Show description

7), the predominant species of an amino acid is negatively charged and migrates toward the anode. The isoelectric point (pI) of an amino acid is the pH at which the molecule has an average net charge of zero and therefore does not migrate in an electric field. The pI is calculated by averaging the pKЈ values for the two functional groups that react as the zwitterion becomes alternately a monovalent cation or a monovalent anion. , glycine and alanine, exist as zwitterions. 7) is protonated to yield an ammonium group (—NHϩ 4 ).

0] is the pI. Thus, pI is the arithmetic mean of pK1Ј and pKЈ2 values and the inflection point between the two segments of the titration profile. The buffering capacities of weak acids and weak bases are maximal at their pKЈ values. 7 (Figure 3-6). Neither these amino acids, nor the α-amino or α-carboxyl groups of other amino acids (which have similar pKЈ values) have significant buffering capacity in the neutral (physiological) pH range. 3). The pK and pI values of selected amino acids are listed in Table 3-2.

A number of processes in living systems result in and depend on the transfer of electrons (oxidation and reduction), and thus their thermodynamic properties are related to their electrical properties and measurement of these electrical properties. Oxidation is the loss of electrons by an atom, ion or hydride (HϪ) ion by a molecule. Reduction is the gain of electrons by an atom, ion or proton (Hϩ) ion by a molecule. In a chemical reaction the transfer of one hydride ion results in the transfer of two electrons; HϪ Hϩ ϩ 2eϪ.