Description
PrymaLab · Research Use Only
PEG-MGF 2mg
A splice variant with a polymer attached · and what that does to the mass spectrum
MGF peptide is not a growth factor. It is a short fragment taken from the tail end of one, and the version supplied here carries a polyethylene glycol chain that the natural sequence does not have. Both of those facts change what a certificate can and cannot tell you.
Specification Table
| Property | Value |
|---|---|
| Compound | PEG-MGF |
| Full name | PEGylated mechano growth factor E-domain peptide |
| What it derives from | The E-domain of IGF-1Ec, an alternatively spliced transcript of the IGF-1 gene |
| Relationship to IGF-1 | It is not IGF-1 and does not carry the IGF-1 receptor binding region |
| Peptide residue count | 24 in the unmodified E-domain sequence |
| Unmodified peptide mass, calculated | Approximately 2,867 g/mol |
| Modification | A polyethylene glycol chain attached to the peptide |
| CAS | None stated. The unmodified E-domain peptide carries a registry number, the PEGylated species does not have a settled one |
| Consequence for mass spectrometry | PEG is a distribution of chain lengths, so the spectrum is an envelope of peaks rather than a single mass |
| Peak spacing in that envelope | Approximately 44 daltons, the mass of one ethylene oxide monomer |
| Nominal fill | 2 mg |
| Appearance | White lyophilized powder |
| Purity | Per lot-specific certificate of analysis |
| Storage, lyophilized | 2-8°C short term, -20°C for extended storage, protected from light and moisture |
Where Does MGF Peptide Come From?
The IGF-1 gene produces more than one transcript, and the compound sold as MGF peptide comes from one of the less common ones.
Alternative splicing generates several IGF-1 variants that share the mature IGF-1 sequence and differ in the carboxy-terminal extension attached to it, which is called the E-domain.
One of those variants, IGF-1Ec, was described in the mid 1990s in work on mechanically loaded muscle, which is where the name mechano growth factor came from.
The full IGF-1Ec protein is IGF-1 plus its particular E-domain, and the compound in this vial is that E-domain on its own.
That distinction is the one most often lost, and it is not a small one. Removing the IGF-1 portion removes the region that binds the IGF-1 receptor.
So MGF peptide is not an IGF-1 analogue and does not act at that receptor, whatever the shared gene of origin might suggest.
What the isolated E-domain does instead has been studied in cell systems, principally in work on satellite cell behaviour, and the receptor it acts through has not been definitively identified.
That is an honest description of the state of the literature rather than a gap in this page. Anyone planning work here should read the primary papers. Summaries routinely describe this compound as an IGF-1 variant, which is the error the paragraph above is about.
Why Is a Polymer Attached?
The native E-domain peptide has a very short life in any biological preparation, and that is the entire reason the PEGylated form exists.
Published estimates put the unmodified peptide in the region of minutes, which is short enough to make many experimental designs impractical.
Polyethylene glycol addresses that in three ways at once, none of which involves changing the peptide sequence.
It increases hydrodynamic radius, which slows renal filtration. It shields the backbone sterically, so proteases reach it less readily. It reduces recognition by clearance mechanisms that read surface features.
The result is a molecule with the same sequence and a considerably longer usable window, which is a well-established strategy rather than anything specific to this compound.
The cost is that the article is no longer a defined chemical entity in the way a synthetic peptide is.
A synthetic hexapeptide has one structure and one mass. A PEGylated peptide has one peptide and a population of polymer chains. That makes it a mixture by construction.
That is not a quality problem and it is a verification problem, and the next section is about what it does to the analysis.
One qualification on the half-life figures in circulation for the unmodified peptide.
Estimates in the region of minutes come from specific preparations, and the number varies with the matrix, the temperature and the assay used to follow disappearance.
Treating any single published figure as a property of the molecule rather than of the experiment is the usual error here.
Why Can a PEGylated Peptide Not Have One Exact Mass?
This is the single most useful check available on this compound and it takes one glance at the certificate.
Polyethylene glycol is manufactured by polymerising ethylene oxide, and the process produces a distribution of chain lengths rather than a single length.
