Structural Biochemistry/Proteins/Purification/Dialysis

This technique is not usable because it takes time and not work well, not good for big molecules. Thus, it is not popular technique to use.

Dialysis
In dialysis a semipermeable membrane is used to separate small molecules and protein based upon their size. A dialysis bag made of a semipermeable membrane (cellulose) and has small pores. The bag is filled with a concentrated solution containing proteins. Molecules that are small enough to pass through the pores of the membrane diffuse out of the bag into the buffer solution, or dialysate. Dialysis is sometimes used to change buffers. The molecules go from an area of high concentration to low concentration. When the level of concentration is equal between the bag and the buffer, there is no more net movement of molecules. The bag is taken out and inserted into another buffer, causing the concentration to be higher in the bag relative to the buffer. This causes more diffusion of molecules. This process is repeated several times to ensure that all or most of the unwanted small molecules are removed (usually done overnight). In general, dialysis is not a means of separating proteins, but is a method used to remove small molecules such as salts. At equilibrium, larger molecules that are unable to pass through the membrane remain inside the dialysis bag while much of the small molecules have diffused out.



Daily Application
The technique of dialysis is used in everyday life for hospital usages. Dialysis mimics one of the functions of a bodily organ, the kidneys. It is used in procedures to filter out the blood's toxins and waste products during kidney failure. During kidney failure, there is a build up of nitrogen-containing waste products (such as urea or creatine) in the body called azotemia, which can be detected from the blood. Patients result to a dialysis when the waste product accumulates on the blood causes metabolic acidosis leading to illness. Two tests are executed through a blood sample and a full day's worth of urine sample. There are two chemicals in the blood that are measured, the urea nitrogen level and the creatinine level. If these two chemicals are found to be high in the blood, then it is an indication that the kidneys are not cleansing bodily waste products efficiently. Certain solutes such as potassium and calcium are carefully calibrated at a concentration similar to the concentration of healthy blood. Another solute is Sodium Bicarbonate which is used as a pH buffer introduced by elevating the solute concentration within the dialysis to neutralize some of the metabolic acidosis occurring within the blood.

creatinine

urea