Peptide Mapping: Protease Digestion and Sequence Coverage
Sixteen daltons above the expected value tells you an oxygen attached itself to the molecule. It does not tell you where. Mapping answers that second question, naming the fragment that carries the extra mass and therefore the residue, or the handful of residues, responsible. Among routine techniques it stands alone in fixing a modification to a position rather than announcing one somewhere along the chain. Intact mass confirms weight; a map confirms order.
Cut, separate, weigh, compare
A protease of known specificity chops the peptide into pieces. Chromatography separates those pieces and the mass spectrometer weighs each one. Since the enzyme’s cut sites follow from the sequence, the full set of fragment masses that a correct sequence would generate can be worked out beforehand.
The test is the comparison. Account for every fragment and every stretch of the chain has been shown to be what it ought to be. Turn up a fragment at an unanticipated mass and the discrepancy has been pinned to that stretch.
Specificity is the entire point
Random cutting would yield a mixture nobody could read. All the value lies in cleaving at defined residues.
| Trypsin | Cleaves after lysine and arginine, though not where proline follows. The default pick, since those residues occur often enough to produce fragments of workable length yet seldom enough that the fragments do not come out trivially short. |
| Chymotrypsin | Cleaves after the bulky hydrophobic residues: phenylalanine, tryptophan, tyrosine. |
| Glu-C | Cleaves after glutamate, and under certain buffer conditions after aspartate as well. |
| Asp-N | Cleaves before aspartate. |
Because each enzyme draws fragment boundaries in a different place, whatever one digest leaves unexamined can frequently be picked up by running a second digest with a different enzyme.
Two caveats that qualify any map
Coverage states what share of the sequence turned up in identified fragments, and it practically never reaches 100%, for reasons baked into the technique. Some fragments come out too small to hold on the column or to detect dependably; a lone residue sandwiched between two lysines yields something analytically worthless. Others are too big and too hydrophobic to elute. Some lack an ionizable group and ionize badly. A region containing no cut sites at all delivers one huge fragment that behaves poorly throughout. The coverage figure is accordingly part of the result rather than a footnote to it. Ninety-five percent with the uncovered stretch identified is a strong outcome. A map quoting no coverage has declined to say what it never saw.
Missed cleavages are the second caveat. No enzyme takes every available site on every molecule. A site adjacent to a proline, beside a modified residue, or tucked inside a structured region may simply be passed over, giving a fragment that spans two of the expected ones. This is ordinary and foreseeable. Interpretation runs against a list holding both the expected fragments and the credible missed-cleavage products, so a surprisingly long fragment generally reflects a skipped site rather than a discovery.
Artifacts the procedure creates for itself
Four things the method can manufacture are worth naming. Tryptic digests commonly sit for hours at 37 degrees near pH 8, and those are exactly the conditions that drive deamidation, so a map can end up reporting a deamidation the digestion itself produced. Those same conditions scramble disulfides, which is why connectivity work is run at lower pH with a faster enzyme wherever that is possible. The protease also digests itself, contributing autolysis fragments to the chromatogram; their masses are known, and they get subtracted. Methionine oxidizes with very little encouragement, so seeing oxidation in a map does not demonstrate that oxidation was present in the material beforehand. Guarding against all four means running a blank digest and comparing against a reference sample carried through identically.
Beyond a stray oxygen
The same localizing logic applies to deamidation, to a protecting group that stayed on, to a substitution, and to disulfide connectivity. For connectivity the trick is to digest between the cysteines and observe which fragments remain tethered to one another, which is what establishes the pairing, as described in disulfide bond formation and scrambling.
Push one step further and fragment the individual peptides inside the instrument, and a modification can be narrowed to a single residue. That approach is covered in reading a sequence from fragment ions.
Why certificates seldom carry one
Expense and proportionality decide it. Mapping means a digestion, a separate chromatographic run and somebody’s interpretation, and for a peptide of fifteen residues the intact mass already pins composition down tightly enough that a map contributes little.
Length changes the calculation. Intact mass loses its decisiveness as chains grow: an 80-residue molecule carrying one substitution can fall within routine mass accuracy of the right answer, and a map settles what the mass leaves open. That crossover is taken up in where peptide analysis becomes protein analysis.
The boundary of the claim
What a map establishes is consistency between the fragments observed and the sequence expected, limited to whatever coverage was reported. Stereochemistry lies outside it, a D-residue producing a fragment of exactly the same mass. Purity lies outside it too, since the experiment runs on material that has already been isolated. And within the uncovered fraction it establishes nothing whatsoever.
Set beside an intact mass and a purity trace, it settles the question neither of those can reach: not the weight of the molecule, but the order of it.
