(Figure ). At very low pH, the -NH2 and -COO -groups are protonated, resulting in an overall +1 charge on the molecule. For simplicity, this protonated form is denoted as H2A + . As the pH increases, the carboxyl group, which has the lower pKₐ (higher acidity), begins to lose its proton, establishing the first equilibrium between the H2A + and HA forms. According to the Henderson-Hasselbalch equation, when the pH equals pK a 1 the H2A + and HA concentrations are equal. As the pH continues to rise, the HA form becomes predominant. At the pI, virtually all glycine exists as the zwitterionic form (HA), where the molecule has no net charge.