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The interpretation of chemical processes in aqueous systems requires the use of modern electronic computers, particularly in the calculation of multicomponent, multiphase equilibria. Commonly, the first concern of solution chemists and aqueous geochemists is to calculate the distribution and activities of species on the assumption that equilibrium exists throughout the aqueous phase. Species distribution can then be used in several areas of analytical and applied chemistry, e.g. to examine the availability of free and reactive ions, to test solubility hypotheses, and to determine the potential bioavailability of nutrients or toxic substances. Species distribution also forms the basis for more complex computations involving solutions which change composition by reaction with other solutions and with gases and solids. Equilibrium calculations of this type are particularly helpful in solving interpretive problems encountered in such fields as chemical and environmental engineering, geochemistry, biochemistry and aquatic ecology.This symposium demonstrates quite clearly that we depend heavily on chemical models, especially computerized models, to interpret aqueous chemical processes. Several computer programs which solve problems of simultaneous chemical equilibria are being used by a rapidly increasing number of investigators and it is necessary to review the inherent assumptions and limitations of these aqueous models. There is a temptation to use these models as ready-made interpretations 1

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The degradation of S--S bonds in 0.2 M-NaOH at 25 degrees C was studied for a series of proteins and simple aliphatic disulphide compounds, by using cathodic stripping voltammetry, ion-selective-electrode potentiometry, spectrophotometry and ultrafiltration. The disulphide bonds that dissociated in 0.2 M-NaOH were usually those that are solvent accessible and that can be reduced by mild chemical reductants. Some unexpected differences were found between similar proteins, both in the number of S--S bonds dissociated and in their rates of decomposition. Chymotrypsin has one S--S bond attacked, whereas chymotrypsinogen and trypsinogen have two. Ribonuclease A has two S--S bonds dissociated, but ribonuclease S and S-protein have three. Denaturation in 6 M-guanidine hydrochloride before alkaline digestion caused the loss of an additional S--S bond in ribonuclease A and insulin, and increased the rate of dissociation of the S--S bonds of some other proteins. The initial product of S--S bond dissociation in dilute alkali is believed to be a persulphide intermediate formed by a beta-elimination reaction. This intermediate is in mobile equilibrium with bisulphide ion, HS-, and decomposes at a mercury electrode or in acid solution to yield a stoichiometric amount of sulphide. Rate constants and equilibrium constants were measured for the equilibria between HS- and the intermediates involved in the alkaline dissociation of several proteins. Elemental sulphur was not detected in any of the protein digests. It is suggested that formation of HS- from a persulphide intermediate involves a hydrolysis reaction to yield a sulphenic acid derivative. The small polypeptides glutathione and oxytocin gave only a low yield of persulphide, and their alkaline decomposition must proceed by a mechanism different from that of the proteins.

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