What does AOD-9604 disulfide bond intact mass verification measure?
AOD-9604 disulfide bond intact mass verification measures whether the undigested synthetic molecule matches the elemental composition of the expected cystine form. The sequence used for that calculation is Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe. Those two cysteine residues are the only sulphur atoms in the chain, so a single intramolecular disulfide is the only intramolecular pairing. The assay does not image the bond. It tests whether the neutral composition, after loss of two hydrogen atoms relative to the dithiol, agrees with the observed mass. Electrospray ionisation reports mass-to-charge. The laboratory either deconvolutes the envelope to a neutral mass or compares an observed m/z with a theoretical m/z at a stated charge. Both are acceptable when the certificate names the convention. The disulfide call is agreement with the oxidised composition, not with the reduced composition. The monoisotopic gap between those compositions is 2.01565 Da. A high-resolution identity window of a few parts per million is much narrower than that gap, so an incorrect redox state should fail a correctly calculated window. Doubly and triply protonated ions are the usual observed species. Sodium and potassium adducts, and acetate or trifluoroacetate clusters, must be labelled as adducts. An adduct changes m/z without opening the cystine. Treating an adduct as a related peptide creates a false impurity; treating a reduced ion as an adduct creates a false pass. Intact mass is also silent on sequence order. Leucine and isoleucine are isobaric, and both occur in this sequence, so a mass match does not assign position 2 or position 4. It also does not prove the absence of a co-eluting isobaric impurity. Chromatographic purity stays a separate result. Gorman and co-workers reviewed mass-spectrometric disulfide determination and placed redox comparison inside the identity argument rather than inside a purity percentage (DOI:10.1002/mas.10025).
How should the theoretical oxidised and reduced masses be calculated?
The cystine form of the stated sequence has the molecular formula C78H123N23O23S2. Calculated with conventional monoisotopic atomic masses, the neutral monoisotopic mass is 1813.8604 Da. The reduced dithiol, C78H125N23O23S2, is 1815.8760 Da. The difference is H2, 2.01565 Da. The certificate should name the atomic-mass set so a reviewer can reproduce the fourth decimal place. Charge-state targets use the proton mass 1.007276 rather than the hydrogen-atom mass. On that basis the cystine [M+2H]2+ ion is at m/z 907.9375 and the [M+3H]3+ ion is at m/z 605.6274. The reduced [M+2H]2+ ion sits about 1.0078 m/z units higher, because the 2.01565 Da neutral shift is divided by the charge. Comparing a doubly charged measurement with a neutral theoretical mass, or a monoisotopic peak with an average-mass value near 1815 Da, creates a false discrepancy of about one to two daltons. That discrepancy is large enough to imitate deamidation, at +0.9840 Da, or to hide the disulfide shift if the wrong reference mass is chosen. Average mass and monoisotopic mass answer different instrument questions. Low-resolution spectra are often read against an average mass near 1815 Da for the cystine form, whereas a high-resolution monoisotopic assignment belongs against 1813.8604 Da. The most abundant isotope should be identified explicitly. At this mass the monoisotopic peak is still straightforward to recognise, but the record ought to state whether the monoisotopic peak, the most abundant peak, or a deconvoluted neutral mass was reported. The acceptance window is an SOP parameter, not a universal legal limit. At about 1814 Da, 5 ppm corresponds to roughly 0.009 Da and 10 ppm to roughly 0.018 Da. Either window tolerates ordinary calibration scatter and still rejects a two-dalton redox error. Unit-resolution data can reveal a two-dalton shift, yet isotope peaks and adducts make the assignment less secure. Cox and colleagues reported detection and in vitro metabolism of AOD9604, which is a reminder to lock the parent composition before related-species masses are interpreted (PMID:25208511).
Why compare a reduced preparation with the unreduced intact mass?
