The pH a peptide sits in decides which of its degradation routes runs fastest, and the right choice is rarely neutral. Neutral water is the default because it is the default, not because it is safe. Deamidation and disulphide scrambling accelerate above pH 7; acid-labile bonds cleave below pH 4; the thiol on a cysteine is quiet at pH 5 and reactive at pH 8. Choosing a buffer is choosing which chemistry to slow down, and it follows from the sequence.
What pH does to each route
| Degradation route | Fastest at | Slowest at | Residues involved |
|---|---|---|---|
| Deamidation | pH > 7 | pH 4–6 | Asn, Gln (esp. Asn-Gly) |
| Thiol oxidation, disulphide exchange | pH > 7.5 | pH < 6 | Cys |
| Acid hydrolysis | pH < 3 | pH 5–7 | Asp-Pro, Asp-X |
| Metal-catalysed oxidation | Neutral to basic | Mildly acidic | Met, Trp, His |
| Aggregation | Near the isoelectric point | Away from it | Whole sequence |
The pattern is that mildly acidic — pH 4 to 6 — is the quietest region for most short peptides. It is not universally right, and the exceptions are why the sequence has to be read first.
Why is mildly acidic usually the safest?
Because three of the five routes above are base-catalysed. Deamidation proceeds through a cyclic intermediate that forms when the backbone nitrogen is deprotonated, which needs basic conditions. Thiol chemistry runs through the thiolate anion, which appears above pH 7. Metal-catalysed oxidation is faster when the metal is bound to a deprotonated site. Drop the pH a couple of units and all three slow down together. The cost is acid hydrolysis, which only becomes significant below about pH 3, so the window between 4 and 6 gets most of the benefit at little cost. How long a solution lasts is largely a function of where in this table it was kept.
When is acidic the wrong choice?
Two cases. A sequence with an Asp-Pro bond is acid-labile and should not sit below pH 4 for long — the bond cleaves, and the peptide is cut in two. And a peptide whose isoelectric point falls in the acidic range will be at its least soluble there: an acidic sequence such as DSIP, with two acidic residues and no basic ones, carries no net charge near pH 4 and aggregates rather than dissolving. For those, neutral or slightly basic is the working choice, and the base-catalysed routes are accepted as the price. Counting the charges is what tells you which case you are in.
Choosing the buffer itself
The pH is one decision; what holds it there is another. Buffers are not inert.
Acetate buffers pH 3.6 to 5.6 and is the usual choice for the mildly acidic window. It is volatile, which makes it compatible with lyophilisation and mass spectrometry, and it introduces nothing that interferes with most assays.
Phosphate buffers around neutral and is the default for cell work because it is what cells expect. It also chelates and precipitates divalent metals, which is a problem for a copper complex and a feature for anything oxidation-prone.
Tris buffers pH 7 to 9, has a primary amine that can react with aldehydes and some coupling chemistries, and shifts pH markedly with temperature — a Tris buffer made at room temperature is a different pH cold.
Plain water is not a buffer. Its pH is whatever the dissolved peptide, its counter-ion and the dissolved carbon dioxide make it, which for a trifluoroacetate salt is usually mildly acidic by accident rather than design.
Does the counter-ion set the pH?
Partly, and unpredictably. A peptide supplied as a trifluoroacetate salt brings TFA with it, and dissolving it in unbuffered water gives a solution whose pH depends on how much TFA came along — which varies by lot. Two vials of the same compound reconstituted in water can differ by a pH unit. That is a reason to buffer rather than to rely on water, and it is one of the less obvious consequences of the counter-ion being inherited from purification.
What about the buffer's ionic strength?
It matters for solubility and for adsorption, in opposite directions. Higher salt screens the charges that keep peptide molecules apart, which can drive aggregation for sequences near their isoelectric point, and it also screens the electrostatic component of binding to a tube wall, which reduces losses for basic peptides. A stock is usually made at low ionic strength for solubility; a working dilution often ends up in physiological salt because the assay demands it. The transition between the two is where a marginal sequence precipitates, and it is worth making that transition as the last step rather than the first.
How should the pH be recorded?
As the buffer, its concentration and its measured pH, not as "PBS" or "water". A stock described as "10 mM sodium acetate, pH 5.0" can be reproduced; one described as "water" cannot, because the pH of that water was set by the lot's counter-ion burden. For anything that will be reported or repeated, the buffer line in the methods is as consequential as the concentration line, and the calculator that produced the concentration has no field for it.
Does the buffer need to be sterile?
For anything that will be entered more than once or kept for more than a day, yes, and buffers are a common route for contamination precisely because they are not thought of as a reagent. A stock buffer made weeks ago and drawn from repeatedly has had the same exposures as any multi-entry vial. Filtering through 0.22 µm at the time of use removes organisms; it does not remove endotoxin, which for cell work is the concern. Buffers for anything cell-based are best made from pharmaceutical-grade water and filtered fresh.
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