Glutathione Has One Bond in an Unusual Place
Count the residues in glutathione and you get three, so tripeptide is a fair description. Look closer at how those residues are joined and one of the two bonds turns out not to be a peptide bond in the ordinary sense. Nearly everything peculiar about handling and analyzing this compound traces back to that one detail.
Where the bond attaches
Written out, glutathione is gamma-Glu-Cys-Gly. The normal arrangement links the alpha-carboxyl of one residue to the alpha-amino group of the next. Glutathione departs from that: the glutamate donates its side-chain carboxyl, the one sitting on the gamma carbon, and keeps its alpha-carboxyl out of the chain entirely.
What remains is a glutamate whose alpha-amino group and alpha-carboxyl both project away from the backbone instead of extending it. Every downstream quirk follows from that geometry.
Two forms, two masses
The reduced species is C10H17N3O6S at 307.08 Da monoisotopic and 307.32 Da average, CAS 70-18-8. GSH is the standard abbreviation, the H marking the free thiol.
Oxidation joins two molecules through a disulfide to give GSSG: C20H32N6O12S2, 612.15 Da monoisotopic, 612.63 Da average, CAS 27025-41-8. Forming that bond costs two hydrogens, which is why the dimer lands at 612.6 instead of 614.6.
Sequencing chemistry does not apply
Edman degradation strips residues one by one from a free alpha-amino terminus, and it depends on that amine being alpha to the carbonyl carrying the chain forward. Glutathione’s free alpha-amine belongs to a residue whose chain bond leaves from the gamma position instead, so the reaction does not run the way it would on a conventional peptide.
Aminopeptidases are blocked by the same geometry, which is the biochemical reason the gamma linkage exists in the first place. Analytically, it means identity rests on mass plus chromatographic comparison against a reference standard, not on sequencing.
Storage is part of the specification
All of the stability difficulty sits on the cysteine thiol, which oxidizes in air to the disulfide-linked dimer. Trace metals catalyze the reaction and neutral to alkaline pH accelerates it. In dry solid form it creeps along; in aerated solution above pH 7 it moves quickly.
Consequently the reduced-to-oxidized ratio in any sample reflects storage and handling history as much as manufacture. A certificate quoting a GSH to GSSG ratio is describing a moment in time, and a solution left on the bench for a week will no longer agree with it.
Preparing solutions in mildly acidic water slows the oxidation relative to neutral buffer, and including a chelator to tie up trace metals slows it further still.
Almost nothing for a C18 column to hold
A free amine, two carboxyls and a thiol hung on a ten-carbon skeleton make for an extremely polar molecule with virtually no hydrophobic surface. Conventional reversed-phase chromatography sends it straight through, at or near the void volume.
The same toolkit that serves other small polar peptides applies: an aqueous-stable bonded phase at very low organic content, ion-pairing chromatography, or hydrophilic interaction chromatography. Under most of these conditions GSSG is retained longer than GSH, being larger and presenting more surface – convenient, since it means the two resolve.
Choosing a detector, and saying which one
With no aromatic residue anywhere in the molecule, 280 nm is dead. The two amide bonds do respond at 214 nm, but weakly for something this small, and glutathione is frequently analyzed at concentrations where the response is marginal.
That pushes laboratories toward three alternatives. Electrochemical detection responds specifically to the free thiol and is blind to the dimer. Pre-column derivatization with a thiol-reactive reagent attaches a chromophore or fluorophore. Mass spectrometric detection is the third. Each answers a slightly different question, so a certificate needs to name the principle used – a thiol-specific method reports nothing at all about the oxidized fraction, by design.
What the label has to specify
The compound appears as GSH, as reduced glutathione, as L-glutathione, and as plain glutathione, the last of which is ambiguous because the bare word does not separate reduced from oxidized. Salt forms are in circulation. So is S-acetyl glutathione, a different compound carrying an acetyl group on the sulfur and weighing 42 daltons more.
State the oxidation state and the salt form. Those two facts, together with the mass, pin down what the vial contains.
A well documented lot
An accurate mass of 307.08 Da for the reduced form; a chromatogram from a method that actually retains it and resolves it from the 612.6 dalton dimer; and an explicit detection principle. Here the storage condition belongs in the specification itself rather than in a footnote.
Related reading
How much of a weighed quantity is the compound and how much is counter-ion depends on salt form, and the effect is larger for small molecules. That is covered in counter-ions and salt form. Another catalog entry that is a cofactor rather than a peptide is discussed in NAD as a coenzyme in cellular energy research.