Protonation constants

Resonance energies of ca. 90, 182 and 330 kJ moF have been estimated for pyrrole, indole and carbazole respectively by comparing their protonation constants with those for selected model compounds (72C1(L)335, 72TL5019). 

Distribution of benzodiazepines in system micellar pseudophase - water was investigated in micellar solutions of sodium dodecylsulfate. The protonization constants of benzodiazepines were determined by the UV-spectophotometry. Values of protonization constants increase with increasing of sodium dodecylsulfate concentration. The binding constants of two protolytic forms of benzodiazepines with a micellar pseudo-phase and P, values were evaluated from obtained dependence. 

The characteristic property distinguishing macrocyclic polyamines from their linear counterparts is seen in successive protonation. One is the higher N basicity to the first proton and another is a sudden drop of N basicities in the later stages of protonation. Table 1 lists the protonation constants (Eq. 1) for the macrocyclic polyamines in comparison with the corresponding values for their linear homologues. When a linear triamine (e.g. dien) 36,37) is cyclized to, say, (9)aneN3, the basicity of the first amine increases (log Kt = 10.59 us 9.70), but the basicity of the second and especially the third amine diminish (log K2 = 6.88 vs 8.95, log K3 < 1 vs 4.25)36)... 

Table 1. Mixed protonation constants of polyamines, (25 °C and 1 = 0.2 M unless otherwise listed)... 

The conditional association constant for [18]aneN4 ( cyclic spermine ) with ATP - at pH 7.5 is calculated from the (3L value (Table 5) and protonation constants (Table 1) to be 2.4x 105 M-1, which is larger than the association constants for the linear spermidine (9 xi 02 M ) and spermine (9.5 x 103 M-1)23). It is also of interest that cyclic spermine is selective for ATP over AMP (ratio association constants is 700), while linear spermine prefers ATP to AMP only by a ratio of 26 to 1 43). The selective complexation of biologically important anions is of particular interest, especially if the ligands are converted into selective anion carriers by attachment of lipophilic hydrocarbon chains. 

The new macrocyclic hexaamine ligand X has the mixed protonation constants, log K of 10.10, 10.01, 8.96 and 8.02 at 25 °C and 1=0.2 M for the four most basic amines. The values for other weaker bases, including the six carboxylates, are all less than 5. From a comparison with logA values of the parent macrocycle [18]aneN6, it was deduced that the initial four protonations occur to the macrocyclic amine bases. Thus, the most abundant species of X at neutral pH is depicted as XI. 

Table II. Signs of Proton-Proton Constants of Carbohydrate Derivatives...

N3-coordinated complexes containing platinum group metal ions have also been synthesized and studied (56,60,61). Steric hindrance has been used to direct binding to N3 in a series of Pd- and Pt-containing complexes of 6,6,9-trimethyladenine (60). Platinum modification was found to have a pronounced effect on the basicity of the adenine moiety. The protonation constants (log Kh values) for the twofold protonation of 6,6,9-trimethyladenine are 4.15 and -0.75, with the initial protonation occurring at N1 followed by N7. The equivalent values for the formation of [Pt(dien)TMA-A3)H]3+ and [Pt(dien)TMA-2V3)H2]4+ are 0.3 and —1.2, respectively. Moreover, the site of initial protonation was found to be N7 (Fig. 19). These observations are supported by theoretical studies (62). 

This protonation equilibrium is characterized by the protonation constant of the complex ... 

The protonation constants of the ligand, Kh are also necessary for the calculation of the complex stability constant, fML ... 

To predict the complexation behavior of ligands at physiological pH, the protonation constants of the ligand have to be considered by using conditional stability constants. Conditional stability... 

This means that the sequestration equilibrium reaction will be pH-dependent. The constant K is known as the conditional equilibrium constant. However, for stability comparisons between complexes of the same denticity, it may be more convenient to compare the equilibrium constant for the proton independent reaction between iron and siderophore. This can also be useful in a theoretical sense, as it allows comparison of complex stability where siderophores have different protonation constants. However, this approach does not account for competition between H+ and Fe3+ for binding, which is always present in a real situation in aqueous solution. 

Using linear regression, it is possible to estimate the protonation constants of the Fe(II) complexes of siderophore complexes where the redox potentials have been measured over a range of pH values (59). This also explains the variation in reversibility of reduction as the pH changes, as the stability of the ferro-siderophore complex is much lower than the ferric complex, and the increased lability of ligand exchange and increased binding site competition from H+ may result in dissociation of the complex before the iron center can be reoxidized. 

