Amino Acids Have Both Acid and Base Properties

Amino Acids Have Both Acid and Base Properties 

The charge properties of amino acids are very important in determining the reactivity of certain amino acid side chains and in the properties they confer on proteins. The charge properties of amino acids in aqueous solution may best be considered under the general treatment of acid-base ionization theory. We find this treatment useful at other points in the text as well. 

Recall that water can be considered a weak acid (or a weak base) because it dissociates into a proton and a hydroxide ion, according to the equilibrium 

Because water dissociates to such a small extent, the concentration of undissociated water is high and does not vary significantly for chemical reactions in aqueous solution. Therefore, the denominator in this equation is effectively constant, with a value of 55.5. The constant Kw for the dissociation of water is redefined by the expression 

In pure water we expect equal amounts of H+ ( hydrogen ion ) and OH- ( hydroxide ion ). From equation (3) we can calculate the concentration of H+ or OH in pure water to be 10 7 M. Therefore, a solution with an H+ concentration of 10-7 M is defined as neutral. An H+ concentration greater than IO-7 M indicates an acidic solution an H+ concentration less than IO-7 M indicates a basic solution. Rather than deal with exponentials, it is convenient to express the H+ concentration on a pH scale, the term pH being defined by the equation 

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