Folding equilibrium data, thermodynamic

Figure 4. Thermodynamic treatment of folding equilibrium data obtained from fluorescence intensity measurements at 0 M ( a ), 0.3 M ( a ) and 0.6 M ( ) sucrose. Lines represent least square fits (r O.98 for all lines). The transition midpoints and values of the free energy change in the absence of denaturant are shown in Table I.

There have been more equilibrium measurements for reactions of carbocations with azide than halide ions. Regrettably, there is little thermodynamic data on which to base estimates of relative values of pARz and pAR using counterparts of Equations (17) and (18) with N3 replacing Cl. Nevertheless, a number of comparisons in water or TFE-H20 mixtures have been made87,106,226,230 and Ritchie and Virtanen have reported measurements in methanol.195 The measurements recorded below are for TFE-H20 and show that whereas pA" 1 is typically 4 log units more positive than pA R. pA Rz is eight units more negative. The difference should be less in water, perhaps by 2 log units, but it is clear that azide ion has about a 1010-fold greater equilibrium affinity for carbocations than does chloride (or bromide) ion. 

Hen egg-white lysozyme, lyophilized from aqueous solutions of different pH from pH 2.5 to 10.0 and then dissolved in water and in anhydrous glycerol, exhibits a cooperative conformational transition in both solvents occurring between 10 and 100°C (Burova, 2000). The thermal transition in glycerol is reversible and equilibrium follows the classical two-state mechanism. The transition enthalpies AHm in glycerol are substantially lower than in water, while transition temperatures Tm are similar to values in water, but follow similar pH dependences. The transition heat capacity increment ACp in glycerol does not depend on the pH and is 1.25 0.31 kj (mol K) 1 compared to 6.72 0.23 kj (mol K)-1 in water. Thermodynamic analysis of the calorimetric data reveals that the stability of the folded conformation of lysozyme in glycerol is similar to that in water at 20-80°C but exceeds it at lower and higher temperatures. 

Thermodynamic and kinetic data snggest that quadruplex stability depends on a nnmber of factors, including the type of structure adopted by the DNA strand (or strands), strand sequence, the size of intervening loops, base and phosphate modifications, pH and the presence of cations [19]. Small molecules may stabilize quadraplex DNA (or facihtate DNA folding into quadruplex structures) due to shifting the competitive equilibrium between the single-stranded or Watson-Crick duplex and quadruplex DNA towards the latter form [19,20]. Inhibition activity of G4 ligands depends mainly on the stability of their complexes with telomeric DNA quadruplexes. 

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