One Free Cysteine Is Enough to Give You Dimers

Two residues make humanin difficult to keep in a single defined state: an exposed N-terminal methionine and one cysteine with no partner. The peptide itself is 24 residues long and is encoded in mitochondrial rather than nuclear DNA.

What the molecule is

The sequence runs Met-Ala-Pro-Arg-Gly-Phe-Ser-Cys-Leu-Leu-Leu-Leu-Thr-Ser-Glu-Ile-Asp-Leu-Pro-Val-Lys-Arg-Arg-Ala – twenty-four residues, both termini free. Monoisotopic mass is 2685.48 Da, average mass roughly 2687.3 Da, CAS 330936-69-1.

The literature calls it a mitochondria-derived peptide because its open reading frame lies inside the mitochondrial genome instead of in nuclear DNA. MOTS-c falls into the same structural category, although the two peptides differ in length and share no sequence similarity whatsoever.

Cysteine 8 has no partner

Position eight holds the molecule’s only cysteine, and nothing inside the molecule is available to pair with it. Left as a free thiol in aerated aqueous solution it oxidizes, and the usual product is a disulfide-linked dimer of two humanin molecules.

Take twice the parent mass, subtract two hydrogens, and the dimer lands near 5369 daltons. Chromatographically it forms its own later-eluting peak. By mass spectrometry it is unmistakable once the charge envelope has been deconvoluted, though in raw form its multiply-charged ions can overlap confusingly with the monomer’s.

The upshot is that a purity value for humanin depends on when it was measured, in a way that a value for a cysteine-free peptide does not. Dimer content on receipt and dimer content after a month in solution are two different numbers, and neither of them is wrong.

Methionine 1 is a separate problem

The N-terminal methionine supplies the second liability. Oxidizing to the sulfoxide adds sixteen daltons and pulls retention earlier, and because the residue sits terminal and exposed it oxidizes more readily than a buried methionine would.

When a sample shows both a plus-sixteen species and a dimer, two separate oxidation processes are running, not one. The controls differ too: methionine responds to exclusion of oxygen and light, while cysteine needs that plus freedom from trace metal catalysts and, in some preparations, a reducing additive.

Four leucines in a row

Positions nine through twelve are consecutive leucines – an unusually hydrophobic stretch for a peptide of this size. It produces two effects: reversed-phase retention stronger than overall composition would predict, and a tendency to aggregate in aqueous solution as concentration rises.

Aggregation presents as a broadened or fronting main peak rather than as an extra species, which makes it tempting to blame the column. Dilute and re-inject to settle the question – peak shape that tracks concentration belongs to the peptide, not the hardware.

Detection and the charge envelope

A single phenylalanine, with no tryptophan or tyrosine anywhere, rules out 280 nm. Detection happens at 214 nm.

At 2687 daltons the electrospray envelope usually spans two to four protons, the triply charged ion falling near m/z 896.8. The readable form is a deconvoluted neutral mass, and at this size that should be compared against the average mass rather than the monoisotopic one, unless the instrument actually resolves the isotope cluster.

S14G, and the 30 dalton test

One analog is studied widely enough to appear alongside the parent: S14G-humanin, also written HNG, in which glycine replaces the serine at position fourteen. Serine at 87.03 giving way to glycine at 57.02 shifts the mass by 30 daltons.

That 30 dalton gap is what distinguishes the two. They are separate compounds, and any document that names one while quoting the other’s mass contains an error somewhere. The variant also turns up listed as HN plus a letter, a form that conveys nothing on its own.

What a confirmed lot shows

A deconvoluted mass near 2687 Da; a 214 nm chromatogram in which the dimer peak is either absent or quantified; and a stated figure for oxidized species. Storage form carries more weight for this peptide than for most – lyophilized material with the cysteine reduced is a defined thing, whereas a standing solution is a mixture whose composition keeps moving.

The other mitochondria-derived peptide

Beyond a shared origin in the mitochondrial genome, humanin and MOTS-c have nothing in common – different lengths, different sequences, different analytical headaches. For MOTS-c, see MOTS-c as a mitochondria-derived peptide and the mitochondrial peptide research overview.