A single unreduced spectrum can match 1813.8604 Da and still leave the covalent state under-determined if the peak assignment is wrong. The direct control is a reduced preparation of the same lot, analysed under the same mass definition. Complete reduction of one intramolecular disulfide must raise the neutral monoisotopic mass by 2.01565 Da, to 1815.8760 Da. Failure to shift indicates that the reducing conditions did not open the cystine, that the original species was already reduced, or that the peak being tracked is not the peptide. Gorman and co-workers described this oxidised-versus-reduced comparison as a core mass-spectrometric route to disulfide evidence (DOI:10.1002/mas.10025). Alkylation after reduction adds a second, orthogonal increment. Iodoacetamide adds 57.0215 Da to each freed cysteine. Two cysteines therefore add 114.0429 Da to the reduced mass, or 116.0586 Da relative to the cystine mass. A split experiment is informative: alkylation without prior reduction should leave an intact intramolecular disulfide largely unlabelled, whereas alkylation after reduction should show the double addition. Incomplete labelling is reported as incomplete reduction or as a blocked thiol, not forced into a pass. The reagent identity, equivalents, pH, time and quench belong in the method note, because those parameters determine whether the mass shift is diagnostic. Dimer ambiguity is the main pairing issue for a two-cysteine peptide. There are no alternative intramolecular regio-isomers of the sort that complicate multi-disulfide conotoxins, where native mass spectrometry has been used to assign cysteine pairing (DOI:10.1021/ac101867e). There is, however, an intermolecular problem. A dimer whose mass is twice the cystine monomer can produce a 4+ ion at the same m/z as the monomer 2+ ion. The discriminator is isotope spacing: 0.50 Da between isotope peaks for a 2+ monomer, and 0.25 Da for a 4+ dimer. Glocker and co-workers showed that sample handling can scramble disulfide linkages and that mass spectrometry of digests can detect that scrambling, so alkaline thiol-exchange conditions must be documented and kept short (DOI:10.1016/1044-0305(95)00250-h). A trisulfide bridge, at about +31.972 Da, is a further covalent neighbour that intact mass will detect if the window is not set loosely around an average mass.
Where does tandem mass spectrometry stop confirming the disulfide?
An intact-mass match and a tandem spectrum answer different questions. Collision-induced dissociation of a peptide that still contains a disulfide often cleaves amide bonds outside the cystine loop more readily than it provides a clean series through the loop. In this sequence the loop between the cysteines is Arg-Ser-Val-Glu-Gly-Ser. Fragment ions that break inside that span can remain tethered by the disulfide and appear as composite internal ions rather than as simple b or y ions. Durand and co-workers examined tandem mass spectrometry of peptide disulfide regio-isomers and set out both the utility and the limits of collision-induced dissociation for disulfide characterisation (DOI:10.1016/j.ijms.2013.04.005). Collisional activation can also open the sulphur-sulphur bond. Fagerquist reported collision-activated cleavage of a disulfide linkage and raised the possibility of intramolecular rearrangement during that activation (DOI:10.1002/rcm.1390). A product ion that appears to prove a particular connectivity may instead reflect a gas-phase rearrangement. A CID spectrum should not be over-read as a map of the solution-phase bond. The intact-mass survey scan, acquired without a fragmentation event, remains the correct place to record the redox state. Electron-based dissociation behaves differently. Uggerud reviewed electron-capture dissociation of disulfide bonds and the preferential cleavage of the sulphur-sulphur linkage under electron capture (DOI:10.1016/j.ijms.2004.06.001). That selectivity is a reason not to treat an ECD spectrum as a disulfide-preserving measurement. Electron-transfer higher-energy collision dissociation has been used to sequence intact peptides that retain an intramolecular disulfide and, in the same experiment, to distinguish isomeric leucine and isoleucine (DOI:10.1007/s13361-017-1857-y). Because AOD-9604 contains both residues, that method class is the relevant one when isomer assignment is required. It is not required to call the cystine mass. Coinage-metal complexes have been surveyed as gas-phase disulfide cleavage reagents on intermolecular model peptides, and they remain a research option rather than a routine certificate field (DOI:10.1002/rcm.3122).
What should an Australian research-lot record show before purchase?
For the disulfide question, the minimum identity block is the full sequence, the expected covalent state, the molecular formula C78H123N23O23S2, the theoretical monoisotopic mass 1813.8604 Da, the observed m/z, the charge state, the deconvoluted mass, the mass error and the mass definition. The archived spectrum should let a second person see the isotope spacing that separates a 2+ monomer from a 4+ dimer. If reduction or alkylation was performed, the reagent, the observed shift and the result against the pre-set shift belong on the same report. If they were not performed, the report should say so, rather than imply that connectivity was mapped. Intact mass should not be blended into the HPLC purity figure. A chromatogram can agree with a correct cystine mass and still miss an isobaric substitution, or it can flag a peak that mass spectrometry then assigns as a trisulfide, a reduced species or an adduct. The two results are concordant only when lot number, sample name and analysis date match across the chromatogram and the mass spectrum. A reviewer should reject a package in which the mass printout carries a different lot identifier from the vial. The document supports laboratory procurement in Australia. Local batch documentation can be checked before a research purchase is accepted. It is not a therapeutic-goods release, and it does not authorise any use outside analytical or other laboratory research. Method qualification remains the receiving laboratory's responsibility: a supplier intact-mass result is evidence about that lot, not a transferred validated method. Where leucine and isoleucine must be distinguished, the record should include an electron-based tandem experiment of the class used to sequence disulfide-intact peptides, or state that isomer assignment was not attempted (DOI:10.1007/s13361-017-1857-y). Detection and in vitro metabolism literature on AOD9604 is the analytical context for keeping the parent mass distinct from related species (PMID:25208511).
