Main chain, defined

When A A or B B, the symmetry is lower and the only possible line repetition groups are s(M/N) and tc for isotactic and syndiotactic polymers, respectively, in both cis and trans configurations. In these cases, two independent torsion angles in the main chain define the regular conformation (Oi and 02 in Figure 2.15). 

To put the errors in comparative models into perspective, we list the differences among strucmres of the same protein that have been detennined experimentally (Fig. 9). The 1 A accuracy of main chain atom positions corresponds to X-ray structures defined at a low resolution of about 2.5 A and with an / -factor of about 25% [192], as well as to medium resolution NMR structures determined from 10 interproton distance restraints per residue [193]. Similarly, differences between the highly refined X-ray and NMR structures of the same protein also tend to be about 1 A [193]. Changes in the environment... 

In this way each amino acid residue is associated with two conformational angles and y. Since these are the only degrees of freedom, the conformation of the whole main chain of the polypeptide is completely determined when the ([) and y angles for each amino acid are defined with high accuracy. 

In the first case, that is with dipoles integral with the main chain, in the absence of an electric field the dipoles will be randomly disposed but will be fixed by the disposition of the main chain atoms. On application of an electric field complete dipole orientation is not possible because of spatial requirements imposed by the chain structure. Furthermore in the polymeric system the different molecules are coiled in different ways and the time for orientation will be dependent on the particular disposition. Thus whereas simple polar molecules have a sharply defined power loss maxima the power loss-frequency curve of polar polymers is broad, due to the dispersion of orientation times. 

The aldotetroses are the four stereoisomers of 2,3,4-trihydroxybutanal. Fischer projections are constructed by orienting the molecule in an eclipsed conformation with the aldehyde group at the top. The four carbon atoms define the main chain of the Fischer projection and are arranged vertically. Horizontal bonds are directed outward, vertical bonds back. 

Materials that exhibit enhanced solubility after exposure to radiation are defined as positive resists. The mechanism of positive resist action in most of these materials involves either main-chain scission or a polarity change. Positive photoresists that operate on the polarity change principle have been widely used for over three decades in the fabrication of VLSI devices and they exhibit high resolution and excellent dry etching resistance. Ordinarily, the chain scission mechanism is only operable at photon wavelengths below 300 nm where the energy is sufficient to break main chain bonds. 

The absolute sensitivity was defined as the number of main chain scissions occurring in one photon molecule when one photon was irradiated on a unit surface (1 cm2) of the film. If the spectral dependence of the quantum yield has been obtained, the absolute sensitivity is calculated following the formula (4). The results obtained are shown in Fig. 9. The practical sensitivity will be the integral of the product of the absolute intensity of the irradiated light and the absolute sensitivity in Fig. 9 over all the wavelength of the spectra. 

The distribution of microstates may be defined as the distribution of spatial dislocations, orientations, and interactions of groups of the main chain and side groups with respect to their most probable values. 

Fig. 6. Standard convention for defining dihedral angles, using four atoms in sequence order either along the main chain or along the major branch of the side chain. Looking along the bond between the central two atoms (in either direction), use the end atom in front as the 0° angle reference. Then the dihedral angle (marked 0) is measured by the relative position of the end atom in back (positive if clockwise, negative if counterclockwise) with respect to the reference atom position.

Experimental determination of the quantum efficiency of photosensitive polymers of the sort that are used in one-component positive resist systems is a more complex experimental undertaking. Here the quantum efficiency is defined as the number of main chain scissions that occurs per photon absorbed. Guillet and coworkers at the University of Toronto have... 

Hydrolysis of mannan-type polysaccharides by P-mannanase is dependent on substitution on and within the main-chain as well as the source of the P-mannanase employed. Characterisation of reaction products can be used to define the sub-site binding requirements of the enzymes as well as the fine-structures of the polysaccharides. Action of c/xt/o-arabinanase and em/o-galactanase on arabinans and arabinogalactans is described. Specific assays for ndo-arabinanase and arabinan (in fruit-juice concentrates) are reported. 

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