic analysis of conformational and structural transformations in the melt at high pressures34 showed that the free volume and free energy minimum required for hydrostatic compression is attained as a result of the transition of the molecules in the melt into a more extended conformation (gauche —> trans transitions) since the extended molecules ensure a more compact packing of the chains at compression. Chain uncoiling leads to a decrease in their flexibility parameter f with increasing pressure p ... 

The transition between crystalline and amorphous polymers is characterized by the so-called glass transition temperature, Tg. This important quantity is defined as the temperature above which the polymer chains have acquired sufficient thermal energy for rotational or torsional oscillations to occur about the majority of bonds in the chain. Below 7"g, the polymer chain has a more or less fixed conformation. On heating through the temperature Tg, there is an abrupt change of the coefficient of thermal expansion (or), compressibility, specific heat, diffusion coefficient, solubility of gases, refractive index, and many other properties including the chemical reactivity. 

One of them assumes the possibility of a compression of polymer coils at average concentrations down to the dimensions less than in the -solvent. The alternative is based on the existence of the wide distribution of macromolecule dimensions in any time.It is rather natural to assume an increase of the probability of intramolecular reaction with an increase of the dimensions of the macromolecule.. e. more extended conformations go to the gel-fraction and more coiled remain in the sol. With the increase of solution concentration the distances between coils are diminished and the critical dimensions, necessary for a transition into the gel are decreased too. This process will be accompanied by a decrease of the average dimensions of molecules in sol. 

Complementary information about the interfacial conformation of the carbosilane dendrimers was obtained from u-A isotherms measured during compression of molecular films at the air/water interface [74]. The full reversibility of the isotherm in Fig. 13 a indicated that the experiment was done under equilibrium conditions, and that the data give direct evidence on the phase behavior. Two transitions, marked by I and II in Fig. 13 a, were observed for the OH terminated carbosilane dendrimer (dendrimer 2 in Fig. 8). The first transition (I) was... 

A hypothetical conformational state defined by a geometrically and electronically strained site within an enzyme thought to facilitate the conversion of an enzyme-substrate complex to the transition state. Vallee and Williams defined the entatic state as an abnormal condition of localized strain transmitted by relief of compression or other steric clashes elsewhere in the enzyme. They suggested that catalytic rate enhancements arise from the heightened reactivity of catalytic group(s) that have experienced unimolecular activation. Williams ... 

Both kinetic and thermodynamic approaches have been used to measure and explain the abrupt change in properties as a polymer changes from a glassy to a leathery state. These involve the coefficient of expansion, the compressibility, the index of refraction, and the specific heat values. In the thermodynamic approach used by Gibbs and DiMarzio, the process is considered to be related to conformational entropy changes with temperature and is related to a second-order transition. There is also an abrupt change from the solid crystalline to the liquid state at the first-order transition or melting point Tm. 

A recent instance225 of reaction 22 (X = NH) involves the reactions of some (het-eroarylchloromethyl)lithium (165) reagents with imines (166) to form 167 and to produce the heteroaryl aziridines (168) as depicted in Scheme 50. Aziridines (167) are obtained in the preferential (or exclusive) conformation E. A tentative explanation of this behaviour is the different steric compression in the transition states affording isomeric E or Z aziridines. 

Our spectroscopic studies of BN in mixture B and in hexane support our contention that ground state conformers are forced by cholesteric mesophases toward extremes of 0 (i.e., closer to 0° or 180° than in hexane solvent). As the two naphthyl groups become more coplanar, their u-overlap increases. Consequently, the 0-0 transitions in absorption (and excitation) occur at longer wavelengths (lower energies) (43). For the same reasons, the cholesteric solvent compresses excited singlets of BN, causing their fluorescence spectra to be red-shifted with respect to those in hexane. 

---