Transfer buffers and membrane choice

Transfer is the step that copies a protein pattern out of a gel and onto a sheet you can probe. The buffer and the membrane decide whether that copy keeps the large proteins, the small ones, or neither. This page compares the common classes. The path from a finished gel through blocking and antibody is western blot from gel to membrane. DNA on agarose is a different matrix, covered in agarose gel electrophoresis for DNA. The comparison here is research method choice, not a clinical immunoassay and not an instrument manual.

Membranes, buffer reagents and general molecular supplies are enquiry classes in the molecular biology catalogue. Name the protein size and the transfer class on the quote request.

The decision the buffer and the sheet are making

You already have an SDS gel and a ladder you can see. The next decision is which proteins must still be on the sheet when the field stops. A 150 kilodalton protein and a 12 kilodalton protein do not want the same time, the same methanol, or the same pore. UniProt is a public place to confirm the mass you think you are moving, including processing that changes it. The gel's apparent mass still wins as the thing you actually have to elute.

Write the constraint in one line: keep a small protein from passing through, or give a large protein time to leave the gel, or do both by changing pore and time rather than hoping one default programme covers the lane.

Three transfer classes

Wet transfer submerges a cassette. The gel and the membrane are sandwiched with wetted papers and pads, and the tank holds a large volume of buffer. Runs are often longer. Large proteins are the usual reason to choose it, because you can give them time and you can often keep the stack cooler. The field has a direction. Reverse the cassette and the proteins travel into the paper you thought was padding.

Semi-dry transfer presses the gel and membrane between plate electrodes with buffer-wetted filter papers and much less liquid. It is faster and uses less buffer. A short programme that is kind to a small protein can leave a large one in the gel. The stack dries at the edges if the papers were barely wet, and a dry patch is a blank patch. It also heats. A warm stack is a reason to shorten or to cool, not a reason to ignore a ladder that only partly moved.

Rapid or fast-transfer systems are a third class. They use a dedicated stack and a programme measured in minutes to about an hour, depending on the instrument. Follow that instrument for the size window it claims. A rapid programme is not a universal field you can copy onto a wet tank, and a wet-tank time is not a setting you type into a rapid device. The consumable stack is part of the method. Substituting loose filter paper because it looks similar is how those runs fail.

Nitrocellulose and PVDF

Nitrocellulose wets in aqueous buffer. It is often enough for abundant proteins, and it is mechanically more fragile when dry. PVDF is tougher and binds more protein per area. It typically needs a brief activation in methanol, or the solvent the sheet specifies, before aqueous buffer will wet it. After activation, do not let it dry before the sandwich is built. A dried PVDF surface can refuse buffer until you activate it again. The western guide's warning stands: an inactivated sheet looks white and stays empty, and that is a transfer failure.

Pore size is the other half of the sheet. A 0.45 micrometre pore is the general choice. A 0.2 micrometre pore is the better choice for small proteins that would otherwise pass through. The test is empirical and simple in concept: a second membrane behind the first, or a total-protein stain that shows protein on the back sheet and not the front, means the protein went through. Neither pore identifies the antigen. Both only decide whether a polypeptide of that size had a chance to stay.

Methanol in a Towbin-style buffer

A Towbin-style buffer is Tris and glycine, and it often includes methanol. Methanol helps many proteins bind to the membrane. It also shrinks the gel slightly and can slow the elution of very large proteins, so the same bottle that saves a 40 kilodalton band can leave a 200 kilodalton band behind. For those large proteins, laboratories sometimes lower the methanol or add a small amount of SDS so the protein leaves the gel, accepting that SDS can make binding worse. Follow the membrane and the instrument rather than a single percentage remembered from a different protein.

Other buffer classes exist for special cases, including higher-pH carbonate-style systems sometimes used for difficult proteins. They are a different recipe family. Do not mix the salts of one family into the habits of another and still call it Towbin.

The photograph on this page is not a blotting tank. It is a pH electrode standing in a beaker of buffer while a gloved hand adds drops from a dropper bottle. That is a buffer check, and transfer buffer deserves one. How to calibrate and read an electrode is preparing a buffer and checking pH. Use the dropper when the method says the buffer is adjusted to a set-point. For a classical Towbin-style mix, measure to confirm you landed in the region that mix is supposed to produce. Titrate it with strong acid only if your SOP says that mix is meant to be adjusted. Extra acid is extra salt, and it changes the run.

Methanol is flammable and toxic. Prepare the buffer in the chemical practice your institution already set. Do not store an open flask of it beside a heat block.

A workflow that branches on size

Equilibrate the gel briefly in transfer buffer if your method asks, so the salt shock is smaller. Activate PVDF if that is the sheet. Cut membrane and papers with clean tools. Gloves matter, because skin protein becomes a lane. Build the sandwich in the orientation the device specifies, roll out air, and mark the side that faced the gel. A bubble is a blank oval later. It is easier to prevent than to interpret.

Place a prestained ladder in a lane you can watch. If the colours never leave the gel, the field, the time, or the orientation failed. If they pass into the papers beyond the membrane, small proteins may have followed them. Stain the membrane for total protein before you celebrate an antibody. Ponceau and related reversible stains show arrival. They do not name the target.

Then branch. Large target still in the gel: more time, less methanol, or a wet tank, one change at a time. Small target missing from the membrane and present on a back sheet: shorter time or a 0.2 micrometre pore. Ladder transferred and sample lanes blank: the load or the sample never entered the gel, which is a gel problem, not a membrane brand problem.

Failure modes that belong to buffer or sheet

High background later is usually block, antibody, or a membrane that dried between steps. A blank oval in every stain is a bubble from the sandwich. A gradient of signal from one edge is often uneven contact or a semi-dry stack that was wetter on one side. Colours from a prestained ladder that transferred backwards tell you the field direction before you blame the primary antibody.

A beautiful membrane with no large-protein colour and a gel that still holds those bands is an elution failure. More time or a buffer kinder to large proteins is the branch. A membrane with no small-protein colour and a back sheet that has it is a pore or time failure. Do not order a new antibody until one of those two checks has been done.

Safety and research limits

Methanol, SDS if you add it, and the gel's acrylamide history are chemical hazards. Power supplies for wet tanks are an electrical hazard. Keep the tank closed and the bench dry. Biological lysates follow the biosafety level of the cells. The WHO Laboratory biosafety manual, 4th edition is background, not permission to transfer a particular organism.

A successful transfer supports "protein of this apparent mass arrived on this sheet." It does not prove antibody specificity, a fold change, or a clinical result.

Heat, evaporation and an open buffer flask

Methanol leaves an open flask faster in a hot room, so the buffer you measured in the morning is richer in salt and poorer in methanol by afternoon. The electrode in the photograph is how you notice a buffer that no longer matches the bottle you think you mixed, provided your SOP includes that check. Cap the flask. Do not top up with water by eye. A semi-dry stack in the same heat runs hotter than the programme assumed. If you have no cold room, prefer a shorter watched run over an unattended stack, and do not start a wet transfer you cannot finish through a power cut. A partial transfer is a partial pattern. It is not a quantitative blot.

What to put in an enquiry

State the apparent-mass window, wet versus semi-dry versus a rapid system you already own, nitrocellulose versus PVDF, and 0.45 versus 0.2 micrometre pore. Say whether the buffer must include methanol and whether large proteins are the constraint. Ask for sheet size and for the activation solvent the PVDF actually requires. The nucleic acid analysis pathway is the neighbouring route on a bench that also runs DNA gels. Ask whether a quotation is possible. A membrane described only as "for western blot" has not yet answered pore size or polymer.