Aliquoting, freeze-thaw cycles, and adsorptive loss

Freezing is not a uniform process. As ice forms, solutes concentrate into a shrinking unfrozen fraction, so a peptide at a comfortable concentration briefly experiences a much higher one — where aggregation is favoured.

Buffer components crystallise at different points, which shifts pH in that unfrozen fraction. Sodium phosphate is the well-known example: disodium phosphate crystallises before the monosodium form, driving a substantial acidic pH shift during freezing. A peptide sensitive to acidic conditions is exposed to them at every cycle even though the buffer reads neutral at both ends.

The growing ice front also presents a large interfacial area, and interfaces promote peptide adsorption and unfolding by the same mechanism that makes foaming damaging.

Peptides adsorb to polypropylene and glass surfaces. In concentrated solution the fraction lost is negligible; in dilute solution it is not, because the amount adsorbed depends on available surface area rather than on how much is in the tube.

The result is a systematic downward bias in effective concentration that grows worse as you dilute — precisely the regime where accuracy usually matters most. Serial dilutions compound the effect at every step.

Low-binding tubes and tips reduce it. A small amount of carrier protein, typically albumin at low concentration, works by competing for adsorption sites, where the assay tolerates the addition. Minimising transfer steps and avoiding large headspace both help by reducing available surface.

Aliquot at the point of first reconstitution, into volumes matched to a single experiment. The objective is that no tube is ever thawed twice, which is only achievable if the volume corresponds to real use rather than to a convenient round number.

Slightly generous is better than slightly short. A tube that runs out mid-experiment forces a second thaw of another tube, which is the outcome the whole exercise exists to prevent.

Very small aliquots have their own cost: the surface-to-volume ratio rises, so adsorptive loss increases proportionally. Somewhere between these pressures is a sensible volume, and it is set by the experiment rather than by a rule.

Thaw on ice or at refrigerator temperature rather than at room temperature or in a warm bath. Slower thawing spends less time in the concentrated, pH-shifted intermediate state.

Once thawed, mix gently — inversion or slow pipetting, not vortexing. Concentration gradients persist after thawing and drawing from an unmixed tube gives an unrepresentative sample.

Inspect before use. Cloudiness or visible particulate after thawing indicates aggregation, and the concentration is no longer what the label claims.