ANALYTICAL METHODS · PRACTICAL GUIDE
MOTS-c and a +16 Da Signal: Work Through an Oxidation Candidate
Find MOTS-c’s two methionines, then follow the mass change from one added oxygen.
A MOTS-c signal appears roughly 16 Da heavier than expected. Oxidation is one possibility to investigate. But to understand that clue, we first need to find the relevant amino acids and check whether the plot shows molecular mass or m/z.
Find the two M letters
A common MOTS-c reference sequence is MRWQEMGYIFYPRKLR. M means methionine. Count from the beginning: the two M residues are at positions 1 and 6.
Use the actual supplied sequence and end groups when evaluating a sample. The reference sequence is a starting point, not a replacement for the material’s specification.
Why one oxygen gives about 16 Da
Adding one oxygen gives a monoisotopic mass increase of 15.994915 Da in the Unimod oxidation record. For two oxygen atoms, the increase is 31.989830 Da. These are often shortened to +16 and +32 in conversation.
Those rounded numbers are useful clues. An accurate comparison uses the more precise value and the correct molecular form.
Why the spectrum may shift by only 8
For matching double-charge ions, divide the mass change by two. One oxygen then shifts m/z by about 7.99746. Two oxygen atoms shift it by about 15.99492.
So “a shift of 16” is unclear without saying which axis and charge you mean.
Can the total mass tell you which M changed?
Not by itself. Oxidation at either methionine can give the same one-oxygen increase in complete mass. Suitable fragment data and chromatographic comparison may help investigate the location and alternatives.
The spectrum also cannot tell you when the change happened. Keep storage and handling records with the analytical data.
Try a three-charge signal
Do not convert peak intensity directly into a weight percentage of oxidation. That requires a suitable quantitative method.
See the answer
15.994915 ÷ 3 ≈ 5.33164. That value alone still does not locate the change.
What to ask about a MOTS-c sample
For MOTS-c, confirm the actual sequence and end groups, and ask which identity or stability-related information is available. This lesson explains a possible mass change; it does not state a batch oxidation level or shelf life.
Discuss material and document requirements with NHD · Continue with the analytical guide