Segmental conformational changes

Volume relaxation is considered to be determined by free-volume relaxation. The agents of this process, segmental conformational changes, are therefore to be coupled to local free-volume states. These states are therefore to be viewed as mobility measures. Thus, we have the connections of mobility rate free volume/= h. (The... 

Citrate synthase in mammals is a dimer of 49-kD subunits (Table 20.1). On each subunit, oxaloacetate and acetyl-CoA bind to the active site, which lies in a cleft between two domains and is surrounded mainly by a-helical segments (Figure 20.6). Binding of oxaloacetate induces a conformational change that facilitates the binding of acetyl-CoA and closes the active site, so that the reactive carbanion of acetyl-CoA is protected from protonation by water. 

This equation makes it possible to obtain the dependence between the degree of closure and the cathodic potential at which the polymer is reduced. The probability ( ) of a conformational change that will allow the reduction and compaction of a segment can be expressed as the inverse of the relaxation time. If all the other terms of Eq. (9) are included in P, then... 

Thus P is a structural parameter ranging between 0 and 1 that acts at the initial moments of the oxidation process of every segment the higher the degree of closure (v), the lower the probability (P) of any spontaneous conformational changes and the greater the anodic overpotential required to create a relaxation nucleus. Under these conditions both magnitudes are related by... 

In solution the molecules of a polymer undergo various segmental motions, changing rapidly from one conformation to another, so that the molecule itself effectively takes up more space than the volume of its segments alone. As we have seen, the size of the individual molecules depends on the thermodynamic quality of the solvent in good solvents chains are relatively extended, whereas in poor solvents they are contracted. 

Conformational Change and Parallel Orientation of Stiff Segments (PSs). 210... 

Agonist-induced conformational changes at the cytoplasmic side of transmembrane segment 6 in the beta 2 adrenergic receptor mapped by site-selective fluorescent labeling. J. Biol. Chem. 276, 9279-9290. 

Loo, T.W., Bartlett, M.C. and Clarke, D.M. (2003) Substrate-induced conformational changes in the transmembrane segments of human P-glycoprotein. Journal of Biological Chemistry, 278, 13603-13606. 

The method is based on the fact that the rate of conformational change required for excimer formation depends on the free volume induced by the segmental motions of the polymer occurring above the glass transition. DIPHANT (compound 3 in Figure 8.3) was used as an excimer-forming probe of three polymer samples consisting of polybutadiene, polyisoprene and poly(dimethylsiloxane).a)... 

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