5x loading buffer western blot preparation: what each component of laemmli buffer does and why laemmli buffer western blot recipes differ, how a western blot loading buffer differs from the western blot running buffer, from running buffer western blot recipes, from the western blot transfer buffer, from transfer buffer western blot and transfer buffer for western blot work, and from the western blot buffer someone means when they say it without qualification, when to reduce, and why boiling some samples destroys the band

What each component does

1. SDS: the uniform negative charge. Sodium dodecyl sulphate coats the unfolded polypeptide with negative charge roughly in proportion to length, which is what makes migration a function of size. Too little and the separation is not by size; far too much and the front distorts.
2. Tris at pH 6.8: the stacking buffer. The sample buffer matches the stacking gel so the sample concentrates into a sharp band before it enters the resolving gel. A sample buffer at the wrong pH gives broad, smeared bands that look like a transfer problem.
3. Glycerol: density so the sample stays in the well. Glycerol makes the sample dense enough to layer under the running buffer. Concentrated stock is viscous, which is the practical reason a 5x buffer is harder to pipette accurately than a 2x one.
4. Bromophenol blue: a visible front. The dye tracks the ion front so you can see the run and stop it before the smallest proteins leave the gel. It is also the fastest way to spot a well that did not load.
5. A reducing agent, added fresh. A thiol reagent breaks disulphide bonds so the protein runs as a single chain. It oxidises in storage, which is why it is added to the aliquot on the day rather than kept in the stock. A stored complete buffer is the commonest reason a sample stops reducing properly.

Buying it against making it

A commercial buffer is consistent between lots, carries a safety data sheet and removes a weighing step from a busy day. Making it is cheaper and lets you adjust the SDS and reducing agent for an awkward sample. Most laboratories do both and use the commercial product as the reference when something stops working.

If you make it, prepare a large batch, aliquot it, and record the date. Small aliquots thawed once avoid both the freeze-thaw cycles and the temptation to top up an old tube.

Concentration and loading

A 5x stock leaves more room for sample volume than a 2x, which matters when the lysate is dilute. It is also viscous enough that pipetting accuracy suffers, so mix thoroughly and give the tip time to drain.

Load equal protein, not equal volume, and quantify the lysate before the buffer goes in. Detergent in the sample interferes with several protein assays, which is why quantification happens first.

Handling the reagents safely

SDS is a respiratory irritant as a powder and should be weighed in a hood or bought as a solution. Thiol reducing agents are odorous and toxic, and belong in a fume hood at every stage.

Every component needs its safety data sheet on file and its container labelled under the hazard communication rule, including buffers made in-house and decanted into a working bottle.

Common questions

How do I make 5x loading buffer for a western blot?

Tris-HCl at pH 6.8, SDS, glycerol and bromophenol blue as a 5x stock, stored in aliquots at minus twenty. The reducing agent goes in on the day of use, because it oxidises in the freezer.

Should I always boil samples?

No. Ninety-five degrees for five minutes suits most soluble proteins. Multi-pass membrane proteins aggregate when boiled and often run better at thirty-seven or seventy degrees, which is a change worth testing before you conclude a protein is not expressed.

Reducing or non-reducing?

Reducing for most work. Non-reducing where the antibody recognises a conformational epitope or where you want to see the assembled complex, in which case the sample must not be heated hard either.

Why are my bands smiling or smeared?

Usually too much protein in the well, or a sample buffer whose pH or salt content has drifted. High salt samples distort the field locally, and diluting into fresh buffer rather than adding concentrated stock to a salty sample fixes most of it.

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