pKa Values Express the Strengths of Weak Acids

The acid strengths of weak acids are expressed as their pK_a. For molecules with multiple dissociable protons, the pK_a for each acidic group is designated by replacing the subscript "a" with a number (Table 3–1). The imidazole group of histidine and the guanidino group of arginine exist as resonance hybrids with positive charge distributed between both nitrogens (histidine) or all three nitrogens (arginine) (Figure 3–2). The net charge on an amino acid—the algebraic sum of all the positively and negatively charged groups present—depends upon the pK_a values of its functional groups and on the pH of the surrounding medium. Altering the charge on amino acids and their derivatives by varying the pH facilitates the physical separation of amino acids, peptides, and proteins