透析：基于扩散的分离方法

A semi-permeable membrane contains pores that impose a molecular weight cutoff, allowing molecules below a certain size to pass through while retaining larger biomolecules. For example, a 10k membrane generally retains molecules larger than 10 kilodaltons. However, the cutoff is not a precise boundary; membranes contain a broad range of pore sizes, so some molecules near the cutoff may be lost.

When a sample is placed in a dialysis membrane suspended in pure water, smaller molecules including buffer ions diffuse freely across the membrane into the dialysate, while larger biomolecules are retained. This process, called desalting, decreases overall buffer concentration. The dialysate can be refreshed multiple times to further displace small molecules and achieve equilibrium.

Density gradient ultracentrifugation uses small particles like sucrose or cesium chloride ions to separate complex biological samples. After separation, these gradient reagents must be removed before the collected sample can be processed. Dialysis efficiently removes these small molecules while preserving the purified biomolecules, making the sample suitable for downstream analysis.

Presoaking the membrane in dialysate makes it easier to handle and removes any preservatives that may be present. This preparation step ensures the membrane is ready for use and prevents contaminants from interfering with the dialysis process or the sample being purified.

After purification, some proteins become misfolded or denatured, losing functionality. Dialysis removes the compounds causing these structural changes by allowing them to diffuse across the semi-permeable membrane into the dialysate. This gradual removal of denaturants allows proteins to refold and regain their biological activity.

Desalting occurs when the dialysate is pure water, causing buffer concentration to decrease as small molecules diffuse out. Buffer exchange occurs when the dialysate contains other small particles; these move into the sample while original buffer components move out, effectively replacing the sample's buffer environment.

Proteins extracted from cell membranes are initially dispersed in detergent. When placed in a dialysis membrane suspended in detergent-free dialysate, the detergent molecules gradually diffuse across the semi-permeable membrane into the surrounding solution. This slow removal allows proteins and lipids to spontaneously form proteoliposomes, reconstituting functional membrane protein complexes.