Reverse conformation

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

Similar conclusions were reached for sulfoxides 157. Conformation 158 was preferred for (RS/SR)-157 but with some contribution from conformer 159. The (RR/SS) dias-tereomers preferred the reverse conformer 161 was preferred to 160161. An attractive force between Ph/Ar and Ph/R was thought to be the primary factor in determining the conformational preference of sulfoxides 152 and 157. MM2 calculations were carried out on a series of molecules of general structure PhCHR—X—R with X equal to CHOH, C=0, S and S=0151. The main conformers of these molecules have the Ph (or aryl) and R (alkyl) groups gauche. The calculations supported the existence of CH-tr attractive interactions with minor contributions from other effects. 

Figure 17. Molecular motor reverse conformational changes (mechanical energy) stimulated by oxidation or reduction of the polymeric chain, a) reduced chain b) oxidized chain.

Preliminary results using fixed polarizer FT-IRRAS to study the melting and hydration of cadmium arachidate on low area metals show that both irreversible and reversible conformational changes can be... 

Tomschik M, Zheng H, van Holde K, Zlatanova J, Leuba SH (2005) Fast, long-range, reversible conformational fluctuations in nucleosomes revealed by single-pair fluorescence resonance energy transfer. Proc Natl Acad Sci USA 102 3278-3283... 

Most enzyme powders are prepared by lyophilisation (freeze drying). However, the lyophilization procedure might inactivate the enzyme to some extent. To avoid this and thereby increase the activity of lyophilized enzymes in dry organic solvents, the lyophilization can be carried out in the presence of lyoprotectants such as sorbitol (Dabulis and Klibanov, 1993). The inactivation is believed to be caused at least partly by a reversible conformational change in the enzyme. This process can be reversed and the enzyme reactivated by the addition of small amoimts of water (Dabulis and Klibanov, 1993). 

The reversible conformational change was seen by the disappearance of unresolved magnetic HFS of Mossbauer spectra after the illumination (decrease of spin-lattice relaxation time) and its restoration after the illumination of bleached samples at 77°K by light of 575 nm wavelength. 

Other indirect (genotypic or phenotypic) modifications of the primary structure (70, 91, 92) as well as reversible conformational transitions (52, 57, 93) have been reviewed by Citri and Pollock (2) and more recently by Pollock (40). 

The immunology of /3-lactamases has been studied fairly extensively 16,23,41, 43, 70,115,121-124) and reviewed elsewhere 70,125). At the molecular level, differences in antigenicity could be usually correlated with differences in composition and physical properties 70). On the other hand, reversible changes in antigenicity—without apparent loss of activity—could be correlated with reversible conformational transitions 52). 

Polyoxybenzoate is a stiff chain, lyotropic liquid crystalline material, as was discussed on the basis of its copolymers with ethylene terephthalate (see Sect. 5.1.4). The crystal structure of the homopolymer polyoxybenzoate was shown by Lieser 157) to have a high temperature phase III, described as liquid crystalline. X-ray and electron diffraction data on single crystals suggested that reversible conformational disorder is introduced, i.e. a condis crystal exists. Phase III, which is stable above about 560 K, has hexagonal symmetry and shows an 11 % lower density than the low temperature phases I and II. It is also possible to find sometimes the rotational disorder at low temperature in crystals grown during polymerization (CD-glass). 

Stereochemical studies were oriented toward finding esters of special structure with reversed conformation and/or with the presence of the second conformer in smaller amounts. As reviewed previously2, these attempts were mostly unsuccessful and claims of the conformation ap were again rebutted. The only exception are certain formates, particularly t-butyl formate, in which more ap form is present in polar solvents6,165. A more recent dipole moment study166 assumed the ap form even in nonpolar solvents in para-substituted phenyl formates in an amount of 10%, but the uncertainty was of the same magnitude. 

This fact was explained in the framework of two models. The first model is based on the concept of dynamic adaptation of a protein matrix in every step of an enzymatic reaction. Concerning the QA — QB transition, fast reversible conformational transitions can provide dipolar relaxation favourable for the media reorganization process (Likhtenshtein, 1976, 1979 a, 1988 a). Such reorganization is necessary to release... 

Figure 3 Photoresponsive polypeptides showing a reversible conformational change.

Kouns, W. C., Kirchhofer, D., Hadvary, P., Edenhofer, A., Weller, T., Pfenninger, G., Baumgartner, H. R., Jennings, L. K., and Steiner, B. (1992). Reversible conformational changes induced in glycoprotein Ilb-IIIa by a potent and selective peptido-mimetic inhibitor. Blood 80, 2539-2547. 

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