Buffers for Peptide Solutions, and When Water Is the Right Answer

Water dissolves a peptide. A buffer holds the solution at a chosen pH while something else tries to change it. Those are different jobs, and the second one is only worth doing when you know which pH you want and why.

What a buffer actually does

A buffer is a weak acid and its conjugate base in solution together. When acid is added, the base takes it up; when base is added, the acid releases a proton. The pH moves far less than it otherwise would.

The capacity to do that is greatest when the two forms are present in roughly equal amounts, which happens when the pH equals the buffer’s pK_a. Move more than about one pH unit away and the capacity falls off sharply.

**So a buffer is only useful within roughly ±1 pH unit of its pK_a.** Choosing a buffer is choosing a pK_a near the pH you want, and a buffer used well outside its range is doing almost nothing while still contributing ionic strength and absorbance.

Choosing the pH before choosing the buffer

For a peptide, two considerations usually decide it.

Solubility. Solubility is at a minimum near the isoelectric point, where the molecule carries no net charge and has least reason to stay solvated. Working a unit or more away from it in either direction is the straightforward way to keep a peptide in solution, and the calculation is in isoelectric point and solubility.

Stability. The main chemical degradation routes are pH-dependent and they do not all point the same way. Deamidation accelerates above neutral. Disulfide scrambling accelerates above neutral. Aspartate-linked isomerisation is fastest in mildly acidic to neutral conditions. Backbone hydrolysis accelerates at both extremes.

For most peptides the practical compromise is mildly acidic — around pH 4 to 6 — which is slow for deamidation and scrambling, and far enough from the extremes to avoid hydrolysis.

The common buffers and what each one brings

• Acetate, pK_a 4.76. Covers the mildly acidic range where many peptides are most stable. Volatile, so it can be removed by lyophilisation — which makes it the usual choice when the solution will be dried again.
• Phosphate, pK_a values at 2.1, 7.2 and 12.3. The 7.2 value makes it the standard near-neutral buffer. It is non-volatile, so it stays behind on drying, and it **precipitates with divalent metals** — a real problem for any metal-containing compound, and the reason a copper complex should not meet a phosphate buffer casually.
• Tris, pK_a 8.1. Covers mildly alkaline conditions. Its pK_a shifts substantially with temperature — roughly 0.03 units per degree — so a solution adjusted at room temperature is at a different pH in a refrigerator. It also carries a primary amine, which reacts with some labelling chemistries.
• HEPES and the other sulfonic-acid buffers, pK_a near 7.5. Designed to avoid the problems above: little metal binding, small temperature coefficient, no reactive amine. They are non-volatile and absorb in the low ultraviolet.
• Ammonium acetate and ammonium bicarbonate. Volatile, removable by lyophilisation, and compatible with mass spectrometry — which is what makes them the buffers of choice when the sample is going into an instrument.

Ionic strength is a separate decision

Buffer concentration sets the buffering capacity. Total salt concentration sets the ionic strength, and that has its own effects: it screens charges, which reduces electrostatic repulsion between molecules and can therefore **promote** aggregation in a peptide that was being kept apart by its own charge.

It also reduces the adsorption of cationic peptides to glass and plastic, which works in the other direction. There is no universal answer; the point is that adding salt is a change with consequences, not a neutral act.

What a buffer costs you

• Ultraviolet absorbance. Most buffers absorb below about 220 nm, so a concentration measurement at 214 nm or a chromatogram at that wavelength inherits a raised baseline.
• Residue on drying. A non-volatile buffer stays in the vial. For a solution that will be lyophilised, that can easily be more mass than the peptide.
• Mass-spectrometry suppression. Non-volatile salts suppress electrospray ionisation badly, which is why a sample for MS is prepared in a volatile buffer or desalted first.
• Chromatographic incompatibility. Phosphate is incompatible with evaporative and charged-aerosol detectors, which need everything in the mobile phase to be volatile.

When plain water is the right answer

Often. If the solution is being made up, used and discarded within a short period, and nothing in it is generating or consuming protons, there is nothing for a buffer to defend against.

The exception worth knowing: unbuffered water is not at pH 7. Dissolved atmospheric carbon dioxide takes purified water to somewhere around pH 5.5, and a peptide with acidic or basic groups moves it further. So “dissolved in water” describes the solvent, not the pH, and the choice of dissolution solvent is discussed in choosing a reconstitution solvent.

Recording it

A stored solution’s buffer, concentration, pH and date belong on its label. The recurring failure is an aliquot in a freezer with a compound name and nothing else, whose behaviour then cannot be compared to anything. The practice is covered in inventory labels and records.