223. Acidity functions of some aqueous acids

Bascombe, K.N.; Bell, R. P.

doi:10.1039/jr9590001096

Cited by 87 publications

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“…Marcus' group report data at 25 and 60°C (17), and Boyd and Wang have studied 4-nitroaniline at 1, 10, 20, 25, 30, 40, and 50°C (18). Additional data at 25°C (19)(20)(21)(22)(23)(24), 17°C (25), and "room temperature", taken as 22°C (26)(27)(28), are also available. The cyanocompounds have been studied by Boyd and Wang at -5, 1, 10, 20, 25, 40, and 50°C (18).…”

Section: Methodsmentioning

confidence: 99%

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Thermodynamics of protonation of weak bases in H₂SO₄-H₂O media, determined using the excess acidity method

Cox¹,

Yates²

1984

Can. J. Chem.

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The excess acidity method has been used to investigate the thermodynamics of the protonation process for those weak bases for which the ionization ratios (or optical densities) at several temperatures have been measured in aqueous sulfuric acid. Standard enthalpies and entropies, at 25 °C in the aqueous reference state, are given for thirteen primary, one secondary, and two tertiary nitroanilines, three cyanocompounds, thirteen triphenylmethanols, three other carbocation precursors, two ketones, nine pyridines, and nine azine N-oxides. Guidelines for estimating pKBH+ at any temperature for other weak bases are discussed.

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Section: Methodsmentioning

confidence: 99%

“…These data were curve fitted as described elsewhere (30) pound was also studied at 25OC by Bascombe and Bell (19).…”

Section: Methodsmentioning

confidence: 99%

Thermodynamics of protonation of weak bases in H₂SO₄-H₂O media, determined using the excess acidity method

Cox¹,

Yates²

1984

Can. J. Chem.

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“…(1984) Catalytic activity was investigated by using Nbenzoyl-L-arginine p-nitroanilide (L-Bz-Arg-Nan) (Sigma Chemical Co.) and a continuous spectroscopic assay at 410 nm essentially as described for the ficincatalysed hydrolysis of DL-Bz-Arg-Nan by Malthouse & Brocklehurst (1976) using e410 = 8.8 x 103 M-l. Ccm (Erlanger et al, 1961) which is essentially pH-independent at pH values above 3 (the pK. ofthe p-nitroanilinium cation is 1.0; Bascombe & Bell, 1959). When corrections are significant they are made by using 6401 = 8.8 x 103/ (1 +0.1[H+]) M1 -cm-'.…”

Section: Materials and Methods Enzymesmentioning

confidence: 99%

Differences in the chemical and catalytic characteristics of two crystallographically ‘identical’ enzyme catalytic sites. Characterization of actinidin and papain by a combination of pH-dependent substrate catalysis kinetics and reactivity probe studies targeted on the catalytic-site thiol group and its immediate microenvironment

1988

Biochemical Journal

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The characteristics of actinidin (EC 3.4.22.14) and papain (EC 3.4.22.2), two cysteine proteinases whose catalytic-site regions appear to superimpose to a degree that approaches atomic co-ordinate accuracy of both crystal structures, were evaluated by determining (a) the pH-dependence in acid media of the acylation process of the catalytic act (k+2/K,) using N-benzoyl-L-arginine p-nitroanilide (L-Bz-Arg-Nan) as substrate and (b) the sensitivity of the reactivity of the catalytic-site thiol group and its pH-dependence to structural change in small, thiol-specific, two-protonic-state reactivity probes (2,2'-dipyridyl disulphide and methyl 2-pyridyl disulphide) where enzyme-probe contacts should be restricted to areas close to the catalytic site. Distortion of the catalytic sites of the two enzymes at pH < 4 was evaluated over time-scales appropriate for both stopped-flow reactivity probe kinetics (s< 1-2 s) and steady-state substrate catalysis kinetics (3-5 min) by using the 2,2'-dipyridyl disulphide monocation as a titrant for non-distorted catalytic sites. This permitted a lower pH limit to be defined for valid kinetic analysis of both types. The behaviour of the enzymes at pH < 4 requires a kinetic model in which the apparently biomolecular reaction of enzyme with probe reagent is separated from the process leading to loss of conformational integrity by a potentially reversible step. The acylation of actinidin with L-Bz-Arg-Nan in acidic media occurs in two protonic states, one produced by raising the pH across pKa < 4 which probably characterizes the formation of -S-/-ImH+ ion pair (pKa approx. 3) and the other, of higher reactivity, produced by raising the pH across pKa 5.5, which may characterize rearrangement of catalytic-site geometry. The pH-dependence of the acylation of papain by L-Bz-Arg-Nan is quite different and is not influenced by protonic dissociation with pKa values in the range 5-6. The earlier conclusion that the acylation of papain depends on two protonic dissociations each with pKa approx. 4 was confirmed. This argument is now more firmly based because titration with 2,2'-dipyridyl disulphide permits the loss of conformational integrity to be taken into account in the analysis of the kinetic data at very low pH. Methyl 2-pyridyl disulphide was synthesized by reaction of pyridine-2-thione with methyl methanethiolsulphonate and its pKa at I = 0.1 was determined by spectral analysis at 307 nm to be 2.8. The pH-dependences of the apparent second-order rate constants (k) for the reactions of actinidin and papain with 2,2'-dipyridyl disulphide and with methyl 2-pyridyl disulphide were determined over a wide range of pH at 25°C and I = 0.1 and the characterizing parameters (pK. values and pHindependent rate constants) were compared. Although the reactivities of actinidin towards the two probe reagents in each of the three reactive protonic states are similar, the reactivity of papain towards the smaller probe reagent (methyl 2-pyridyl disulphide) is considerably greater than its reactivity towards ...

