What Do MOTS-c ESI-MS Fragmentation Patterns Confirm About Sequence Identity?
MOTS-c ESI-MS fragmentation patterns confirm bond-level agreement between observed product ions and the declared sequence MRWQEMGYIFYPRKLR. They do not measure a biological activity, and they are not a purity percentage. The primary structure reported for this mitochondrial-derived peptide is sixteen residues, methionine at the N-terminus and arginine at the C-terminus (PMID:25738459). Later literature continues to describe that same peptide as a discrete sequence-defined entity (PMID:27216708; PMID:36233287). For a supplier certificate, that sequence is the reference. Intact mass asks whether the precursor matches the calculated neutral monoisotopic mass of about 2173.108 Da. Fragmentation asks which residue occupies which position.
Electrospray ionisation from an acidic water-acetonitrile mobile phase yields multiply protonated precursors. Three arginine residues, one lysine and a free N-terminus make [M+3H]3+ near m/z 725.377 and [M+4H]4+ near m/z 544.284 the practical precursors. Isolation of one charge state, then collision-induced or higher-energy collisional dissociation, releases b ions from the N-terminus and y ions from the C-terminus. The pattern is that set of m/z values, charges and relative intensities, read against a theoretical list from the same sequence.
A single matching product ion is not identity. The laboratory should require a contiguous series, a stated mass-accuracy window, and concordance between the selected precursor and the intact-mass result on the same lot. Low-energy spectra do not separate isoleucine at position 9 from leucine at position 15, because those residues share a monoisotopic residue mass. That limit belongs in the reported result, not in a later clarification. Doubly charged product ions must be labelled with charge; folding them into a singly charged column creates false mismatches. Internal fragments may support an assignment only after the b and y series is already coherent. Within those rules, the pattern supports sequence consistency with the declared 16-mer. It does not address human use, and it does not replace an orthogonal chromatographic purity result.
How Are Theoretical b and y Ions Calculated for the MOTS-c Fragment Map?
Calculate the fragment map; do not copy it from an unrelated file. Use monoisotopic residue masses. Sum the sixteen residues of MRWQEMGYIFYPRKLR and add water to obtain the neutral mass, about 2173.108 Da. The singly protonated ion is about m/z 2174.115. Doubly through quintuply protonated precursors fall near m/z 1087.561, 725.377, 544.284 and 435.629. State on the certificate whether the reported figure is monoisotopic or average. Near 2.2 kDa the average mass is roughly 1.5 Da higher, so mixing conventions can invent an out-of-specification result or conceal a real shift.
Singly charged b ions equal the N-terminal residue sum up to the cleaved bond, plus one proton. Singly charged y ions equal the C-terminal residue sum, plus water, plus one proton. Illustrative 1+ values are b2 (MR) near m/z 288.149, b3 (MRW) near 474.228, b11 near 1505.670, y1 near 175.119, y2 (LR) near 288.203, y5 (PRKLR) near 669.452 and y8 near 1092.668. These are reading aids. Regenerate them in the processing software from the residue-mass table locked to the method. A 10 ppm window at these masses is only a few hundredths of a dalton, so fourth-decimal drift matters.
Two checks catch transcription errors before any sample is run. The full-length b ion is not a useful product ion; the intact sequence returns as the precursor. For singly charged pairs, b_n plus y_(16-n) equals the singly protonated peptide mass plus one proton. If a hand-built table fails that complementarity test, the table is wrong, not the sample. Doubly charged fragments sit near half the singly charged m/z and must be labelled with charge so they are not scored as mismatches. a-ions, formed by loss of carbon monoxide (about 27.995 Da) from b ions, may be annotated only after the b and y assignment is secure. They support the map. They are not the identity backbone, and they should not be used to fill gaps in a weak series.
Which Ions Make MOTS-c ESI-MS Fragmentation Patterns Diagnostic?
Diagnostic value follows from where this sequence prefers to break. Proline at position 12 promotes cleavage of the amide bond on its N-terminal side, between Tyr11 and Pro12, releasing b11 and y5. The y5 ion, PRKLR, carries proline and the C-terminal basic cluster, so it is often intense. y5 near m/z 669.452 is useful only together with neighbours such as y3, y4 and y6. A single nominal mass can be an internal fragment or noise.
