Proton activity

It has been reported that exchange of protons activated by enolization can be performed directly in a glass inlet system of the mass spectrometer prior to analysis by heating the sample at about 200° with deuterium oxide vapor for a few minutes. " Exchange has been observed with 2-, 3-, 6-, 11- and 17-keto steroids, but the resulting isotopic purity is usually poor,... 

Protonic Activation Using Anhydrous Hydrogen Fluoride. 216... 

Protonic Activation with Acids Other than HF. 221... 

For the purposes of this chapter, an arbitrary distinction is made between protonic and thermal activation, wherein protonic activation is caused by the action of acid at room temperature or lower, and thermal activation refers to the use of elevated temperatures with or without the addition of acid. In fact, in both cases, the initial steps in the postulated mechanisms are protonation of the C-2 oxygen atom followed by elimination of the aglycone to yield a ketohexofuranosyl or pyranosyl cation, which is the reactive intermediate in certain circumstances, this might be in equilibrium with the derived glycosyl fluoride. 

Diheterolevulosans, 209-211, 240 Dihexulose dianhydrides, 207 -266, see also Caramels Di-D-fructose dianhydrides 13C NMR spectra, 245-246 conformation, electronic control, 224-228 conformational rigidity, energetic outcomes, 228 hexulopyranose rings, 226 historical overview, 210-213 H NMR spectra, 248 -249 intramolecular hydrogen-bonds, 227 isomerization, 231 -232 1,2-linked, ero-anomeric effect, 224-225 listing, 240-241 nomenclature, 208-210 optical rotations and melting points, 242-243 protonic activation... 

Sometimes the term normal hydrogen electrode (and respectively normal potential instead of standard potential) has been used referring to a hydrogen electrode with a platinized platinum electrode immersed in 1 M sulfuric acid irrespectively of the actual proton activity in this solution. With the latter electrode poorly defined diffusion (liquid junction) potentials will be caused, thus data obtained with this electrode are not included. The term normal hydrogen electrode should not be used either, because it implies a reference to the concentration unit normal which is not to be used anymore, see also below. 

Assuming adsorption to behave according to the Langmuir adsorption isotherm, we get Eq. (1.22b). Both the rate constant of proton activation and the equilibrium constant of adsorption K q depend on cavity details. 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

Fig. 2.11. The temperature dependence of cation/proton activity ratios of geothermal well discharges in Japan. The lines in the figure are recalculated temperature dependences of cation/proton ratios in Icelandic geothermal waters. The dashed curve in B represents the reaction 1.5 K-feldspar + H+ = 0.5 K-mica + 3 quartz (or chalcedony) + K+ (Chiba, 1991). Open circle Takigami, open triangle Kakkonda, open square Okuaizu, solid circle Kirishima, solid triangle Sumikawa, solid square Nigoiikawa.

The common method of estimation of the interfacial potential in microheteroge-neous systems is to use pH indicators adsorbed at for instance charged micelles. The local interfacial proton activity is related to the bulk activity and to an interfacial potential by Boltzmann s law according to the equation [17,70-72]... 

Bacterial cell walls contain different types of negatively charged (proton-active) functional groups, such as carboxyl, hydroxyl and phosphoryl that can adsorb metal cations, and retain them by mineral nucleation. Reversed titration studies on live, inactive Shewanella putrefaciens indicate that the pH-buffering properties of these bacteria arise from the equilibrium ionization of three discrete populations of carboxyl (pKa = 5.16 0.04), phosphoryl (oKa = 7.22 0.15), and amine (/ Ka = 10.04 0.67) groups (Haas et al. 2001). These functional groups control the sorption and binding of toxic metals on bacterial cell surfaces. 

Tendency for epoxide protonation. When the epoxide is formed in vivo it is believed that protonation of the oxygen occurs. This idea is supported by the fact that there are subcellular regions with high proton activity [49-52]. Protonation of an epoxide oxygen weakens the C-0 bonds and promotes ring opening [53-57]. 

In any aqueous solution, the pH is a measure of the hydrogen ion or proton activity. However, in many if not most cases, pH is treated as the concentration of protons in solution rather than their activity. Soil solutions are no different except that the measurement is much more complex. The complexity arises from two sources ... 

There is room for further analysis in many traditional areas, as pointed out above during the discussion of enolization. Also, it is noted that the employment of transition state pKf values is very close to the use of the proton activating factors and deprotonating factors, introduced by Stewart (Stewart and Srinivasan, 1978 Stewart, 1985). It is to be hoped that the two approaches can be consolidated in a common view of acid-base catalysis. 

In surface-complexation models, the relationship between the proton and metal/surface-site complexes is explicitly defined in the formulation of the proposed (but hypothetical) microscopic subreactions. In contrast, in macroscopic models, the relationship between solute adsorption and the overall proton activity is chemically less direct there is no information given about the source of the proton other than a generic relationship between adsorption and changes in proton activity. The macroscopic solute adsorption/pH relationships correspond to the net proton release or consumption from all chemical interactions involved in proton tranfer. Since it is not possible to account for all of these contributions directly for many heterogeneous systems of interest, the objective of the macroscopic models is to establish and calibrate overall partitioning coefficients with respect to observed system variables. 

Therefore, if the excited-state lifetimes r0 and Tq are known, the plot of ( / 0)/( / ) versus [H30+] yields the rate constants k3 and k i. However, it should be emphasized that corrections have to be made (i) the proton concentration must be replaced by the proton activity (ii) the rate constant k 3 must be multiplied by a correction factor involving the ionic strength (if the reaction takes place between charged particles), because of the screening effect of the ionic atmosphere on the charged reactive species. 

It is obvious from the discussion that one can take advantage of equations of the type (11.69), and its generalization to multiple sites, to deduce information about the number of protons actively participating in the transition state, see Fig. 11.7. 

Perhaps the most important parameter involved in aqueous-organic mixtures is their effective protonic activity (denoted by pH or pan). This parameter has been measured for most commonly used buffers in all selected mixtures down to their freezing point (Hui Bon Hoa and Douzou, 1975 Douzou ei al., 1976). Values of pH depend on solvent and temperature in a way that varies for different buffers, but with the data available a medium of known pH under any desired condition may be prepared. An example of the effect of solvent and temperature is provided by Tris-HCl buffer a solution of this at pH 8.0 in water at 20 C will be pH 10.5 in 50% (v/v) ethanediol at -40 C (Douzou et al, 1976). On the other hand, neutral buffers such as phosphate undergo... 

Finally, we note that all transfers to alcohol-water mixtures or additions of alcohol to crystal mother liquor involve changes in the proton activity of the solution. Care must be taken to ensure that the pH does not change too much, or the crystal may be disrupted. Worse still, the enzymatic activity may be abolished. Control of proton activity in mixed solvents is discussed in Section III,D. If dielectric effects are controlled and pH is properly adjusted, the microenvironment of a crystalline protein will correspond closely to that of aqueous solution at room temperature. Such correspondence is essential for temporal resolution of individual steps in a catalytic reaction. 

Acid-base equilibria are quite sensitive to the presence of any organic solvent, but since the interpretation of measured pH values is limited to pure water solution, it has been both necessary to define a proton activity (termed pan) for any mixture and to use conveniently modified electrodes to carry out the potentiometric determinations of pan in the range of normal and subzero temperatures. 

The potentiometric determinations have been achieved by the use of a glass-calomel electrode assembly described elsewhere (Lanoque et ai, 1976). The potential of a glass electrode, reversible with respect to H, immersed in a solution whose proton activity is ah is given by... 

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