The Role of Histidine-Containing Compounds on the Buffering Capacity of Muscle

A buffer is defined as a compound that in small concentrations can impart a resistance to large pH changes. Common buffers contain two substances, a conjugate base and a conjugate acid. There are acidic buffers that contain a weak acid and the salt of a weak acid (the conjugate base) and basic buffers that contain a weak base and the salt of a weak base (the conjugate acid). The combination of a conjugate base and acid can resist large changes in pH by absorbing H+ and OH-. For example, the conjugated base can absorb H+ to form the conjugate acid. Since this reaction decreases the amount of free H+ in solution, the pH change is decreased compared to a solution with no buffer. The pHs of buffered systems do change upon the addition of an acid or a base. However, this change is much smaller than seen in unbuffered systems. The amount of pH change is dependent on the ratio of conjugate base/conjugate acid. The conjugate base/conjugate acid ratio for a buffer is 1 when the pH = pKa. The ability of a buffer to resist pH change is greatest when the pH is at or near the buffer’s pKa. Figure 1 shows the ability of acetic acid to resist pH change when titrated with a strong base. As can be seen, the maximum resistance to pH change is near the pKa of acetic acid (4.76).