Reducibility stability

Disulfides. The introduction of disulfide bonds can have various effects on protein stability. In T4 lyso2yme, for example, the incorporation of some disulfides increases thermal stability others reduce stability (47—49). Stabili2ation is thought to result from reduction of the conformational entropy of the unfolded state, whereas in most cases the cause of destabili2ation is the introduction of dihedral angle stress. In natural proteins, placement of a disulfide bond at most positions within the polypeptide chain would result in unacceptable constraint of the a-carbon chain. 

The axial-equatorial conformational equilibria for 2-fluoro- and 2-chlorotetrahydropyran have been investigated with several MO calculations, including calculations at the MP2/6-31G level. The MP2/6-31G calculations give values of 3.47 and 2.84kcal/mol, respectively, for the energy favoring the axial conformer. Solvent effects were also explored computationally and show the usual trend of reduced stability for the axial conformation as solvent polarity increases. 

The predictions of relative stability obtained by the various approaches diverge more widely when nonbenzenoid systems are considered. The simple Hiickel method using total n delocalization energies relative to an isolated double-bond reference energy (a + fi) fails. This approach predicts stabilization of the same order of magnitude for such unstable systems as pentalene and fulvalene as it does for much more stable aromatics. The HMO, RE, and SCF-MO methods, which use polyene reference energies, do much better. All show drastically reduced stabilization for such systems and, in fact, indicate destabilization of systems such as butalene and pentalene (Scheme 9.2). 

The relatively slower rearrangement of alkynyl and cyano substituents can be attributed to the reduced stability of the intermediate derived by cyclization of the triply bonded substituents ... 

The anomalously reduced stabilities of certain nonalternant hydrocarbons and higher members of [4n+2] annulenes arise from their seemingly peculiar geometrical structures in which a strong bond distortion often accompanied by a molecular-symmetry reduction occurs. 

The addition of two ortho-methyl groups to cumyl derivatives also results in a decrease in the Hammett reaction constant from p+ = —4.3 for cleavage of X-[8]-(4-nitrobenzoate) to p+ = —3.0 for cleavage of X-[10]-(4-methoxy-benzoate).27 This provides additional support for the conclusion that rotation about the CAr—Ca bond, which will minimize destabilizing steric interactions between methyl groups in the transition state, also reduces stabilizing resonance electron donation from the ring substituent to the benzylic carbon (Scheme 9). 

Incidents where presence of impurities, often in trace (catalytic) amounts, has significantly reduced stability or enhanced reactivity are under the entries ... 

A possible explanation of the reduced stability of the lower organic solvates may be connected with the effect of reduced solvation upon the oxygen balance of the salts. Uncolvated metal perchlorates have an apparent balance of +700%, and solvation with 4 mols of acetonitrile reduces this to -46.3%, and with 4 or 6 mols of DMSO to —43% and —28.6% respectively. Desolvation to 2.18 mols of acetonitrile, or to 2.0 mols of DMSO, would give products of zero oxygen balance, with maximum energy release potential [8],... 

The Heidelberg group (Diehl et al., 2001) chose (carboxymethyl) trime-thylammonium hydroxide, commonly known as betaine (Figure 4.33) as a substitute for DMSO. Why these researchers selected betaine and did not consider other additives was not discussed. However, we do know from other works (see cited references) that this compound is effective in reducing stability differences between A T and G C base pairing during hybridization, much like the action of tetramethylammonium chloride or formamide (Rees et al., 1993). Thus, betaine also provides a simple means to denature DNA and maintain such probes in a single-stranded state. 

QIC Stability Testing for New Dosage Forms Extends main stability guideline for new formulations of already approved medicines and defines circumstances under which reduced stability data can be accepted... 

Chitosan-stabilized Au NPs can be selectively synthesized on surfaces like poly (dimethylsiloxane) (PDMS) films using HAuC14 as precursor. The computation of surface plasmon bands (SPBs) based on Mie theory and experimental results indicates that the particles are partially coated by chitosan. The proposed mechanism implies that chitosan acts as a reducing/stabilizing agent. Furthermore, PDMS films patterned with chitosan could induce localized synthesis of gold nanoparticles in regions capped with chitosan only [110]. 

K135L <5> (<5> isoform B, mutation in dimeric interface, defective DNA-binding and reduced stability of enzyme [45]) [45]... 

Reduced stability and fraying of the chain is commonly expected near its end. However, the chain end sequence has a total Stokes shift in our time window that is distinctly less than the Stokes shift in the normal sequence. One way to explain this unexpected result is to note that the probe only has normal DNA on one side, so the amplitude of its contribution would be reduced. Any extra contribution from the solvent beyond the... 

The irradiation of bis(nonafluoro-tert-butyl)peroxide in octafluorocyclopen-tene or decafluorocyclohexene yields the corresponding vicinal bis(nonafluoro-tert-butyl) diethers in 55 and 63%, respectively [30] Negligible amounts of dimeric or oligomeric products are obtained. Conversely, the similar reaction with bis(tn-fluoromethyl)peroxide yields primarily 2,2 -bis(tnfluoromethoxy)dicycloalkanes (equation 12). The dissimilar behavior is believed to be due to the reduced stability of the trifluoromethyloxy radical and thus its reduced population, which favors oligomerization [30]. 

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