A nominal 5 kDa PEG is a population centred near 5 kDa containing chains of many different sizes, all differing by whole numbers of the 44 dalton monomer.
Attach that population to a peptide and the mass spectrum shows an envelope: a series of peaks spaced 44 daltons apart, with an intensity distribution reflecting the polymer.
That envelope is the correct and expected result. It is what a properly analysed PEGylated species looks like.
A certificate reporting a single exact mass for a PEGylated peptide is therefore reporting something that cannot be true of the material.
It usually means one of three things. The mass quoted is the unmodified peptide rather than the conjugate, the figure is theoretical rather than measured, or no mass spectrometry was performed at all.
The right questions are what the average molecular weight of the PEG was, what its polydispersity index was, and whether an envelope was observed with the expected spacing.
A supplier who can answer those has characterised the material. One who quotes a single number has not. A great many suppliers of MGF peptide sit in that second category.
What Else Should the Certificate Show?
Six further fields alongside the PEG questions, and the first is about which article is in the vial.
Whether the peptide sequence stated is the E-domain alone or a construct including part of the IGF-1 sequence. Those are different articles sold under overlapping names.
The attachment site of the polymer. PEG chemistry can target the amino terminus or lysine side chains, and a chain on a different residue gives a molecule with the same average mass and different behaviour.
Peptide content as a proportion of total mass. On a PEGylated article the polymer contributes a large share of the weight, so a nominal 2 mg fill contains substantially less than 2 mg of peptide.
That figure matters more here than the ordinary counterion correction does, and it is frequently absent.
Purity by chromatography with the trace supplied. PEGylated species elute broadly rather than as sharp peaks, which is expected and which also makes a summary percentage close to meaningless.
Whether any unmodified peptide remains in the preparation, since incomplete conjugation leaves a fast-clearing species alongside the intended one.
Lot number and synthesis date, tying every figure to the vial that ships.
How Much Peptide Is in a 2mg Vial?
The answer is a good deal less than 2 mg, and the arithmetic is worth doing before the vial is reconstituted rather than after.
A conjugate consists of peptide plus polymer, and both contribute to the weight on the label.
The unmodified E-domain peptide is roughly 2,867 daltons. A polymer chain commonly used for this kind of modification runs in the region of 5,000 daltons.
On those figures the peptide is around 36 percent of the conjugate by mass and the polymer is the rest.
A nominal 2 mg fill of that conjugate contains somewhere near 0.7 mg of peptide.
Then the ordinary corrections apply on top. Residual water and counterion take a further share, so the usable figure falls again.
None of this is a defect. It is what a PEGylated article is, and the same arithmetic applies to every conjugate on the market.
What matters is that the polymer size is not fixed across suppliers. A 2 kDa chain and a 20 kDa chain give conjugates with peptide fractions differing by a factor of five for the same nominal fill.
So two vials both labelled 2 mg PEG-MGF can differ several-fold in how much peptide they contain, and nothing on the front label distinguishes them.
The certificate figure to ask for is peptide content as a proportion of total mass, and a supplier who cannot state the polymer size cannot supply it.
How Should the Vial Be Handled?
Storage is conventional and reconstitution needs more care than for a small peptide.
Sealed lyophilized powder holds at 2 to 8 degrees Celsius for short periods and at minus 20 for extended storage, protected from light and moisture.
Bring the vial to ambient temperature before opening. PEGylated material is hygroscopic and readily takes up atmospheric water onto a cold cake.
Reconstitute slowly down the vial wall and swirl gently. Polymer-modified peptides can be slow to go fully into solution, and the temptation to shake is the thing to resist.
Give it time rather than agitation. A cake that has not fully dissolved after gentle swirling usually needs a few minutes standing rather than force.
Aliquot on first reconstitution. Freeze-thaw cycling damages the peptide portion regardless of what the polymer is doing.
Adsorption is less of a problem than for an unmodified peptide, since the polymer shields the surface, and low-binding consumables remain the sensible default at low working concentrations.
