Conformation conversion

Uptain, S. M., Sawicki, G. J., Caughey, B., and Lindquist, S. (2001). Strains of PSI1 are distinguished by their efficiencies of prion-mediated conformational conversion. 

One can see that in the neutral derivative the pentamethylcyclopenta-dienyl rings assume a staggered conformation. Conversely, in the monocation they are disposed in an eclipsed conformation.5 A comparison between the bonding distances of decamethylferrocene and the decamethylferrocenium ion (already reported in Chapter 2, Table 1) reveals that the rotation of the rings following the electron removal is accompanied by a lengthening of the Fe-C distance by about 0.05 A. This is approximately equal to that measured in the case of ferrocene. 

Figure 4.1. Thermodynamic cycles, which illustrate states of enzyme ATCase during calorimetric measurements of enthalpies accompanying the binding of progressively increasing quantities of the substrate analog PALA. At the outset, the ATCase is 100% in the T conformation at the conclusion of the transformation, the ATCase (PALA)6 is 100% in the R conformation and has six bound PALA molecules. When the extent of binding of PALA is between 0 and 6, the extent of conformational conversion is between 0% and 100%. The horizontal broken arrow at the top of the diagram indicates the process that is accompanied by the enthalpy that we want to know. The vertical broken arrow at the right represents a pure...

These results of course are inconsistent with Dowd s finding of a triplet ground state from the matrix ESR spectrum. At present, the discrepancy is rationalized with the hypothesis that the triplet is metastable in the matrix and that its conformational conversion to the more stable singlet is slow, thus allowing the observation of the ESR signal and the linear Curie plot. Other cases of what seem to be con-formationally based triplet metastabUity in matrix ESR had been observed previously in 19931° ° and will be discussed later, together with further examples (see Section 7.2.1). 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature.219 This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be non-separable, there must be rapid interconversion of one chair form to another (in which all axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of about 10 kcal/mol (42 kJ/mol)220 and is very rapid at room temperature.221 However, by... 

Medium effect on thermodynamic parameters of conformational conversions in some 6- to 8-membered 0-, S-, and O.S-heterocycles 93ZOB721. 

At the energies required for conformational conversions and other exchange processes which are amenable to study by NMR spectroscopy, the reacting molecules have state densities which are much lower than those of molecules undergoing isomerization and decomposition reactions which are generally found to obey RRKM kinetics. Whether these systems can be modeled with RRKM theory is a question of current interest. Table 8 lists molecules for which pressure-dependent gas-phase chemical exchange rate constants have been obtained. 

Rabinovitch and co-workers found that the Lindemann mechanism is adequate for modeling the pressure dependence of bimolecular region unimolecular rate constants for extracting collision efficiencies for the methyl isocyanide isomerization [122]. For the conformer conversion of molecule A at constant temperature, it can be written as,... 

V. Pavone, E. Benedetti, B. Di Blasio, C. Pedone, A. Santini, A. Bavoso, C. Toniolo, M. Crisma, and L. Sartore, ]. Biomol. Struct. Dyn., 7, 1321 (1990). Critical Main-Chain Length for Conformational Conversion From 3)0-Helix to a-Helix in Polypeptides. 

Figure 12.2 cis-trans conformational conversion upon formation the Au-Ln arrays by incorporating gold(I) acetylide diphosphine complexes with Ln(hfac)3 units [122], (Redrawn from H.B. Xu et al, Conformation changes and luminescent properties of Au-Ln (Ln = Nd, Eu, Er, Yb) arrays with 5-ethynyl-2,2 -bipyridine, Inorganic Chemistry, 47, 10744, 2008.)... 

Cyclic olefin copolymer (COC, poly(norbomene-co-ethylene)) catalyzed by ansa-ethylenebis-(indenyl)zirconium dichloride (Et[Ind](2)ZrCl2)/methyl-aluminoxane (MAO) gives rise to phenomenal increase of Tg upon annealing. Unaveraged solid-state NMR resonance of the CH carbons in the norbor-nene (53 and 49 ppm) evidenced the different conformation conversions. ... 

The effect of the solvent on the abundance of the conformers of 2-meth-oxyoxane is demonstrated in Table XIV, where molar fractions of the axial form are compared with available experimental data already shown in Table VI. For a number of solvents, the agreement is remarkably good. Although the results indicate a decreased abundance of the a form of 2-methoxyoxane with increase in the dielectric constant of the solvent, the dependence is not a simple one. The calculations also reproduce such subtle factors as the pronounced effect of chloroform when compared with other solvents of similar polarity and, conversely, a relatively weak effect of dimethyl sulfoxide in comparison to less-polar solvents. The analysis of the role of individual solvation energy terms in the total energy suggests that the conformationally most important term is the contribution of electrostatic interactions that stabilize the ap conformations. Conversely, the dispersion term shows only a shght conformational dependence. 

Prion Protein and Its Conformational Conversion A Structural Perspective... 

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