The C-terminus retains charge. Basic side chains at positions 13, 14 and 16 usually make the y series richer than the b series in positive-ion collision-induced dissociation. A practical read looks for a contiguous y run across the proline junction, then asks whether b2 and b3 confirm the MRW start. Tryptophan at position 3 is distinctive: its immonium ion near m/z 159.092 helps exclude a gross substitution there. Tyrosine, phenylalanine and methionine immonium ions, near m/z 136.076, 120.081 and 104.053, support composition only. A spectrum of immonium ions alone has not confirmed sequence, even if the precursor mass matched.
Intensity ratios are method-dependent. Collision energy, precursor charge and formic acid concentration change which bond wins, so lock precursor m/z, isolation width, activation type and a normalised collision-energy range, and compare the sample with a qualified reference in the same sequence. Match rules can require a minimum of assigned amide bonds out of fifteen, plus a mass-error limit such as 10 ppm on a high-resolution analyser. Selected-reaction monitoring on a triple quadrupole can confirm chosen transitions. It does not replace a full product-ion scan when the risk is an unexpected sequence error. Unmatched peaks stay on the report. An assigned series beside a large unexplained ion at a related-substance mass is an investigation, not a pass.
How Do Mass Shifts Localise Related Substances on a MOTS-c Fragment Map?
A precursor shift says the composition changed. The fragment pattern says where. Methionine oxidation adds about 15.995 Da. Positions 1 and 6 are both methionine, so a +16 Da precursor is ambiguous until product ions are read. If b2 and b3 are unshifted and y11 is shifted, the oxygen sits at Met6, because y11 is the shortest y ion containing Met6. If b2 is already shifted and the short y ions are not, the oxygen sits at Met1. A shift of about 31.990 Da fits oxidation at both sites only when the early b series and the y11 region each move by one oxygen. A bare +16 Da label wastes that information.
Glutamine at position 4 can deamidate, adding about 0.984 Da. The shift should appear from b4 upward and from y13 upward, not in y1 to y12. On a low-resolution instrument that delta hides in the isotope envelope. High-resolution intact mass plus the b4 and y13 pair is the appropriate check. A glutamine-to-lysine error at position 4 is smaller: lysine is only about 0.036 Da heavier than glutamine. A reciprocal lysine-to-glutamine swap at position 14 can cancel the intact-mass change. b4 moves if position 4 changed; y3, which contains Lys14, moves if position 14 changed. Mass-only release is a weak identity test for this sequence.
Truncations are straightforward if the extracted-ion list includes them. Loss of N-terminal methionine lowers the neutral mass by about 131.040 Da and removes the original b2. Loss of C-terminal arginine lowers it by about 156.101 Da and removes y1 near m/z 175.119. Keep adducts out of the impurity column. Sodium sits about 21.982 Da above the singly protonated peptide, trifluoroacetic acid about 113.993 Da above it, and acetic acid about 60.021 Da above it. Those signals track the main fragment skeleton or fade when source desolvation is harsher. An N-formyl difference, only where that variant was declared, adds about 27.995 Da and shifts the b series rather than the short y ions. These are assignments for the analysed vial, not a handling claim.
What Should an Australian CoA Record With MOTS-c ESI-MS Fragmentation Patterns?
An Australian buyer should reconstruct the identity decision from the certificate alone. Record the declared sequence, monoisotopic versus average convention, measured mass and ppm error, precursor m/z and charge, activation method, and a product-ion table of observed m/z, charge, theoretical m/z, error and label. A spectrum image without a table cannot be re-checked. The lot number must match the vial and the chromatographic section. A pattern from another lot is not evidence for the vial in hand.
System suitability sits on the same report. Inject a qualified reference of the declared sequence in the same run, and accept sample data only if that reference meets the coverage rule. Blanks document carry-over: this basic peptide can linger in an electrospray source and donate y ions to the next vial. Name the method, polarity, analyser and calibration status. If only selected transitions were monitored, say so. Do not imply full coverage. Area-normalised HPLC purity at 214 nm does not confirm sequence, and a matching fragment pattern does not quantify poorly ionising related substances. Net peptide content, counterion and water answer different questions.
