Acid-base activity, changed

It is a well-known fact that upon covalent immobilization at the surface, the dissociation constant of the acid-base indicator changes by as much as 3 pK units. This shift clearly illustrates the dramatic effect that the interphase has on the ionization equilibria. It is perhaps the most serious problem with optical sensors that the surface concentration of any species is related to its corresponding bulk activity value through an adsorption isotherm which, with the exception of Henry s law, is a highly nonlinear and variable relationship. It is also known (Davies and Rideal, 1963) that the surface pH is different from the bulk value due to the electrostatic repulsion. 

Changes in acid-base activity. Water bound to a metal ion frequently is more acidic than free water, and coordination to proteins enhances this effect still more. This results in M — OH species that can then react with other substrates. Mg and Zn are common metal ions that serve this function. 

Figure 4. Model 1 from Acid-Base Equilibrium Changes in Ocean pH activity. Graph adapted from (17).

Poly(L-malate) decomposes spontaneously to L-ma-late by ester hydrolysis [2,4,5]. Hydrolytic degradation of the polymer sodium salt at pH 7.0 and 37°C results in a random cleavage of the polymer, the molecular mass decreasing by 50% after a period of 10 h [2]. The rate of hydrolysis is accelerated in acidic and alkaline solutions. This was first noted by changes in the activity of the polymer to inhibit DNA polymerase a of P. polycephalum [4]. The explanation of this phenomenon was that the degradation was slowest between pH 5-9 (Fig. 2) as would be expected if it were acid/base-catalyzed. In choosing a buffer, one should be aware of specific buffer catalysis. We found that the polymer was more stable in phosphate buffer than in Tris/HCl-buffer. 

Compounds absorbed by active uptake mechanisms (e.g., glucose and Gly-Pro) and compounds known to be substrates for efflux transport (e.g., digoxin, verapamil) were also included in the list. The applied concentration (10-500 pM) only had minor effects on the permeability values. Thus, the choice of concentration was not critical for this set of compounds with respect to the relationship between permeability and fraction absorbed in humans. Changing the pH on the apical donor side had significant effects on the Papp values of several compounds, the effects being in agreement with the acid-base properties of the compounds. The... 

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