Frequently asked questions
Does an intact-mass match prove the AOD-9604 disulfide connectivity?
An intact-mass match supports the elemental composition of the cystine form, including the two-hydrogen deficit. With only two cysteines, only one intramolecular pairing exists, so the remaining ambiguities are a reduced chain, a mixed disulfide, a trisulfide and a dimer that can share the monomer m/z. Connectivity beyond that composition call needs a reduced or alkylated control and, where sequence ions are required, a tandem experiment interpreted with the known limits of collisional disulfide cleavage.
What mass shift confirms reduction of one AOD-9604 disulfide?
Complete reduction of one intramolecular disulfide raises the neutral monoisotopic mass by 2.01565 Da, from 1813.8604 Da for C78H123N23O23S2 to 1815.8760 Da for the dithiol. On a doubly charged ion the same change is about 1.008 m/z units. Subsequent iodoacetamide labelling of both freed cysteines adds a further 114.0429 Da to the reduced mass. The certificate should state the reagent and whether a non-reduced alkylation control was included.
Can HPLC purity replace AOD-9604 disulfide intact-mass verification?
No. Chromatographic area percent describes peak distribution under a stated method. It does not measure the two-hydrogen deficit of a disulfide, and it does not separate a 2+ monomer from a 4+ dimer at the same m/z. Intact mass does not replace HPLC either, because an isobaric co-eluting impurity can hide under a correct mass. Lot release documentation should keep the two results separate and tie both to the same lot number, sample name and analysis date.
Why are leucine and isoleucine invisible to intact mass?
Leucine and isoleucine have the same elemental composition, so exchanging them does not change the intact mass. AOD-9604 contains both residues, at the second and fourth positions of the working sequence. Calling the cystine mass therefore does not assign those positions. Discrimination requires fragment ions that report the side-chain difference, such as the electron-transfer higher-energy collision dissociation approach demonstrated on disulfide-intact peptides. If that experiment was not run, the lot record should say that isomer assignment was not attempted.
Is this verification a guide to non-research use?
No. AOD-9604 disulfide bond intact mass verification is an identity and covalent-state control for a research material. The numbers above are calculation and documentation checks for laboratory lots. They are not instructions for any use outside analytical or other laboratory research, and they are not a therapeutic-goods specification. A receiving laboratory still qualifies its own method before relying on a supplier mass result for inventory booking.
References
- PMID:25208511 — Detection and in vitro metabolism of AOD9604 — Drug Test Anal — 2015
- DOI:10.1002/mas.10025 — Protein disulfide bond determination by mass spectrometry — Mass Spectrometry Reviews — 2002
- DOI:10.1021/ac101867e — Disulfide Bond Assignments by Mass Spectrometry of Native Natural Peptides: Cysteine Pairing in Disulfide Bonded Conotoxins — Analytical Chemistry — 2010
- DOI:10.1016/1044-0305(95)00250-h — Characterization of disulfide linkages and disulfide bond scrambling in recombinant human macrophage colony stimulating factor by fast-atom bombardment mass spectrometry of enzymatic digests — Journal of the American Society for Mass Spectrometry — 1995
- DOI:10.1016/j.ijms.2013.04.005 — Tandem mass spectrometry (MSn) of peptide disulfide regio-isomers via collision-induced dissociation: Utility and limits in disulfide bond characterization — International Journal of Mass Spectrometry — 2013
- DOI:10.1002/rcm.1390 — Collision‐activated cleavage of a peptide/antibiotic disulfide linkage: possible evidence for intramolecular disulfide bond rearrangement upon collisional activation — Rapid Communications in Mass Spectrometry — 2004
- DOI:10.1016/j.ijms.2004.06.001 — Update on electron capture dissociation of disulfide bond — International Journal of Mass Spectrometry — 2004
- DOI:10.1007/s13361-017-1857-y — EThcD Discrimination of Isomeric Leucine/Isoleucine Residues in Sequencing of the Intact Skin Frog Peptides with Intramolecular Disulfide Bond — Journal of the American Society for Mass Spectrometry — 2018
- DOI:10.1002/rcm.3122 — Can metal ions be used as gas‐phase disulfide bond cleavage reagents? A survey of coinage metal complexes of model peptides containing an intermolecular disulfide bond — Rapid Communications in Mass Spectrometry — 2007
Research use only
This article is provided for laboratory research and educational purposes only. Products referenced are not for human or veterinary use. ClaraScience makes no therapeutic, medical, or efficacy claims, and nothing here constitutes medical advice.