False calls are mostly bookkeeping. Picking the isotopic centroid instead of the monoisotope makes a specification near 2173.108 Da look 1 to 2 Da heavy; triply charged spacing should be about 0.333 Da. A wide isolation window around m/z 725.377 can co-isolate an oxidised or sodiated neighbour and corrupt coverage. In-source y ions from a harsh orifice voltage are neither impurities nor collision-cell sequence proof. Isoleucine at position 9 and leucine at position 15 remain unproven by ordinary b and y ions. Honest wording is consistent with the declared sequence, not residue-level proof of both. Leave unassigned peaks unassigned. Published experimental work uses MOTS-c as a sequence-defined test article (PMID:29983246; PMID:31293078). Fragment-level agreement with MRWQEMGYIFYPRKLR is a traceability statement for research material only. It is not a therapeutic-goods specification, and it transfers no experimental outcome onto the lot.
Frequently asked questions
What monoisotopic mass anchors MOTS-c ESI-MS fragmentation patterns?
The neutral monoisotopic mass calculated for MRWQEMGYIFYPRKLR is about 2173.108 Da, so [M+H]+ is about m/z 2174.115. Practical precursors are [M+3H]3+ near 725.377 and [M+4H]4+ near 544.284. These are calculator outputs for the declared sequence, not measured lot results. The certificate must say whether a reported mass is monoisotopic or average, because the average value sits roughly 1.5 Da higher.
Why is y5 a useful ion in a MOTS-c product-ion spectrum?
Proline at position 12 favours cleavage between Tyr11 and Pro12, which releases y5 (PRKLR) near m/z 669.452 and b11 near m/z 1505.670. The C-terminal basic residues help y5 retain charge. y5 alone is not identity. Neighbouring y ions and an intact precursor that matches the same lot should accompany it. Intensity still depends on collision energy and charge state, so compare with a reference run under the same method.
Can fragment ions localise the two methionine oxidation sites?
Yes, if the product-ion series is complete enough. A +15.995 Da precursor is ambiguous because Met1 and Met6 can both oxidise. Unshifted b2 and b3 with a shifted y11 localise the oxygen to Met6. A shifted b2 with unshifted short y ions localises it to Met1. A +31.990 Da precursor fits both sites only when each series moves by one oxygen. Report the localisation, not only the precursor delta.
Do fragment patterns replace HPLC purity on a research certificate?
No. Fragment patterns test sequence consistency. Area-normalised HPLC purity at a stated wavelength estimates chromatographic related substances under that method. A peptide can match MRWQEMGYIFYPRKLR and still show truncation or oxidation peaks in the chromatogram. Conversely, a high area percent does not prove which residue is at position 4 or 14. Keep the two results in separate rows, with lot numbers that match, and do not average them into one quality score.
Are isoleucine and leucine proven by a standard CID spectrum of MOTS-c?
No. Isoleucine at position 9 and leucine at position 15 have the same residue mass, so b and y ions from low-energy collision-induced dissociation do not distinguish them. Immonium ions do not either. A research certificate should not claim independent proof of both residues unless a secondary method was used. Appropriate wording is that the spectrum is consistent with the declared sequence, with isoleucine and leucine assigned by that declaration rather than by the fragment masses.
Which adducts should not be reported as MOTS-c sequence variants?
Sodium, trifluoroacetic acid and acetic acid adducts are common electrospray companions of a peptide salt, not residue substitutions. Approximate deltas above [M+H]+ are 21.982 Da, 113.993 Da and 60.021 Da. They should share the main component fragment skeleton or diminish when desolvation is harsher. Sequence variants such as des-Met or Met oxidation move specific b or y ions and remain after adduct-suppressing source settings. Keep adducts in an artefact row on the certificate.
References
- PMID:25738459 — The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance — Cell Metab — 2015
- PMID:27216708 — MOTS-c: A novel mitochondrial-derived peptide regulating muscle and fat metabolism — Free Radic Biol Med — 2016
- PMID:29983246 — The Mitochondrial-Encoded Peptide MOTS-c Translocates to the Nucleus to Regulate Nuclear Gene Expression in Response to Metabolic Stress — Cell Metab — 2018
- PMID:31293078 — The mitochondrial-derived peptide MOTS-c is a regulator of plasma metabolites and enhances insulin sensitivity — Physiol Rep — 2019
- PMID:36233287 — MOTS-c, the Most Recent Mitochondrial Derived Peptide in Human Aging and Age-Related Diseases — Int J Mol Sci — 2022
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.
