Amino acids Properties

Amino acids in aqueous solution contain weakly acidic α-carboxyl groups and weakly basic α-amino groups. In addition, each of the acidic and basic amino acids contains an ionizable group in its side chain. Thus, both free amino acids and some amino acids combined in peptide linkages can act as buffers. Acids may be defined as proton donors and bases as proton acceptors. Acids (or bases) described as weak ionize to only a limited extent. The concentration of protons ([H+]) in aqueous solution is expressed as pH, where pH = log 1/[H+] or –log [H+]. The quantitative relationship between the pH of the solution and concentration of a weak acid (HA) and its conjugate base (A−) is described by the Henderson-Hasselbalch equation.
Equation derivation
Consider the release of a proton by a weak acid represented by HA:

The salt or conjugate base, A−, is the ionized form of a weak acid. By definition, the dissociation constant of the acid, K_a, is:

[Note: The larger the K_a, the stronger the acid, because most of the HA has dissociated into H+ and A−. Conversely, the smaller the K_a, the less acid has dissociated and, therefore, the weaker the acid.] By solving for the [H+] in the above equation, taking the logarithm of both sides of the equation, multiplying both sides of the equation by −1, and substituting pH = −log [H+] and pK_a = −log K_a, we obtain the Henderson-Hasselbalch equation: