Two Correct Masses for the Same Molecule

Here is a discrepancy worth pausing over: a report where the measured figure and a database figure sit about one dalton apart. That is the size of a real modification. It is also the size of a bookkeeping error, because two different conventions for stating a peptide’s mass are in circulation and both of them are correct. Which one you are holding is decided long before the certificate is printed.

The instrument chose for you

Resolution settles it. Where an instrument can pull the individual isotope peaks apart, the first peak of the cluster is the monoisotopic species, and that is the value reported. Modern time-of-flight and orbital-trap instruments manage this for peptides of ordinary size.

Where it cannot, the whole cluster arrives as one broad signal, and the centroid of that envelope approximates the average mass. That approximation is what lands on the report. How the two ionization techniques differ in the spectra they generate is treated in MALDI and ESI.

Size eventually defeats even good resolution. Isotope peaks crowd closer together as atom count climbs, so for large molecules the cluster stops resolving and average mass becomes the only practical figure.

Why a single molecule has more than one mass

Nature does not supply elements in one flavor. Carbon is mostly carbon-12 with roughly 1.1% carbon-13 alongside it; hydrogen, nitrogen, oxygen and sulfur each contribute their own minor isotopes in their own proportions. Take a population of chemically identical molecules and its members differ in mass according to which isotopes each one happens to have picked up.

The gap grows with the molecule

Atom count drives the spread between them. A short peptide of a few residues separates the two conventions by a fraction of a dalton, small enough to overlook. Stretch to 29 residues and it is a dalton or two. Reach a 43-residue protein and it can exceed three.

Now recall the scale of the things worth detecting. Deamidation moves mass by 0.98 daltons, as covered in deamidation and the 0.98 dalton shift. An amide and a free acid are separated by roughly one. Set a measured monoisotopic value against a calculated average value, or the other way round, and the artifact you manufacture is the same magnitude as the modification you set out to find.

Two checks before trusting any comparison

First: confirm that measured and calculated are the same species of number. A certificate pitting a high-resolution measurement against a calculated average mass will display a mismatch that says nothing whatever about the material in the vial.

Second, on electrospray data, watch the charge state. What gets reported as a mass-to-charge ratio is the molecular mass plus the attached protons, all divided by how many protons there are. The deconvoluted neutral mass is the figure that belongs in a comparison with a calculated value. The division of labor between methods is set out in HPLC and mass spectrometry in purity verification.

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