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Section: Introductionmentioning

confidence: 99%

“…The general utility of this function has received much investigation in the ensuing years (2)(3)(4)(5)(6)(7)(8)(9)(10). Its applicability has been found to be quite limited (5)(6)(7)(8)(9)(10)(11) and this has led to the introduction of other acidity functions (12)(13)(14)(15)(16)(17)(18)(19)(20). Differences between the various functions have been attributed to hydration effects (21)(22)(23)(24)(25)(26)(27)(28)(29)(30) in attempts to generalize the Hammett (H,) acidity function by identifying its relationship with the others.…”

Section: Introductionmentioning

confidence: 99%

The Measurement of the Activity Coefficient Ratios of Protonated and Neutral Forms of Hammett Indicators in Several Strong Acids

Essig¹,

Marinsky²

1972

Can. J. Chem.

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The activity coefficient ratio for several indicators has been estimated for the protonated and neutral forms of the same indicator molecule over a large range of acidity in H2S04, HCIO,, and HCI systems. The activity coefficient (f,) of the neutral form of a number of Hammett indicators was determined via solubility measurements. The observed linear relationship between log f, and PK;~' then permitted estimate by extrapolation of log f, not experimentally accessible for the more basic indicators. Solubility studies of the pentacyanopropenide (PCP-) salts of these more basic protonated indicators were referred to a reference standard, tetraethylammonium pentacyanopropenide (TEA+PCP-) according to the method developed by Boyd to obtain the activity coefficient ratio fBH+&,,+.By synthesizing the term fB,+lf, Cf,,,-lf,EA+)llZ the approach to a meaningful value of fBH+/fB is believed to be facilitated. This function is uniquely related to the water activity of the various acid systems examined to support the suggestion by Hammett that fBH+lf, is a constant for primary aniline indicators at a specified acid concentration. de fBH+/fB. Cette fonction est uniquement en relation avec I'activitt de I'eau dans les divers systkmes acides examines pour soutenir la suggestion de Hammett que fBH+/fB est une constante pour les indicateurs aniline primaire a une concentration acide fixte.

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223. Acidity functions of some aqueous acids

Cited by 87 publications

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Thermodynamics of protonation of weak bases in H₂SO₄-H₂O media, determined using the excess acidity method

Thermodynamics of protonation of weak bases in H₂SO₄-H₂O media, determined using the excess acidity method

The Measurement of the Activity Coefficient Ratios of Protonated and Neutral Forms of Hammett Indicators in Several Strong Acids

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223. Acidity functions of some aqueous acids

Cited by 87 publications

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Thermodynamics of protonation of weak bases in H2SO4-H2O media, determined using the excess acidity method

Thermodynamics of protonation of weak bases in H2SO4-H2O media, determined using the excess acidity method

The Measurement of the Activity Coefficient Ratios of Protonated and Neutral Forms of Hammett Indicators in Several Strong Acids

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Thermodynamics of protonation of weak bases in H₂SO₄-H₂O media, determined using the excess acidity method

Thermodynamics of protonation of weak bases in H₂SO₄-H₂O media, determined using the excess acidity method