Humanin

What Humanin is, structurally

Humanin is a twenty-four residue peptide with a free N-terminal amine and a free C-terminal acid. The sequence reconciles with the formula exactly: twenty-four backbone amides plus ten side-chain nitrogens, three from each of the three arginines and one from the lysine, giving the thirty-four the formula states.

The two sulfurs are not a matched pair, and that governs the handling. One is the methionine at position one, sulfur in a thioether between two carbons. The other is the cysteine at position eight, a free thiol. With only one cysteine present, no intramolecular disulfide can form under any conditions. The free thiol can still react with another molecule, giving a disulfide-linked dimer at roughly twice the mass.

So the molecule carries two oxidation-sensitive sulfurs of different kinds with different products: the methionine gives a sulfoxide, adding sixteen, while the cysteine gives a dimer, roughly doubling the mass.

The second feature worth noting is four consecutive leucines at positions nine to twelve. Leucine is strongly hydrophobic, and four in a row with nothing polar between them is an uninterrupted hydrophobic face on an otherwise well-distributed peptide. Such segments drive self-association, making aggregation a realistic failure mode here rather than a theoretical one.

Charge is mixed but net positive: three arginines and a lysine against a glutamate, an aspartate and the C-terminal acid. The peptide is water soluble, though no quantitative figure is quoted here. Phenylalanine at position six is the only aromatic residue, and there is no tryptophan or tyrosine anywhere.

Reconstitution and handling

Sterile water is the default solvent, and with no metal centre present PBS or an assay buffer are reasonable. Add diluent down the vial wall, swirl gently, and let it stand until fully clear. Do not vortex: a chain this length with a hydrophobic run in the middle aggregates readily at an air-liquid interface under shear.

Limit air exposure and keep headspace small. Both sulfurs are oxidation-sensitive and neither is protected, so this is a real precaution rather than a generic one.

Adsorption is worth allowing for at low concentration, and low-protein-binding tubes reduce measured loss. The tetraleucine segment gives a genuine hydrophobic surface, so affinity for plastic is higher than a uniformly polar sequence would show.

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. Reconstituted, hold at 2 to 8 degrees C, or aliquot and freeze rather than thawing repeatedly. Aliquot before the first freeze: a chain this long can aggregate on cycling, and aggregation is not undone by warming.

Three distinct liabilities apply, more than most peptides carry. The methionine oxidises to a sulfoxide. The cysteine thiol also oxidises, but its product is a disulfide-linked dimer rather than a modified monomer, and thiol oxidation is base-catalysed, so alkaline conditions accelerate it specifically. The aspartate at seventeen can cyclise and reopen as isoaspartate at unchanged mass.

Air exposure drives the first two, pH the second, and warmth the third. Cold, near-neutral, low-headspace storage addresses all three, which is fortunate, because no single condition would.

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

How Humanin is tested

Reversed-phase HPLC establishes chromatographic purity as area percent. Detection has to sit in the low UV near 214 nm. The single phenylalanine is the only aromatic residue in twenty-four, and phenylalanine is a weak chromophore, so 280 nm is not a usable channel here despite the peptide's size.

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 is substantial. Residual trifluoroacetate, truncated chains and deletion sequences are the impurity classes to resolve, and a synthesis running through four consecutive leucines is a plausible place for deletions to arise.

Mass spectrometry confirms identity and separates the liabilities, since each has a different signature. Methionine sulfoxide appears sixteen units above the molecular ion. A disulfide-linked dimer appears near 5372, roughly twice the parent. Isoaspartate appears nowhere, being isobaric, and must be caught chromatographically. Reading all three means looking in three places, not at one peak.

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

Handling FAQ

Can this form an internal disulfide bridge? No. The sequence contains one cysteine, and a bridge needs two. The second sulfur belongs to the methionine at position one, which is a thioether and cannot participate. The free thiol can still react with a second molecule to give a dimer.

Why is 280 nm not usable on a peptide this large? Because size does not guarantee aromatic content. Only one of the twenty-four residues is aromatic, a single phenylalanine, and phenylalanine absorbs weakly. There is no tryptophan or tyrosine to provide the usual 280 nm signal.

Full specifications for Humanin.