Change of conformation

During crystallization, polymer chains incorporated in crystals assume hehcal conformations [1-3] as described in Chapter 2. 

The melting process occurs with an increase in disorder, an increase in entropy (ASt = AHt/Tm) (7-17 JK moT ), changes of volume (1-20%), and a sigmoidal change of heat capacity under the endothermic peak, which usually for flexible macromolecules depends on the degree of 

The conformational contribution Cv(conf) can be calculated based on the Ising-like model [2,21] by making use of a previous suggestion by Strobl [2] and applying the derived equation for flexible macromolecules by Wunderlich and Pyda [21]. 

Rgure 9.6 shows a comparison of the experimental and calculated heat capacity with all contributions (Cv(vib), Cv(conf), Cv(ext)) at the liquid-like state of amorphous PE above the glass transition, with good agreement of experimental and calculated Cp [21]. Tlie experimental heat capacities data Cp(exp), in the temperature range from 250 to 600 K above the melting temperature 414 K, also agrees with the calculated Cp [21]. 

More det on applications of conformation changes to heat capacity for other polymers and polymer-water systems can be found in References [21,25-27]. 

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An important property of chain molecules is that a major contribution to the standard entropy is conformational in nature, i.e. is due to hindered internal rotations around single bonds. This property is most relevant to cyclisation phenomena, since a significant change of conformational entropy is expected to take place upon cyclisation. Pitzer (1940) has estimated that the entropy contribution on one C—C internal rotor amounts to 4.43 e.u, A slightly different estimate, namely, 4.52 e.u. has been reported by Person and Pimentel (1953). Thus, it appears that nearly one-half of the constant CH2 increment of 9.3 e.u. arises from the conformational contribution of the additional C—C internal rotor. 

The empty state is the LL state on the top left comer of Fig. 4.18. The binding of a ligand on any of the subunits will shift its conformation completely from L to H without affecting the conformation of the second subunit. Binding of the two ligands will shift the entire polymer to the state HH. Thus, in each binding process there is a total change of conformation of one subunit hence the term sequential model. 

In the second model, the sequential model (Fig. 5-15b), proposed in 1966 by Daniel Koshland and colleagues, ligand binding can induce a change of conformation in an individual subunit. A conformational change in one subunit makes a similar change in an... 

The steroid derivative ouabain (pronounced wah -bane from waa bayyo, Somali for arrow poison ) is a potent and specific inhibitor of the Na+K+ ATPase. Oubain binds preferentially to the form of the enzyme that is open to the extracellular side, locking in two Na+ ions and preventing the changes of conformation necessary to ion transport. Another very potent toxin, palytoxin (produced by a coral on the Hawaiian shoreline), also targets the Na+K+ ATPase, but it binds... 

The classical statistical theory of rubber elasticity1) for a Gaussian polymer network which took into account not only the change of conformational entropy of elastically active chains in the network but also the change of the conformation energy, led to the following equation of state for simple elongation or compression 19-2,1... 

According to the theory of rubber elasticity, the elastic response of molecular networks is characterized by two mechanisms. The first one is connected with the deformation of the network, and the free energy change is determined by the conformational changes of the elastically active network chains. In the early theories, the free energy change on deformation of polymeric networks has been completely identified with the change of conformational entropy of chains. The molecular structure of the chains... 

The acid-catalysed epimerization reaction often contributes to the change of conformation that alters the sterical shape of a compound. This may have a severe effect on pharmacological properties as with reserpine (1) and isoreserpine (2). The same seems to apply to the C-3 epimers yohimbine (78) and pseudoyohimbine (79). Yohimbine (78) blocks ai-receptors, whereas pseudoyohimbine (79) has little affinity for this... 

In addition, trimethylsilyl ether 197a and triethylsilyl ether 197b have conformations different from those of compounds 198a and 198b247, which bear a bulky PhsSi or a (z -Pr) vSi group, respectively. The change of conformation is due to the steric hindrance from these two bulky silyl groups. 

M 35] [protocol see [119]] A protein conformation kinetic study of the small protein ubiquitin was performed both in the continuous and in a stopped-flow mode at low reactant consumption [119], The bifurcation mixer was used prior to an IR flow cell for data monitoring. The change of conformation from native to the A-state was followed when adding methanol under low pH conditions to the protein solution. In the continuous mode, long data acquisition could be made and the reaction time was determined by the flow rate and the volume interconnecting zone between the mixer and IR flow cell, which was small, but not negligible. In the stopped-flow mode, the reaction time resolved was dependent on the time resolution of the FTIR instrument. 

This rearrangement implies a change of conformation, from 46 to 47, corresponding to a rotation around the bond between carbon atoms 2 and 3 in the trithiapentalene structure. If this rotation is prevented, for instance, if the carbon atoms are part of a six-membered carbon ring, the carbanion (46) is stable enough to condense with carbon disulfide in reasonable yield.74 This reaction is described in the synthesis of extended structures (Section V). 

The change of conformation of the N-substituted quinolizidine ring from trans to cis (as a result of quaternization of nitrogen) causes a change of stereochemistry of C-7 or C-7. In consequence, C-17 changes from a diaxial relationship in thiobinupharidine (16) to an axial-equatorial one or, in the case of cis-AB, cis-A B thiobinupharidine dimethiodide (41), to an equatorial-equatorial relationship. This causes a signal shift for C-17 downfield by 5 and 10 ppm, respectively (43). 

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