Acid-Base Equilibria Are Shifted by Electrostatic Fields

Acid-Base Equilibria Are Shifted by Electrostatic Fields 

Acid-base equilibria can be shifted in the presence of electrical fields. For example, the pK of an acid group changes if the acid is near a charged surface, such as a membrane or a protein. ions will be repelled near a positively charged surface, causing an acid attached to the surface to give up its protons more easily than in the absence of the surface. This makes the acid appear to be stronger. Let s make this more quantitative. 

The subscript a on indicates that this is an acid dissociation constant (not an association constant Ki, is the dissociation constant for bases). Taking the logarithm of both sides of Equation (22.30) for Ka gives the Henderson-Hasselbalch equation. 

An additional useful relationship is Ka(x) = Ka(°°), which is shown as follows. The condition for electrochemical equilibrium is 

EXAMPLE 22.3 Histidine in a spherical protein. Histidine is a basic amino acid with pXa(°o) = 6.0. Suppose that a histidine is attached to the surface of a spherical protein of radius a = 30 A in water. Suppose that the protein has 

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