In biochemical testing, when we select buffers Tris, MOPS, CAPS, etc., the first consideration is the dissociation constant pKa
and buffer range of the buffer. In addition, the salt effect of the detection system cannot be ignored, and it will also affect
the experimental results. Make an impact.
Tromethamine Tris buffer
Most biological systems are affected by salt. In addition to meeting the pH range required by the system, the buffer selected for
detection should also ensure that the salt effect is as small as possible. In special circumstances, salt will be added during
the experiment to make adjustments to increase the salt effect. Buffers such as Tris, Bicine, MOPS, etc. are usually composed of
conjugate acid-base pairs, which are weak electrolytes. If the system contains strong electrolytes (strong acids, strong bases)
and weak electrolytes come into contact with them, salt effects will occur. When the solubility decreases, it is the saltingout
effect; when the solubility is reduced, it is the saltingout effect (saltingin).
Definition of salt effect:
When a strong electrolyte that does not have the same ions as the weak electrolyte is added to the weak electrolyte solution, the
total concentration of ions in the solution increases, and the interaction between ions is enhanced, which reduces the chance
of the weak electrolyte dissociated anion and cation from combining to form molecules. , So that the concentration of weak
electrolyte molecules decreases, the ion concentration increases correspondingly, and the dissociation degree increases. This
effect is called the salt effect.
For example, Tris, MOPS, HEPES, etc. are weak electrolytes, while the hydrochloric acid and sodium hydroxide used for buffer
pH adjustment are strong electrolytes. If the concentration of the latter is too high, it may increase the dissociation degree
of the buffer and affect the buffer performance. pH balance. Therefore, in some experiments, Tris acetic acid or Tris glycine
is used instead of Tris hydrochloric acid. Acetic acid and glycine are both weak electrolytes, and hydrochloric acid is a strong
electrolyte.
Adding 0.1 mol/L NaCl solution to 0.1 mol/L acetic acid solution, sodium chloride is completely ionized into sodium ions and
chloride ions, so that the total number of ions in the solution increases sharply, and the electrostatic interaction between
the ions is enhanced. At this time, acetate ions and hydrogen ions are surrounded by many different-sign ions (sodium ions,
chloride ions). The chance of combining acetate ions and hydrogen ions to form acetic acid is reduced, which increases the
ionization degree of acetic acid (from 1.34% to 1.34%). 1.68%).
Now there are a lot of Tris buffer systems, such as Tris glycine, Tricine, Bis-Tris, etc., in addition to meeting the required
pH of the system, it can also meet the appropriate salt effect of different detection systems.
