Proton-Binding Equilibrium

Discrete affinity spectra are composed of a few, well-characterized binding sites with known binding constants this is the case of common polyprotic acids, such as phosphoric acid 

For proton binding to soil minerals, in most cases, reactions 11.8 and 11.9 are considered in the modeling, which is a spectrum consisting of two sites of equal height (/= 0.5 for each one) this is termed the 2-pK model. However, it is also employed as the so-called 1-pK model, where the protonation equilibrium is represented by 

FIGURE 11.2 Affinity spectra (represented as fraction of sites against the logarithm of association constant) for (a) phosphoric acid (b) an equimolar mixture of acetic and dipico-linic acids. 

In some cases, two types of proton-binding sites are considered, such as in the case of goethite, where singly and triply bound surface OH have been distinguished by Rahnemaie, Hiemstra, and Van Riemsdijk (2006), using the 1-pK model for each, namely 

The total surface coverage for discrete affinity spectra is simply the weighted sum of the coverages of the diverse sites, as in Equation 4.53  

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When a metal cation or an anion binds to the surface of a soil colloid, especially oxide particles, it always occurs in competition with proton-binding equilibrium. For transition cations such as Cu(II), Pb(II), and so on, both monodentate binding and bidentate binding have been proposed, that is. 

The most dramatic rate retardations of proton transfers have been observed when the acidic or basic site is contained within a molecular cavity. The first kinetic and equilibrium studies of the protonation of such a basic site were made with large ring bicyclic diamines [72] (Simmons and Park, 1968 Park and Simmons, 1968a). It was also observed (Park and Simmons, 1968b) that chloride ion could be trapped inside the diprotonated amines. The binding of metal ions and small molecules by macrocyclic compounds is now a well-known phenomenon (Pedersen, 1967, 1978 Lehn, 1978). In the first studies of proton encapsulation, equilibrium and kinetic measurements were made with several macrobicyclic diamines [72] using an nmr technique. 

EQUILIBRIUM MODELS OF PROTON BINDING BY HUMIC SUBSTANCES... 

The slower reaction, where the proton reacts with a ground-state molecular proton detector, widens the observation range. It can detect charge modulation of a protein, and the state of the water on an interface. The submicrosecond time resolution can discriminate proton exchange between donor-acceptor moieties located on the same structure from reaction with bulk proton. Similarly, it can resolve a complex proton-binding dynamics into initial and postprotonation events and it determines the contribution of each step to the overall measured equilibrium constants. 

It has been noted previously(22) that HCN and other ligands supplying an exchangeable proton bind to carbonic anhydrase by donation of a proton and association as an anion. We attribute the ability of HCO3 to bind to the basic form of carbonic anhydrase to this effect. The relevant equilibrium is... 

After a second flash, approximately 1.2-1.6 protons are taken up, from pH 5 to 9. The amplitude declined steadily at pH above 8, and essentially equal stoichiometries, i.e., 1 H+ per P+, were taken up on each flash, at pH 9.5. At even higher pH, the net H+ taken up after two flashes fell below 2, due to the unfavorable one-electron equilibrium, Qa Qb < > Qa Qb" blocking turnover of a fraction of the RCs on the second flash. Ubiquinone (Q-10) titration of the second flash proton binding gave a half saturating quinone concentration of about 4 iM. This is comparable to other titrations of Qb activity under these conditions [21]. 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

Depending on pH, increasing the acidity of the solution either makes the potential required to yield a fixed turnover frequency more oxidizing by 60 mV/pH or does not affect it. This pH dependence is in most cases the same as that of the Fe /n (.Q jpjg jjj jjjg absence of a substrate. These identical pH dependences suggest a pre-equilibrium between the ferric and ferrous forms of the catalyst followed, by a kinetically irreversible step that does not involve proton or electron transfer (e.g., O2 binding). 

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