Why modifications are made
Unmodified peptides are cleared quickly and degraded easily. A C-terminal amide removes a negative charge and slows carboxypeptidase attack. An N-terminal acetyl group does the same at the other end. A fatty acid chain promotes albumin binding and extends circulation dramatically, which is the mechanism behind the long half-lives of several modern therapeutic peptides.
Others are analytical rather than pharmacological. A biotin tag enables affinity capture, a fluorophore enables imaging, and stable isotope labelling enables quantitative mass spectrometry against an internal standard.
The mass changes worth memorising
A handful of numbers account for most of what you will see on a peptide mass spectrum. Recognising them turns an unexplained mass difference into a specific hypothesis you can test, rather than an anomaly to be noted and set aside.
The direction is part of the number. Every entry below is signed, because a gain and a loss of the same magnitude have entirely different explanations, and the sign is what distinguishes an oxidation from an amidation.
- •C-terminal amidation: minus 0.98 Da
- •N-terminal acetylation: plus 42.01 Da
- •Each disulfide bridge: minus 2.02 Da
- •Oxidation, one oxygen: plus 15.99 Da
- •Deamidation: plus 0.98 Da
- •Phosphorylation: plus 79.97 Da
- •Palmitoylation: plus 238.23 Da
- •Biotinylation: plus 226.08 Da
Pegylation is different
Polyethylene glycol chains are polydisperse: a nominal 5 kDa PEG is a distribution of chain lengths centred on 5,000 daltons, not a single molecular species. A pegylated peptide therefore has no single molecular weight and shows up in a mass spectrum as a broad envelope rather than a peak.
The number reported for a pegylated conjugate is a nominal average, and the width of the distribution is a property of the PEG reagent. This is a genuine limit on how precisely a pegylated product can be characterised by mass.
Isotope labelling
Uniform carbon-13 or nitrogen-15 labelling shifts the mass by a fixed amount per labelled atom: 1.00336 daltons per carbon and 0.99703 per nitrogen. The shift for a whole peptide is that figure multiplied by the atom count, which is why a labelled peptide's mass depends on its composition rather than on a single modification.
The chemistry is unchanged, which is the point: a labelled internal standard behaves identically to the analyte through sample preparation and chromatography, and differs only in mass.
How the modified mass is calculated
The base sequence mass, then one addition or subtraction per selected modification. Modifications that apply per residue are multiplied by the count of that residue in the sequence.
base MW = SUM(residue masses) + 18.0153 modified MW = base MW + SUM(selected modification deltas) per-residue mods scale with the residue count: phosphorylation = +79.9663 x number of selected S/T/Y oxidation = +15.9949 x number of Met isotope labelling: 13C = +1.00336 x carbon count 15N = +0.99703 x nitrogen count
- Compute the unmodified mass. Residue masses plus one water, the same calculation the molecular weight tool performs, so the two agree before any modification is applied.
- Apply terminal modifications once. Acetylation and amidation act on a terminus, so each applies once regardless of sequence length.
- Scale residue modifications by count. Phosphorylation, oxidation and deamidation act on specific residues, so the delta is multiplied by the number of those residues you select.
- Subtract for disulfide bridges. 2.016 daltons per bridge. The number of bridges is capped at half the cysteine count, since a bridge needs two.
- Count atoms for isotope labelling. Carbon and nitrogen counts are summed per residue from a composition table, then multiplied by the isotope mass difference. This is why the labelled shift is sequence-dependent rather than fixed.
What this method cannot tell you
- •Pegylation has no exact mass. The figure shown is a nominal average for a polydisperse reagent.
- •It computes mass only. Whether a modification is chemically feasible at a given site, and what it does to activity, are separate questions.
- •Modifications are treated as independent. In practice one can block another, and the order of synthesis steps matters.
- •Isotope labelling assumes uniform incorporation. Partial labelling gives a distribution rather than a single shift.
Modification mass calculator: frequently asked questions
Replacing the C-terminal carboxylic acid with an amide. It removes a negative charge, makes the peptide 0.98 daltons lighter, and slows degradation by carboxypeptidases.
Many natural bioactive peptides are amidated, and the modification is often necessary for full activity rather than merely helpful for stability.
42.011 daltons. It also removes the positive charge on the terminal amine, which changes the isoelectric point.
This is why thymosin beta-4's catalogue weight of 4,963 is 42 daltons above the 4,921 its bare sequence gives.
Each bridge removes 2.016 daltons, the two hydrogens lost when two thiols oxidise.
The maximum number of bridges is half the cysteine count, rounded down.
Attaching a polyethylene glycol chain, usually to extend circulation time by increasing hydrodynamic size and reducing renal clearance.
PEG reagents are polydisperse: a nominal 5 kDa PEG is a distribution of chain lengths, not one molecular species. The conjugate therefore has an average mass and a distribution width rather than a single weight.
79.966 daltons per phosphate group, on serine, threonine or tyrosine.
It also adds two negative charges at neutral pH, which usually matters more biologically than the mass change does.
Attaching a sixteen-carbon fatty acid, adding 238.23 daltons. It promotes binding to serum albumin, which slows clearance substantially.
This is the mechanism behind the extended duration of several modern therapeutic peptides. It also makes the peptide markedly more hydrophobic and harder to dissolve.
Uniform carbon-13 adds 1.00336 daltons per carbon atom and nitrogen-15 adds 0.99703 per nitrogen.
The total shift depends on the sequence's atom composition, so a labelled peptide's mass has to be computed from its own composition rather than from a fixed offset.
As internal standards for quantitative mass spectrometry. A labelled peptide is chemically identical to the analyte, so it behaves the same through extraction and chromatography, and differs only in mass.
That lets the instrument measure the ratio between the two directly, which cancels most sources of variation in sample handling.
Adding a biotin tag, 226.08 daltons, so the peptide can be captured by streptavidin. It is the standard handle for pull-down and immobilisation.
The biotin-streptavidin interaction is one of the strongest non-covalent bonds known, which is what makes the capture effectively irreversible.
Often substantially. Lipidation makes a peptide markedly more hydrophobic; pegylation makes it more soluble; acetylation and amidation each remove a charge and generally reduce solubility a little.
The solubility predictor works from the unmodified sequence, so a heavily modified peptide needs its behaviour established empirically.
Yes, and the mass changes simply add. A peptide can be acetylated, amidated and bridged at once.
The chemistry is less additive than the arithmetic: modifications can block each other's sites and the order of synthesis steps matters.
Mass spectrometry. The intact mass should shift by exactly the expected amount, and fragmentation locates which residue carries it.
An intact mass matching the modified weight confirms that something of the right mass was added. Only the fragment series says where.
Oxidation adds an oxygen, 15.995 daltons, mostly to methionine. Deamidation converts an amide to an acid, adding 0.984 daltons, on asparagine or glutamine.
Both are degradation rather than deliberate modification, and both make the peptide slightly more polar, so both elute earlier on a reverse-phase column.
Any modification that adds or removes a charged group does. Acetylation removes the positive N-terminus, amidation removes the negative C-terminus, phosphorylation adds two negative charges.
In a short peptide these shifts are large, because the termini are a big share of the total ionisable groups.
Because a modification at or near the binding region can block the interaction. Adding mass is easy; adding it somewhere harmless is the hard part.
This is why lipidation and pegylation are attached at positions established by structure-activity work rather than wherever the chemistry is convenient.
Sometimes. If the non-standard residue differs from a standard one by a fixed mass, entering the standard residue and applying that mass difference gives the right total.
It gives the correct mass and the wrong sequence, so treat it as an arithmetic convenience rather than a description of the molecule.
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