The observed protonation constant describing the proton transfer on the aqua oxo complexes as a function of the total metal complex concentration, [M] ( obs)aq = d MOH2]/[M]df, is therefore given by Eq. (12)... 

Selected equilibrium constants and thermodynamic quantities of the mononuclear species are listed in Table I and the distribution of mononuclear ions as a function of pH is shown in Fig. 1. Values have also been reported which show the variation of the protonation constant of HVO, - with variation of added KC1 from 0 to 2 M (15). 

It has been suggested that an increase in the coordination number of vanadium from 4 to 5 already takes place in the second protonation step, i.e. when [H2V04] is formed (21). For reactions (1) and (2), however, the protonation constants and thermodynamic parameters are comparable with those reported for P04 and As04 , providing firm evidence that reaction (2) is not accompanied by incorporation of water in the vanadate ion (15, 17). Further, the estimated thermodynamic quantities for reaction (6), AH° = -39 kJ/mol and AS0 = —51 J/(mol K), obtained by extrapolation from the experimental values for reactions (1) and (2) and those for the three protonation steps of P04 and As04 , are not typical of a simple protonation reaction (17). For such a reaction the entropy change is normally a positive quantity often amounting to 100 50 J/(mol K) and the enthalpy... 

An increase in the coordination number of molybdenum takes place in the second protonation step, which has a dramatic effect on the value of K2. Instead of the typical decrease of 3 to 5 log units from the first to the second protonation constant, K2 has in this case about the same value as Kx. In fact, these unusual values for the protonation constants compared to those of other oxyanions, along with the thermodynamic parameters AH° and AS0, were the basis on which the change in coordination number in the second protonation step was first proposed (54). Previously the small difference between the first and second pK value was interpreted in terms of an anomalously high first protonation constant, assumed to be caused by an increase in the coordination number in the first step (2, 3, 54-57). 

To indicate six-coordination the neutral monomeric molybdic acid is sometimes formulated as Mo(OH)6, but the alternative formulations Mo02(OH)2(H20)2 or Mo03(H20)3 are most likely closer to the truth (58-60) the latter formulation is used in this chapter. An interesting linear relationship has been found (61) between values of the known first protonation constants of the oxoanions of groups 5, 6, and 7 and... 

The values for the ionization and protonation constants (0.21 and 0.24 respectively) indicate that [Mo205(OH2)6]2+ is the major dimeric species in 1.0 M acid with about equal concentrations of the other two dimers. The dimeric cationic species show a characteristic absorption band in the UV at —245 nm. 

As in the case of molybdate the first and second protonation constants of tungstate have about the same values. It can therefore be assumed, after the analogy of molybdate, that an increase in the coordination number of tungsten also occurs in the second protonation step (54, 58, 61). The equilibria are therefore formulated in the same way... 

The small difference between the successive pK values (cf. values below) of tungstic acid was previously explained in terms of an anomalously high value for the first protonation constant, assumed to be effected by an increase in the coordination number of tungsten in the first protonation step (2, 3, 55). As shown by the values of the thermodynamic parameters for the protonation of molybdate it is actually the second protonation constant which has an abnormally high value (54, 58). An equilibrium constant and thermodynamic quantities calculated for the first protonation of [WO, - pertaining to 25°C and zero ionic strength (based on measurements from 95° to 300°C), namely log K = 3.62 0.53, AH = 6 13 kJ/mol, and AS = 90 33 J, are also consistent with a normal first protonation (131) (cf. values for molydate, Table V). 

Fig. 16. Distribution of mononuclear species as a function of pH, calculated from protonation constants log Kx = 3.5 and log K2 = 4.6.

The protonation constant for reaction (33) has been determined from 183W chemical shifts at 20°C as log K = 4.59 0.03. The protonation is complex and a rearrangement of all the protons probably takes place. A possible explanation for the observed kinetics involved is that one of the [H3Wi2042]9 isomers bears three internal protons and the other has one external and two internal protons. The exchange of a proton between the internal and external sites would then be the rate-determining step (141). 

All three methods lead to consistent surface complexation constants in clay minerals (27, 28) and zeolites (82). The surface protonation constants of the organic compounds such as amines can be defined and determined similarly (41, 83) and are also found to be enhanced in the clay interface. 

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