Transfer buffers must enable both effective elution of proteins from the gel matrix and binding of the protein to the membrane. While different gel types and blotting applications call for different transfer buffers, the choice of buffer also depends on the membrane being used and the physical characteristics of the protein of interest. This section provides an overview of different transfer buffers, such as Tris/Glycine, CAPS, Dunn carbonate buffers and discontinuous buffer systems. It also provides general guidelines for selection of buffers and membranes according to gel types and applications.
Related Topics: Protein Blotting Methods, Protein Blotting Equipment, Membranes and Blotting Papers, and Transfer Conditions.
Transfer buffers contain a conductive, strong buffering agent (for example, Tris, CAPS, or carbonate) in order to maintain the conductivity and pH of the system during transfer. In addition, alcohol (for example, methanol or ethanol) may be included in the transfer buffer to promote binding of proteins to membranes, and SDS may be added to promote elution of proteins from gels.
Regardless of the transfer buffer selected, when preparing and using transfer buffers:
The most common transfers are from SDS-PAGE gels using the buffer systems originally described by Towbin (1979). Standard Towbin buffer contains 25 mM Tris, 192 mM glycine, pH 8.3, 20% methanol and, occasionally, 0.025–0.1% SDS.
A buffer similar in composition to the standard Towbin buffer is the Bjerrum Schafer-Nielsen buffer (48 mM Tris, 39 mM glycine, pH 9.2, 20% methanol), which was developed for use in semi-dry applications.
CAPS-based transfer buffer (10 mM CAPS, pH 11, 10% methanol) may be preferable for transfers of high molecular weight proteins (for example, >150 kD) and in cases where the glycine component of Towbin buffer may interfere with downstream protein sequencing applications.
A unique feature of semi-dry blotting is the ability to use two different buffers during transfer, known as a discontinuous buffer system. In a semi-dry transfer, the buffer reservoirs are the filter paper on either side of the gel, which are independent (discontinuous). In a discontinuous system, methanol is included in the buffer on the membrane (anode) side of the blot assembly and SDS is used on the gel (cathode) side, taking advantage of the positive effects of each buffer component. A discontinuous buffer system using a Tris-CAPS buffer can greatly increase the efficiency of protein transfer by semi-dry blotting. This system uses 60 mM Tris, 40 mM CAPS, pH 9.6, plus 15% methanol in the filter paper on the anode side and 0.1% SDS on the cathode side. Concentrated, premixed anode and cathode buffers are available for purchase. For more information about the use of a discontinuous buffer system in semi-dry transfer, see Bio-Rad bulletin 2134.
In some cases, using a carbonate buffer (10 mM NaHCO3, 3mM Na2CO3, pH 9.9, 20% methanol) may produce higher efficiency transfers and improve the ability of antibodies to recognize and bind to proteins. Carbonate buffer has also been recommended for the transfer of basic proteins (Garfin and Bers 1989).
The mobility of proteins during electrophoretic transfer from native gels will depend on the size and pI of the protein of interest relative to the pH of the buffer used.
Proteins in native gels, as well as acidic and neutral proteins, require buffers that do not contain methanol. Gels for isoelectric focusing, native PAGE, and those containing basic proteins or acid-urea may be transferred in 0.7% acetic acid. When using acetic acid for transfer, the proteins will be positively charged, so the membrane should be placed on the cathode side of the gel.
General guidelines for transfer buffer and membrane selection by gel type.
General guidelines for transfer buffer and membrane selection by application.
A Note Regarding SDS and Alcohol SDS and alcohol play opposing roles in a transfer. SDS in the gel and in the SDS-protein complexes promotes elution of the protein from the gel but inhibits binding of the protein to membranes. In cases where certain proteins are difficult to elute from the gel, SDS may be added to the transfer buffer to improve transfer. SDS in the transfer buffer decreases the binding efficiency of protein to nitrocellulose membrane; PVDF membrane can be substituted for nitrocellulose when SDS is used in the transfer buffer. Addition of SDS increases the relative current, power, and heating during transfer and may affect the antigenicity of some proteins.
Alcohol (methanol or ethanol), on the other hand, removes the SDS from SDS-protein complexes and improves the binding of protein to nitrocellulose membrane but has some negative effects on the gel itself. Alcohol may cause a reduction in pore size, precipitation of some protein, and some basic proteins to become positively charged or neutral. All of these factors will affect blotting efficiency.
Note: Only high-quality, analytical grade methanol should be used in transfer buffer; impure methanol can increase transfer buffer conductivity and result in poor transfer.
Garfin DE and Bers G (1989). Basic aspects of protein blotting. In Protein Blotting: Methodology, Research and Diagnostic Applications, B.A. Baldo et al., eds. (Basel, Switzerland: Karger), pp. 5–41.
Towbin H et al. (1979). Electrophoretic transfer of proteins from polyacrylamide gels to nitrocellulose sheets: procedure and some applications. Proc Natl Acad Sci USA 76, 4350–4354.
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