P21 (P021)

What P21 is, structurally

P21 is not a straightforward peptide. Four of its parts are amino acids, taken from a short stretch of a human protein, and two are not: an acetyl cap at one end, and at the other a rigid cage-shaped carbon group, an adamantyl group. The CAS index name for 1246751-68-7 confirms it, N-acetyl-L-alpha-aspartylglycylglycyl-N-(3-carbamoyladamantan-1-yl)-L-leucinamide.

Aspartate, glycine, glycine and leucine contribute 342.35 of the mass, the acetyl cap 43.05 and the adamantane carboxamide cap 193.27, summing to 578.66 against the stated 578.67. Around fifty-nine percent is peptide and the cage alone is close to a quarter, so the mass per amino-acid-equivalent runs far higher than four residues would suggest.

All six nitrogens are amide nitrogens, four along the chain and two at the capped end, so none is basic: the N-terminus is acetylated rather than free, and there is no lysine, arginine or histidine. Six of the eight oxygens are carbonyls; the other two are the aspartate side-chain carboxylic acid, the sole ionisable group in the molecule. It is a monoprotic acid, pKa near 4: one negative charge at neutral pH, none below pH 3, positive charge at no point.

Solubility should accordingly be poorer than for an equal-mass peptide. The cage is ten carbons of saturated hydrocarbon with no hydrogen-bonding capacity, leucine adds nonpolar bulk, and capping both ends removes the amine and carboxylate a free tetrapeptide would carry. Being rigid and compact rather than a flexible tail, though, the cage gives a dissolution problem, not lipopeptide surfactant behaviour. There is no sulfur in C27H42N6O8, closing disulfide bonding, thiol oxidation and methionine sulfoxide formation entirely.

Reconstitution and handling

Sterile water is the default solvent, and a near-neutral phosphate buffer is equally sound, no metal centre being present to compete for it. Holding a reluctant vial at or just above neutral keeps the carboxylate ionised, the right direction here. Add diluent slowly down the inside wall, swirl gently rather than vortexing, and let the solution clarify. Should both fail, a water-miscible co-solvent followed by dilution into buffer is conventional, though none is named at source.

No reducing agent is needed, there being no sulfur to act on. Adsorption is worth managing at low concentration: binding here is driven by the cage rather than by charge, so low-protein-binding polypropylene is the default, and as an anion it binds negatively charged glass less readily than a basic peptide.

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 or below, sealed, dry and dark, and let a cold vial warm before opening. Reconstituted, hold at 2 to 8 degrees C, or aliquot and freeze rather than thaw repeatedly.

The named liability is the aspartate-glycine bond at the head of the chain, the fastest-isomerising motif in peptide chemistry, glycine having no side chain to obstruct its backbone nitrogen closing onto the aspartate side-chain carbonyl. This aspartate is at position one with a residue after it, so the route is genuinely open, unlike a C-terminal aspartate, which has no following nitrogen. The succinimide intermediate is eighteen units lighter and reopens to aspartate and isoaspartate, isobaric with the parent. Both pH extremes accelerate it; cold, near-neutral storage is the control.

No asparagine or glutamine is present, so deamidation does not arise, and the saturated cage offers nothing to oxidise or hydrolyse.

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

How P21 is tested

Reversed-phase HPLC establishes chromatographic purity as area percent. No residue is aromatic and the cage is fully saturated, so there is no chromophore above roughly 230 nm: detection near 210 to 214 nm on the amide bonds is the only real option, and 280 nm shows nothing. Retention is nonetheless stronger than a short polar peptide's, from the cage and the leucine side chain.

Area percent is not content by weight, though with no basic site there is no trifluoroacetate counter-ion inflating it.

Mass spectrometry confirms identity against 578.67. With no basic site nothing protonates readily, so a deprotonated ion near 577.66 in negative mode is the more natural measurement, with a sodium adduct near 601.66 the positive-mode alternative. Isoaspartate is isobaric with the parent and invisible to mass spectrometry, so chromatography carries that separation alone.

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

Handling FAQ

A vial is slow to dissolve. Does adding acid help, as it does for some peptides? No, it will probably make matters worse. The only ionisable group is the aspartate carboxylic acid, so acidifying towards pH 3 neutralises the molecule's single charge. Stay at or just above neutral.

Why does the stated sequence read as six residues when this is a tetrapeptide? Ac-DGGLAG-NH2 is shorthand in which the trailing AG is the adamantylated cap, not alanine and glycine. The formula settles it: a real hexapeptide would be C21H35N7O9 at 529.55, not 578.67.

Full specifications for P21 (P021).