AOD-9604 vs HGH Fragment 176-191: Cyclised and Linear Forms Side by Side

What separates these two compounds is chemical state, not sequence: the fragment is the bare C-terminal stretch of growth hormone, while AOD-9604 carries an extra tyrosine at the N-terminus and arrives with its two cysteines joined in a disulfide bridge. Their molecular weights spell the relationship out — 1817.12 g/mol for the fragment versus 1815.08 g/mol for AOD-9604, a gap corresponding almost exactly to the two hydrogen atoms lost when a disulfide closes. So when comparing AOD-9604 with the hGH 176-191 fragment, the variables that matter are conformation and analytical traceability rather than primary structure.

We supply each as lyophilized powder in 5 mg and 10 mg vials with lot-matched HPLC certificates — AOD-9604 and HGH Fragment 176-191 — both catalogued under fat-loss and metabolic peptides.

The origin of both molecules

Structure–function studies of human growth hormone during the 1980s and 1990s suggested that its effects on fat tissue arise from the C-terminal region, while the surfaces needed to dimerise the growth hormone receptor and induce IGF-1 sit elsewhere in the 191-residue protein. Cutting out the C-terminal stretch offered a direct test of that separation: should a 16-residue piece reproduce adipose-related readouts without receptor-driven growth signalling, then the two functions really are distinct domains rather than facets of one integrated response.

The isolated stretch is HGH Fragment 176-191. AOD-9604 is the variant carried into development: appending a tyrosine at the N-terminus and closing the intramolecular disulfide between the two cysteines fixes the peptide in the loop shape this region adopts inside the folded hormone, instead of leaving a floppy linear chain. Intact growth hormone is itself a prescription biologic, listed here purely for reference as somatropin and never sold as a research reagent.

The disulfide bridge is the entire comparison

In a short peptide, two cysteines can realistically occupy three states: free and reduced, joined into an intramolecular ring, or cross-linked between molecules as dimers and larger oligomers. A cyclic peptide and its linear equivalent are separated by a mere 2 Da — trivially overlooked on a mass spectrum glanced at rather than read — yet they can differ considerably in the geometry a receptor sees, in how readily they aggregate, and in stability.

For that reason, the AOD-9604 versus fragment question is more an analytical matter than a pharmacological one. Buy the fragment, leave it in a dilute solution permeable to oxygen, and a portion oxidises spontaneously, drifting toward the AOD-9604-like state with nobody recording the change. The reverse also holds: place AOD-9604 into a reducing environment — a buffer containing DTT or beta-mercaptoethanol, say — and the ring opens. Any comparison that fails to confirm the redox state of both materials at the moment of use is comparing labels rather than molecules.

What the published record supports

Reported work on both compounds lives largely in cell and rodent systems. Adipocyte culture is the commonest setting, with glycerol and free fatty acid release measured as indicators of lipolysis, alongside lipogenic enzyme activity and, in some designs, involvement of beta-adrenergic signalling. Rodent studies have looked at fat-depot mass and metabolic markers. A framing that recurs throughout is the contrast with intact growth hormone: does the fragment produce effects in adipose tissue without the insulin antagonism and IGF-1 elevation that accompany the whole hormone?

Two caveats belong on any page describing this literature honestly. Results across the fragment literature have not been consistent, and several groups have reported effects weaker than — or absent compared with — the earlier reports, and that heterogeneity is part of why these compounds remain research tools. Separately, AOD-9604 has undergone investigational clinical evaluation, and outcomes from those regulated programmes concern pharmaceutical-grade product given under supervision; they say nothing about a reagent vial and appear here only to characterise the state of the record.

Matching the compound to the research question

Pick AOD-9604 when conformation has to be fixed

Where an experiment hinges on one reproducible molecular species — a binding study, a structure–activity series, anything in which lot-to-lot drift in geometry would be fatal — the cyclised material is the better-defined reagent. It is also the right pick for reproducing published AOD-9604 studies specifically, since both the added tyrosine and the closed ring formed part of what was examined.

Pick the fragment when the native sequence is the point

Where the question concerns the hGH C-terminal region as it exists within the hormone, the unmodified fragment is the truer reagent: no additional residue, no imposed modification. It is likewise the right starting material for a group that wants to govern oxidation state itself, running reduced and oxidised arms alongside each other.

Use both when cyclisation is the variable

Together the pair forms an unusually clean two-point series: one sequence, differing by a single added residue and a single ring closure. Testing them side by side asks directly how much of the reported activity rests on conformation rather than primary sequence. Groups extending beyond growth-hormone fragments usually add an unrelated metabolic probe such as 5-Amino-1MQ to separate pathway-specific effects from generic adipocyte responses; our background article on the lipolytic fragment covers that context.

Differences in handling, reconstitution and storage

Both are short and water-soluble, and both are much easier to manage than the acylated incretin analogues — but free thiols make the fragment the more demanding of the two. Keep sealed lyophilized vials of either frozen, and let them warm to room temperature before opening so that condensation does not form on cold powder. Reconstitute by running the diluent down the vial wall and letting the cake dissolve without shaking.

The fragment's particular need is control of oxygen exposure. Reduced cysteines oxidise in aerated aqueous solution, more rapidly at neutral to alkaline pH and in the presence of trace metals, so prepare working solutions fresh, keep them slightly acidic where the assay permits, and avoid leaving them standing in a partly filled tube. AOD-9604, already closed, is comparatively unaffected but should still be kept away from reducing buffers. Concentration remains a bench calculation: 2 mL of diluent in a 5 mg vial gives 2.5 mg/mL (2,500 mcg/mL), so 0.1 mL holds 250 mcg. With only 2 g/mol between them, mass and molar concentrations are effectively equivalent across the pair — a rare convenience in comparative work. Consult our reconstitution guide and storage guide, and aliquot both so neither endures repeated freeze-thaw.

Purity, identity and certificate checks

The certificate carries more weight than usual for this pair, since 2 Da separates the two compounds and a purity percentage cannot tell them apart. Read the mass-spectrometry value carefully against what is expected — 1815.08 g/mol for AOD-9604, 1817.12 g/mol for the fragment — and treat a monoisotopic number quoted without stated resolution as inadequate. In the chromatogram, watch for an early- or late-eluting satellite of comparable area, which in a cysteine-containing peptide often means a mixture of redox states, and for high-mass shoulders indicating disulfide-linked dimer. Confirm that the lot number on the certificate matches the vial you hold. Our COA guide explains what a complete document should contain.

Regulatory position

Each compound is supplied as a research chemical for laboratory use only and not for human consumption. Neither is an approved medicine in the United States or the EU; AOD-9604 has been assessed in investigational settings and holds food-ingredient status in certain jurisdictions, but neither status extends to the reagent sold here. Full-length growth hormone is a prescription-only biologic, listed in our catalogue for reference alone. For broader context, see our metabolic research overview.