Solubility & difficult compounds

01 When water is enough

Most of the catalogue dissolves in bacteriostatic water without argument. Short sequences carrying charged residues — the tri- and tetrapeptides, the secretagogues, the repair peptides — go into solution within seconds of the diluent reaching them, and nothing on this page applies to them.

That is worth stating plainly, because the existence of a guide about difficult compounds tends to make everything look difficult. It is a minority of products. If the product page does not say otherwise, bacteriostatic water is correct and the reconstitution guide covers everything you need.

The compounds below are the exceptions, and they fail in specific, predictable ways. Knowing which failure you are looking at is most of the fix.

02 Why some compounds resist

A peptide dissolves when water molecules can surround it more favourably than it can associate with itself. Three things commonly prevent that.

The first is hydrophobicity. Sequences rich in aromatic or aliphatic residues — tryptophan, phenylalanine, leucine, isoleucine — present a surface that water orders poorly, and the molecules aggregate with each other instead. Deliberately lipophilic compounds such as Dihexa are hydrophobic by design, because that is what carries them across membranes.

The second is the isoelectric point. Every peptide has a pH at which its positive and negative charges cancel, leaving no net charge. At that pH it is least soluble, because there is no electrostatic repulsion keeping molecules apart. A peptide that refuses to dissolve in neutral water is often simply sitting near its pI, and moving the pH in either direction fixes it.

The third is size and structure. A folded protein such as IGF-1 LR3 has a defined tertiary structure with a hydrophobic core, and it is far more particular about its solvent than a short unstructured chain. It is also far easier to destroy: the structure is the activity.

03 Moving the pH

Where the problem is proximity to the isoelectric point, a dilute acid or base restores net charge and with it solubility.

Dilute acetic acid — commonly 0.1 M — is the usual first choice, and it is the conventional diluent for IGF-1 LR3 and a reasonable option for LL-37. It protonates basic residues, gives the molecule a net positive charge, and keeps the molecules repelling each other. The peptide is then diluted into the working buffer immediately before use.

The important detail is that the acid is a solvent for the stock, not the working medium. Concentrated stock first, dilution second. Reversing that order — adding a little acid to a large volume of neutral buffer and hoping — does nothing, because the pH never moves far enough to matter.

Watch the point of dilution. A peptide that is happily dissolved at pH 3 can crash out the moment it meets a neutral buffer, particularly if the dilution is done quickly into a small volume. Add slowly, with mixing, and look at what you are making.

Never guess at a diluent for a compound whose product page specifies one. Where a specific solvent is required it is stated in the handling notes on that page, and it is stated because bacteriostatic water will not work.

04 DMSO stocks

For genuinely hydrophobic compounds — Dihexa and FOXO4-DRI are the two clearest cases in the catalogue — no aqueous diluent will work, and the standard approach is a concentrated stock in dimethyl sulfoxide, diluted into the aqueous working medium afterwards.

DMSO dissolves almost anything, which is why it is used, but it introduces two problems worth knowing about in advance.

It is strongly hygroscopic. It pulls water out of the air every time the container is opened, and a stock that has absorbed water is a weaker solvent than the one you prepared. Compounds that dissolved fine on the first day precipitate out on the fifth. Aliquot into single-use volumes, seal tightly, and stop reopening one tube.

It also carries into the experiment. DMSO has biological effects of its own at concentrations well below where it is obviously toxic, so the final solvent fraction has to be low enough not to confound the result — and a vehicle control containing the same fraction is the only way to know it did not.

05 Peptide that dissolved and then vanished

A preparation can be fully dissolved and still lose most of its content before it reaches the assay. Strongly charged peptides adsorb onto glass and standard polypropylene, and at low working concentrations the amount lost to the container wall is not a rounding error — it can be the majority of what you added.

LL-37 is the worst offender in the catalogue; Kisspeptin-10, SS-31 and VIP are all affected. The pattern is the same in each case: a concentrated stock behaves normally, and dilute working solutions quietly under-deliver.

Low-binding tubes and tips are the direct fix and are worth using as a default for anything dilute. Preparing working dilutions immediately before use rather than storing them removes most of the remaining exposure, since adsorption is time-dependent. Where a protocol permits it, a carrier protein such as BSA occupies the binding sites instead of your peptide.

This is invisible on a certificate of analysis. The vial contained what it said it contained; the loss happened in your tube.

06 Metals, chelators and cofactors

Two compounds in the catalogue depend on a bound metal ion, and both can be inactivated by a buffer choice that leaves them chromatographically intact.

GHK-Cu is a copper complex, and the copper is the active part. Chelating agents strip it — EDTA in particular, which is present in more buffer formulations than people expect. The blue colour is a free stability indicator: a solution that has faded or greened has lost the complex, whatever the label says.

Thymulin requires zinc, and the binding is reversible. A chelator will remove it, and so, more slowly, will prolonged storage in dilute solution. Thymulin can therefore lose activity while still assaying as pure, because purity measures the peptide and the peptide is still there.

The general rule: check what your buffer contains before assuming it is inert. Reducing agents, chelators and free thiols are all capable of destroying a compound without changing how much of it is present.

CJC-1295 with DAC is a third case for a different reason. Its maleimide linker reacts with free thiols, so a buffer containing DTT or beta-mercaptoethanol will consume the linker and the peptide will no longer conjugate to albumin.

07 Compounds that need something other than water

The handling notes on each product page carry the full detail. This is the short version of which ones are not routine.

IGF-1 LR3

Dilute acetic acid, then dilute into working buffer. Never vortex — it is a folded protein and agitation destroys the structure.

Dihexa

DMSO stock, then aqueous dilution. Lipophilic by design; will not dissolve in water alone.

FOXO4-DRI

DMSO stock. Long sequence with substantial hydrophobic content.

LL-37

Sterile water or dilute acetic acid, not saline. Low-binding consumables throughout.

Matrixyl

Wet with a water-miscible co-solvent such as propylene glycol before bringing to volume. Lipidated and foams readily.

GHK-Cu

Water is fine, but no chelators and no reducing agents. Watch the blue colour.

Thymulin

Water is fine, but no chelators. Zinc binding is reversible and the complex is the active form.

Glutathione

Water, prepared fresh. The thiol oxidises in hours; avoid alkaline buffers and metal contact.

NAD+

Water, but unstable above neutral pH and hygroscopic as a powder. Prepare close to use.

Cerebrolysin

Supplied as a solution. Nothing to reconstitute; do not freeze.

08 When it will not dissolve

Before escalating to a stronger solvent, work through the ordinary causes. Most failures are procedural rather than chemical.

• Give it time. Several minutes of standing dissolves more than several seconds of agitation, and agitation costs you material.
• Check the temperature. A vial straight from the freezer dissolves slowly and draws condensation into the stopper.
• Check the product page. If a specific diluent is listed, no amount of technique will make bacteriostatic water work.
• Do not sonicate or heat as a first resort. Both will dissolve a peptide and both can denature it, and you will not be able to tell which happened.
• Do not shake. Foam is denatured peptide at the air–liquid interface, and it does not return to solution.
• If a solution clouds after dilution rather than during reconstitution, the stock was too concentrated for the volume it went into — remake it more dilute.
• If material persists after everything above, stop and contact us with the batch number rather than forcing it. A vial that will not dissolve as specified is a vial we want to look at.