Thermodynamic Studies on the Protonation Equilibria of Some Hydroxamic Acids in NaNO₃ Solutions in Water and in Mixtures of Water and Dioxane

Abstract

The protonation equilibria for nine hydroxamic acids in solutions have been studied pH-potentiometrically via a modified Irving and Rossotti technique. The dissociation constants (pK a values) of hydroxamic acids and the thermodynamic functions (ΔG°, ΔH°, ΔS°, and δ) for the successive and overall protonation processes of hydroxamic acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (the mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol dm-3 NaNO3, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature, and ionic strength on the protonation processes of hydroxamic acids is presented and discussed to determine the factors which control these processes.

Thermodynamic and structural study of complexation of phenylboronic acid with salicylhydroxamic acid and related ligands

“…Also unknown are p K a values for these ligands which are needed to characterize the complex stability at variable pH. As one can see from Table p K a of SHA‐OMe is very close to p K a1 of SHA in agreement with assignment of first deprotonation of SHA to phenolic OH and p K a1 of SHA‐NMe is significantly lower than that of SHA as is typical for N‐substituted hydroxamic acids . Both methylated derivatives form anionic deprotonated complexes with PBA with β 1,1,‐1 values slightly smaller in case of SHA‐OMe and even larger in case of SHA‐NMe than that for SHA.…”

“…As one can see from Table 1 pK a of SHA-OMe is very close to pK a1 of SHA in agreement with assignment of first deprotonation of SHA to phenolic OH [15] and pK a1 of SHA-NMe is significantly lower than that of SHA as is typical for Nsubstituted hydroxamic acids. [17] Both methylated derivatives form anionic deprotonated complexes with PBA with β 1,1,-1 values slightly smaller in case of SHA-OMe and even larger in case of SHA-NMe than that for SHA. The K obs values for both derivatives in pH range 7-9 are smaller than K obs for SHA only by a factor of 2 ( Figure 1).…”

Protonation equilibria studies of the standard α-amino acids in NaNO3 solutions in water and in mixtures of water and dioxane

“…The pK a values were calculated adopting the Irving and Rossotti technique as described in our previous work [24][25][26]. Computations related to the estimation of dissociation constants were performed by regression analysis of titration curves using the least-squares computer ESAB [16,17] and PKPOT programmes [18].…”

Calorimetric studies of the interactions of metalloenzyme active site mimetics with zinc-binding inhibitors

“…The significantly lower affinity for 8-HQ than free Zn 2+ likely reflects the lower charge density on the BPA-bound zinc ion thus a weaker bond with 8-HQ, which gave rise to an overall weaker affinity of [Zn(BPA)] 2+ than free Zn 2+ for 8-HQ. 16 With a pKa value of 9.28, 44 AHA is expected to undergo deprotonation upon binding to Zn 2+ , contributing to the difference between the logKa and logβ values. As mentioned earlier, the slightly more basic pH than the 6.80 used for the 8-HQ study was necessary in order to increase AHA affinity for [Zn(BPA)] 2+ for accurate determination of the binding parameters.…”