MOTS-c

What MOTS-c is, structurally

MOTS-c is a sixteen-residue peptide with a free N-terminal amine and a free C-terminal acid.

The formula reconciles exactly. Twenty-eight nitrogens comes from sixteen backbone amides, three for each of the three arginines, one for the lysine, one for the tryptophan indole and one for the glutamine side chain.

The two sulfurs are the point most likely to be misread. They are the methionines at positions one and six, not cysteines: the deposited structure shows two thioethers and no S-S bond, and the sequence contains no cysteine. So although the formula shows two sulfurs, this molecule cannot form a disulfide under any conditions and needs no reducing agent. Two sulfurs says nothing on its own about whether a peptide is cyclic.

What the methionines do instead is give two independent oxidation sites, and the tryptophan at position three adds a third. All three take up oxygen and all three add sixteen mass units, which matters more for interpretation than handling and is covered under testing.

Charge is strongly cationic: three arginines and a lysine against a single glutamate, giving a substantial net positive charge across the working range. It is water soluble, though no quantitative figure is quoted here.

Aromatic content is high for a peptide of this length: a tryptophan, two tyrosines and a phenylalanine, four rings in sixteen residues.

Reconstitution and handling

Sterile water is the default solvent, and with no metal centre present PBS or an appropriate assay buffer are equally suitable. Add diluent slowly down the inside wall of the vial, swirl gently, and let the solution clarify. Do not vortex.

No reducing agent is required or appropriate. There is no disulfide and no free thiol, so a reducing agent has nothing to act on and only adds a reactive species to the preparation.

Limit air exposure and work in subdued light. With two methionines and a tryptophan, three separate residues are vulnerable to oxidation and one is additionally photosensitive, so both precautions are specific rather than generic.

Adsorption is worth allowing for at low concentration, with low-protein-binding tubes reducing measured loss. Four positive charges make untreated glass a likelier route than polypropylene, though four aromatic rings give a hydrophobic route too.

This is laboratory preparation chemistry, not dosing, administration, or protocol guidance of any kind.

Storage and stability

Store the lyophilized powder at -20 degrees C, sealed, desiccated and protected from light, and let a cold vial warm before opening. Reconstituted, hold at 2 to 8 degrees C, or aliquot and freeze rather than thawing repeatedly.

Oxidation dominates, and unusually there are three sites rather than one. Both methionines oxidise to sulfoxides and the tryptophan indole oxidises independently. Air, light and trace metals drive all three, light mattering most for the tryptophan.

A second, slower route is deamidation of the glutamine at position four, which converts a neutral side chain to an acidic one and adds about one mass unit. It needs water, so the solid is protected while solutions are not, and it accelerates with warmth and alkaline pH.

Nothing else competes. There is no cysteine, so no thiol chemistry and no dimerisation, and no aspartate, so no isomerisation.

These are storage conditions for the material, not dosing or administration guidance.

How MOTS-c is tested

Reversed-phase HPLC establishes chromatographic purity as area percent. Four aromatic rings, including a tryptophan, make 280 nm a genuinely strong detection channel alongside the low-UV amide measurement at 214 nm.

Area percent is a chromatographic measure rather than peptide content by weight. Solid-phase synthesis delivers a salt, commonly trifluoroacetate or acetate, and with four basic side chains the counter-ion load on this peptide is high.

Mass spectrometry confirms identity, and the oxidation signature needs more care than on a single-site peptide. Three residues can each take up oxygen, so oxidised material appears at sixteen, thirty-two or forty-eight above the molecular ion, and a plus-sixteen peak does not say which site was hit. Localising it requires fragmentation. On most peptides that satellite has one obvious explanation; here it has three.

Deamidation of the glutamine adds about one mass unit, which on a molecule of 2175 sits within the parent's isotope envelope and needs adequate resolving power to separate.

These describe general methodology, not a claim about any particular batch.

Handling FAQ

The formula shows two sulfurs. Is there a disulfide? No. Both sulfurs belong to methionines, at positions one and six, and a disulfide requires two cysteines. The sequence contains none, and the deposited structure confirms two thioethers and no S-S bond. No reducing agent is needed at any point.

What does a peak sixteen mass units above the main one mean here? Oxidation, but at one of three possible sites: either methionine, or the tryptophan. Intact mass cannot say which. On a peptide with a single oxidisable residue that satellite is unambiguous; on this one it is not, and fragmentation is needed to localise it.

Full specifications for MOTS-c.