Deletion Sequences: The Impurity Made By The Synthesis Itself

Solid-phase peptide synthesis builds a chain one residue at a time, and each cycle has two chemical steps that must go effectively to completion: removal of the protecting group from the growing chain, then coupling of the next activated amino acid. Neither step is ever perfectly quantitative. Whatever fraction of chains fails to couple at cycle nine is capped or carried forward, and the next cycle adds residue ten to it regardless.

The result is a chain that reads correctly from both ends and is simply one residue short in the middle. It is not degraded material and it is not a different peptide. It is the target sequence with a hole in it, which is why it is described as a deletion or a des-X analogue, after the residue that is absent.

Truncation is the related but distinct case: the chain stopped growing early, so a run of residues is missing from one terminus rather than a single residue from the interior. Truncated chains usually differ from the target by enough mass and enough hydrophobicity to be separated. Single deletions frequently do not.

Why a very high per-step yield is still not enough

The arithmetic is the whole story. Full-length yield is the per-cycle efficiency raised to the number of couplings, so long sequences punish small inefficiencies severely. The figures below assume a uniform per-cycle efficiency, which real syntheses do not have, but the shape of the curve is correct.

Deletions are not spread evenly across those steps. They concentrate at sequence positions where coupling is sterically hindered or where the growing chain has aggregated on the resin: beta-branched residues such as valine, isoleucine and threonine, consecutive hindered residues, and stretches prone to forming beta-sheet on the support. A synthesis with an overall crude purity of 70% by area at 214 nm can carry most of its deletion burden at two or three specific positions.

The mass signature, and why it is the reliable one

A deletion changes the molecular mass by exactly the residue mass of the missing amino acid, which is the amino acid mass less the mass of water lost in forming the bond. Those differences are small in absolute terms but unambiguous by mass spectrometry, because they are fixed numbers.

Leucine and isoleucine share a residue mass to three decimal places, so intact mass alone cannot distinguish which of the two is absent in a sequence containing both. Resolving that requires fragmentation by tandem mass spectrometry, and the position of a deletion within the chain requires fragmentation regardless of which residue is involved. An intact mass measurement says a residue is missing; it does not say where from.

The failure mode: co-elution counted as product

Here is the specific thing that goes wrong. A des-glycine or des-alanine analogue of a 30-residue peptide differs from the target by one small, uncharged residue. Reverse-phase retention is driven by the hydrophobic surface the chain presents to a C18 stationary phase, and removing glycine changes that surface almost not at all. On a standard gradient the two species can arrive within a few hundredths of a minute of each other, or under the same peak entirely.

When that happens, the impurity is integrated into the main peak. The reported figure rises rather than falls, because area that belongs to a deletion sequence has been assigned to the target. The chromatogram looks cleaner than the material is, and nothing in the trace signals the problem: a single symmetrical peak is exactly what a good result looks like.

The countermeasures are orthogonality and mass detection. A second gradient at a different pH, or a different stationary phase, will often shift a co-eluting pair apart because the selectivity mechanism changes. Liquid chromatography coupled to a mass detector removes the ambiguity at source: two species under one ultraviolet peak still show two masses. This is the reason purity by ultraviolet area and identity by mass are reported as separate results rather than combined into one.

What a purity figure covers, and what it leaves out

• Area percent at 214 nm responds to the peptide bond, so it counts chains roughly in proportion to their length. A deletion analogue absorbs about as strongly as the target and is counted at nearly full weight when it is resolved.
• Non-peptide material, including counter-ions, residual water and salts, absorbs weakly or not at all at 214 nm and is largely absent from the figure.
• Deletion sequences that co-elute are inside the main peak, not in the impurity total.
• Nothing in a chromatographic purity figure addresses where in the sequence any impurity sits.

This is the reason a chromatographic figure and net peptide content are not the same quantity. A chromatographic result describes how the peptide-like material in the sample is distributed between peaks. It does not describe how much of the vial's mass is peptide at all.

Where we stand on this

The purity figures Aurum publishes are chromatographic figures from independent assay by reverse-phase HPLC, and identity is assayed by mass spectrometry. Those figures are HPLC purity, not net peptide content, which Aurum does not publish. Nor is a deletion-impurity profile among the specifications Aurum publishes: an orthogonal second-gradient result, and fragmentation data locating an impurity within a sequence, are not part of the published record. Where a number of that kind is required, the analysis has to be commissioned against the specific lot.