Record lot, peptide content as distinct from total fill, diluent volume and date. On this compound the peptide content figure is the one that makes the preparation reproducible.
One further practical point about comparing lots.
Two lots of a PEGylated article can differ in polymer size as well as in the ordinary ways, and a change of polymer changes the peptide fraction of every subsequent preparation.
Recording the stated PEG average molecular weight per lot, alongside the usual fields, is what makes a lot change visible rather than mysterious.
It is one number and it is the one most likely to move between orders.
Published Literature
Selected references on the splice variant, on the isolated E-domain and on the polymer chemistry.
- Yang S, Alnaqeeb M, Simpson H, Goldspink G. Journal of Muscle Research and Cell Motility. 1996;17(4):487-495. DOI: 10.1007/BF00123364
- Goldspink G. Journal of Anatomy. 1999;194(Pt 3):323-334. DOI: 10.1046/j.1469-7580.1999.19430323.x
- Hill M, Goldspink G. Journal of Physiology. 2003;549(Pt 2):409-418. DOI: 10.1113/jphysiol.2002.035832
- Veronese FM, Pasut G. Drug Discovery Today. 2005;10(21):1451-1458. DOI: 10.1016/S1359-6446(05)03575-0
Frequently Asked Questions
What is MGF peptide?
The E-domain of IGF-1Ec, an alternatively spliced transcript of the IGF-1 gene, supplied here as an isolated 24-residue peptide carrying a polyethylene glycol chain.
Is it a form of IGF-1?
No. The isolated E-domain does not include the mature IGF-1 sequence and therefore does not carry the region that binds the IGF-1 receptor, whatever the shared gene of origin suggests.
Where does the name come from?
From work in the mid 1990s on mechanically loaded muscle, where the IGF-1Ec transcript was described. Mechano growth factor is the name that came out of that work.
What receptor does the isolated E-domain act through?
It has not been definitively identified. Effects have been described in cell systems, principally in satellite cell work, and the receptor question remains open in the published record.
Why is a polymer attached?
Because the unmodified peptide has a very short life in biological preparations, estimated in the region of minutes, which makes many experimental designs impractical.
How does PEG extend that?
Three ways at once. It increases hydrodynamic radius so renal filtration slows, it shields the backbone from proteases, and it reduces recognition by clearance mechanisms that read surface features.
Why can a PEGylated peptide not have one exact mass?
Because polyethylene glycol is manufactured as a distribution of chain lengths. Attached to a peptide it produces an envelope of peaks spaced by the 44 dalton monomer rather than a single mass.
So what does a single exact mass on a certificate mean?
Usually one of three things. The figure is for the unmodified peptide, it is theoretical rather than measured, or no mass spectrometry was performed on the conjugate at all.
What should be asked instead?
The average molecular weight of the PEG, its polydispersity index, and whether an envelope was observed with the expected 44 dalton spacing. A supplier who can answer has characterised the material.
Does the polymer affect how much peptide is in the vial?
Substantially. The polymer contributes a large share of total weight, so a nominal 2 mg fill contains considerably less than 2 mg of peptide, and that figure is often absent from the certificate.
Does the attachment site matter?
Yes. PEG chemistry can target the amino terminus or lysine side chains, and a chain on a different residue gives the same average mass and different behaviour.
How should it be reconstituted?
Slowly down the vial wall with gentle swirling. Polymer-modified peptides can be slow to dissolve, and a cake that has not cleared usually needs standing time rather than shaking.
Compliance Statement
PEG-MGF is sold exclusively for laboratory research use. It is not a drug, food, or cosmetic product, and it is not a dietary product of any kind. It is not approved by the FDA or any comparable authority for human or veterinary use, the receptor through which the isolated E-domain acts has not been definitively identified in the published record, the isolated E-domain is not IGF-1 and does not carry the IGF-1 receptor binding region, and no compound in this range is offered for any human or veterinary purpose. This product is not intended to diagnose, treat, cure, or prevent any disease. It must not be given to humans or animals. Purchase is restricted to qualified researchers and institutions operating within applicable laws. All handling is the responsibility of the purchasing laboratory.
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