Mechanism of DNA (Southern) and protein (Western) blotting on cellulose nitrate and other membranes.

The transfer of DNA fractions from hydrophilic gels to nitrocellulose membranes (Southern blotting) which was soon followed by the description of an analogous procedure for RNA (Northern blotting), and somewhat later for proteins (Western blotting), has rapidly become an important separation and characterization method in molecular biology, genetic engineering, and immunological detection. Surface tension measurements have shown that the interfacial attraction between DNA and cellulose esters (-delta G132) in aqueous media can be considerable. The weaker binding energy of proteins to cellulose nitrate and to cellulose acetate may be compared to hydrophobic interaction chromatography, as on account of the somewhat lower [-delta G132] values, it often is necessary to "fix" them more tightly onto nitrocellulose by using high salt concentrations. The binding energy of RNA to both cellulose esters also is rather low. In addition to the effect of high ionic strength, the effect of adding methanol, and the effects of denaturation, heating and drying on the energy of attachment of the biopolymers to cellulose esters, have been studied. Cationized nylon membranes have been advocated recently, especially for electrophoretic transfer of nucleic acids (in which process high salt concentrations cannot easily be used). With positively charged nylon membranes, the attachment mainly occurs through the electrostatic attraction between the strongly negatively charged nucleic acids (or proteins) and the positively charged membrane. Also, more apolar membranes (of polyvinyl difluoride) have been proposed, which manifest a strong interfacial (hydrophobic) attraction to all the above biopolymers (regardless of their electrostatic charge). However, with these two novel membrane types it is no longer possible to exploit the large difference in binding energy between DNA and RNA, which makes cellulose nitrate membranes so uniquely suited for RNA-DNA hybridization assays.