The Interpretation of Hydrogen Ion

The Interpretation of Hydrogen Ion Titration Curves of Proteins Charles Tanford... 

Tanford, C. 1962. The interpretation of hydrogen ion titration curves of proteins. In Advances in Protein Chemistry, Vol. 17, C. B. Anfinsen, K. Bailey and J. T. Edsall (Editors). Academic Press, New York. 

This interpretation is confirmed by the solution behaviour of CisDAO/SDS mixtures. Solutions of these two components have been shown to be turbid and birefrlngent, and the addition of SDS to C18DA0 results in the production of filament-like structures and an increase in the bulk pH value, suggesting the formation of a new species between protonated CieDAO and SDS, which is also responsible for the surface tension lowering ( 1.) The increase in bulk pH value is then a consequence of the consumption of hydrogen ions in the production of cationic amine oxide, and the protonated amine oxide and the long chain sulfate precipitate out stoichiometrlcally. 

Under aqueous conditions, the concentration of hydrogen ion in solution is usually expressed in terms of the hydrogen ion concentration or activity, or in terms of pH units. The pH of a non-aqueous solution or co-solvent solution is more difficult to measure, since pH only relates to purely aqueous conditions. The pH of a water miscible solution can be measured with an electrode, however interpretation of the pH value must be done with care. 

Measurements made by calibration of electrodes with lUPAC aqueous RVS or PS standards to obtain pH(X) are perfectly valid. However, the interpretation of pH(X) in terms of the activity of hydrogen ion is complicated by the non zero residual liquid junction potential as well as by systematic differences between electrode pairs, principally attributable to the reference electrode. For 35%o salinity seawater (S = 0.035) calculated from pH(X) is typically 12% too low. Special seawater pH scales have been devised to overcome this problem ... 

There seem to have been only two investigations on dediazoniations in a protic solvent, where the observed products indicate that, in addition to DN + AN solvolysis, an aryne is likely to be present as a metastable intermediate. Broxton and Bunnett (1979) have found that 3-nitroanisole is formed in the dediazoniation of 2-nitroben-zenediazonium ions in methanol in the presence of methoxide ions. This has to be interpreted as a product arising from 3-nitro-l,2-benzyne as an intermediate. The occurrence of the aryne mechanism in poly (hydrogen fluoride)-pyridine mixtures, as discovered by Olah and Welch (1975), is mentioned in Section 8.2. 

Adin and Sykes S have re-examined the hydrogen-ion dependence of the V(III)- -Cr(lI) system over a broad range of H concentrations from 0.45 M down to 0.016 M. They confirm the type of acid dependence quoted by Espenson and support the interpretation given by Haim, equations (2.16) to (2.18). At 25 °C and n = 0.5 M, the experimental parameters q and r are 0.50 and 0.10, respectively, so that k = 3.57x 10 l.mole". sec"S and k Jk = 0.1 mole.P. Adin and Sykes S find no evidence for the existence of the more complex hydrogen-ion dependence originally suggested by Sykes in a re-analysis of Espenson s data °. 

Metal ion catalyzed autoxidation reactions of glutathione were found to be very similar to that of cysteine (76,77). In a systematic study, catalytic activity was found with Cu(II), Fe(II) and to a much lesser extent with Cu(I) and Ni(I). The reaction produces hydrogen peroxide, the amount of which strongly depends on the presence of various chelating molecules. It was noted that the catalysis requires some sort of complex formation between the catalyst and substrate. The formation of a radical intermediate was not ruled out, but a radical initiated chain mechanism was not necessary for the interpretation of the results (76). 

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

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