Hydrogen exchange rate constants

Figure 1.5 An example of a pH profile of (a) the chemical hydrogen exchange rate constant (k ) and (b) the exchange half-life (t, of an amide hydrogen in a random coil-like poly-ot-alanine peptide at 20°C (k. =4l.7M min andk., o = I-I2x 10 M min ). Adapted from Ref [29]...

In the course of our investigations to develop new chiral catalysts and catalytic asymmetric reactions in water, we focused on several elements whose salts are stable and behave as Lewis acids in water. In addition to the findings of the stability and activity of Lewis adds in water related to hydration constants and exchange rate constants for substitution of inner-sphere water ligands of elements (cations) (see above), it was expected that undesired achiral side reactions would be suppressed in aqueous media and that desired enanti-oselective reactions would be accelerated in the presence of water. Moreover, besides metal chelations, other factors such as hydrogen bonds, specific solvation, and hydrophobic interactions are anticipated to increase enantioselectivities in such media. 

Electrocatalytic Oxidation of Hydrogen The rate constant of the hydrogen oxidation reaction (HOR), as measured by the exchange current density jo (i.e. the current... 

A value for the self-exchange rate constant for O2/OJ has been deduced by Espenson et The reaction of 02( A) with 1 probably involves the formation of HOOI, followed by a fast reaction with I to yield + H202. Intermediates in the reaction of 62 with Et2S can be trapped efficiently with trimethylphosphite. The kinetics of the reaction of (ArO)3P with (Ph0)3P-03 to form O2 and (ArO)3PO show a spontaneous and an (ArO)3P-catalyzed pathway. A complex mechanism is involved in the Cu(II)-catalyzed decomposition of hydrogen peroxide in the range pH 11 to 12. A superoxide Cu(I) complex is involved. ... 

The last assumption that is required for the time correction process to be reliable is that the reaction is approximately first order with an effective rate constant that is a linear function of the chemical exchange rate constant. As discussed in Section 17.3.1, the assumption of first-order kinetics with an approximately EX2-like mechanism for disordered proteins may not be accurate. The reliability of the time correction method is only as good as the extent to which the hydrogen exchange kinetics follow an EX2-like mechanism. Taking all of these factors into account, it is clear that the use of low pH labeling to reach msec HX must be approached with some measure of caution. 

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