SS-31: structure, nomenclature and handling

The sequence is D-arginine, 2,6-dimethyltyrosine, lysine, phenylalanine, with an amidated carboxyl terminus — written D-Arg-Dmt-Lys-Phe-NH2. It weighs approximately 640 daltons.

The design principle is visible in the ordering. Positions 1 and 3 are basic residues, arginine and lysine, each carrying a positively charged side chain at neutral pH. Positions 2 and 4 are aromatic, dimethyltyrosine and phenylalanine. The chain therefore alternates basic, aromatic, basic, aromatic — a deliberate motif that gives one face of the short molecule a concentrated positive charge and the other a hydrophobic surface. Together with the amidated terminus, which removes the negative charge a free carboxylate would carry, the peptide has a net charge of about +3 across only four residues.

Two of the residues resist enzymatic breakdown. The arginine is present in the D configuration rather than the natural L form, and proteases have evolved to act on L-amino acid sequences. The dimethyltyrosine is a tyrosine bearing two methyl groups on its aromatic ring — a non-proteinogenic residue that no natural enzyme is shaped to process.

The molecule is characterized for its association with cardiolipin, a phospholipid of the inner mitochondrial membrane. That association is a biophysical property of the charge and hydrophobicity pattern described above rather than a receptor interaction, which is what distinguishes this compound from most of the catalogue.

The material is catalogued as SS-31, and under the international non-proprietary name elamipretide. Development codes MTP-131 and the trade name Bendavia refer to the same molecule, as does the notation D-Arg-Dmt-Lys-Phe-NH2. As with most short synthetic peptides, it is typically isolated as an acetate salt, which affects the mass weighed out without altering identity.

The SS prefix belongs to a numbered series of related tetrapeptides sharing the alternating motif — SS-02, SS-20 and others carry different residues at the aromatic and basic positions. They are distinct compounds with distinct masses, so a listing giving only a series number is worth resolving against the certificate.

Mass spectrometry confirms the intact molecular mass near 640 daltons. For this peptide the measurement also confirms the amidated terminus: a free-acid impurity, where the amide failed to form during synthesis, is heavier by approximately one dalton, a small but defined difference that adequate resolving power detects.

High-performance liquid chromatography resolves the main peak from related substances and yields the purity figure. The characteristic related substance is the diastereomer: the D-arginine at position 1 can invert to the L form during synthesis, producing a compound of identical molecular mass that mass spectrometry cannot distinguish at all. It separates chromatographically, which is why the two methods are run in combination rather than either being treated as sufficient.

A second consideration is specific to the modified aromatic residue. Dimethyltyrosine is not a stock reagent in the way ordinary amino acids are, and an incompletely methylated residue — plain tyrosine in its place — gives a peptide lighter by a defined increment. That too is a mass question with a defined answer.

HEEZ Research supplies SS-31 in a single format: a 10 mg lyophilized vial. There is no second strength in the current catalogue, so the format decision that arises with two-size products does not apply here.

The material is supplied as a lyophilized powder. Freeze-drying removes water and leaves the peptide as a solid, markedly more stable than the same peptide in solution, and stable enough at ambient temperature to ship without refrigeration in transit.

Reconstitution converts the dried cake into a working solution, and it is a standard laboratory step rather than a product specification. Which solvent is used, and at what final concentration, is determined by the protocol in force at the receiving facility.

What HEEZ specifies is the contents of the vial: lyophilized SS-31 in a single 10 mg format, stated on the label and on the batch certificate of analysis. Where a protocol is expressed in molar rather than mass terms, the additional figure it needs is the molecular weight, which the certificate records for the batch supplied.

The high net positive charge makes the peptide readily water-soluble, and at four residues it presents essentially no surface for aggregation. One consequence of that charge is worth noting: strongly cationic peptides adsorb to negatively charged surfaces, including some glass and untreated plastics, so low-binding labware is a reasonable default where small quantities are being measured.

Sealed lyophilized SS-31 is ambient-stable for the duration of transit, so no cold chain or insulated packaging is required for shipping. On receipt the vial goes to refrigeration and is kept out of direct light.

Reconstituted material is held refrigerated and used within the window the receiving facility's protocols specify, and repeated freeze-thaw cycling is avoided. Solutions are treated as short-lived relative to the lyophilized form.

By composition the peptide is comparatively robust: no cysteine or methionine removes the common oxidation routes, and no asparagine or glutamine removes deamidation as a degradation pathway. The residues warranting attention are the two aromatics: phenolic side chains of the tyrosine class oxidise under harsh conditions and are light-sensitive over extended periods, which is reason enough to keep solutions covered. The D-configuration and methylation that resist enzymatic cleavage confer no advantage against these physical routes.

The certificate records what an independent facility measured for one specific batch. Its value lies in being batch-specific, so the first check is that the lot number on the document matches the vial in hand.

The identity section compares observed molecular mass against a stated theoretical mass near 640 daltons. The theoretical figure should correspond to the amidated peptide carrying the dimethylated aromatic residue — the two synthesis shortfalls described above each shift it by a defined amount.

The purity figure is the HPLC main-peak area as a percentage of total peak area, and the attached chromatogram shows how that figure was arrived at. Peaks eluting close to the main peak are the ones to note, since a position-1 diastereomer of identical mass separates by a small retention difference rather than a large one.

Peptide content, or net peptide content, is sometimes reported separately. It accounts for counter-ions and residual water, and it is the figure determining how much actual peptide a nominal 10 mg represents.