Peptide molecular formula and weight: deriving and checking both from the sequence

Two numbers that are both derived

A product record for a research peptide carries a molecular formula and a molecular weight, and both look like facts the supplier looked up. They are not. Both follow arithmetically from the sequence, and a buyer with the sequence, a table of residue masses and ten minutes can derive them independently. That is the useful thing about them. A formula that does not match the sequence, or a weight that does not match the formula, is an error someone made while assembling the record, and it is visible from the outside without a laboratory.

The weight has a further role. The certificate of analysis for a lot should report an observed mass from mass spectrometry, and that observed mass is read against the theoretical weight derived here. If the record's weight is wrong, the comparison is wrong, and a lot that is actually correct can look wrong, or a lot that is wrong can look right. So the derivation is worth doing once, carefully, for every sequence a laboratory buys, and worth keeping.

This page walks the arithmetic, works it through for a real sequence, explains why two different weights are quoted for the same peptide, and sets out what a counterion and residual water add to the mass in a vial. Nothing here is chemistry a buyer needs a background to follow. It is bookkeeping with atoms.

From residues to a formula

When amino acids join into a chain, each peptide bond forms by condensation and releases one molecule of water. A residue is what remains of each amino acid after that loss: the free amino acid minus H2O. The chain of n residues has lost n − 1 waters to form n − 1 bonds, so a linear peptide with a free amino terminus and a free carboxyl terminus has the formula of all its residues added together plus one water, the one that was never lost because the ends are unjoined.

The residue formulas are fixed and well tabulated. Glycine is C2H3NO, alanine C3H5NO, proline C5H7NO, and so on through the twenty standard residues. Add the residues, add H2O, and the result is the molecular formula of the free peptide. Modifications change the sum in predictable ways: an N-terminal acetyl adds C2H2O, a C-terminal amide replaces the terminal OH with NH2 and so swaps one oxygen for one nitrogen and adds a hydrogen, and a disulfide between two cysteines removes two hydrogens.

Worked through for BPC-157, whose sequence is Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val: three glycines, one glutamate, four prolines, one lysine, two alanines, two aspartates, one leucine and one valine. Summing the residue formulas gives C62H96N16O21; adding one water gives C62H98N16O22, which is the formula on the PubChem record and on every correct product listing. A record that shows a different formula for the same sequence has either a different sequence or a mistake.

From a formula to a weight

Molecular weight is the formula multiplied out with atomic weights. Which atomic weights is the question the next section answers; for now, take the conventional average values: carbon 12.011, hydrogen 1.008, nitrogen 14.007, oxygen 15.999 and sulfur 32.06. These are the standard atomic weights published by the IUPAC Commission on Isotopic Abundances and Atomic Weights, rounded to the precision a peptide weight needs.

The quicker route is to sum average residue masses directly, which is what every peptide calculator does. The standard residue masses are tabulated to two decimals and the arithmetic is a column of additions.

PubChem lists BPC-157 at 1419.5 g/mol. The two-hundredths of a difference is rounding in the residue table, and a product record quoting anything between 1419.5 and 1419.6 is consistent with the sequence. A record quoting 1418.7 is not wrong either; it is quoting a different kind of mass, which is the next point.

Average mass and monoisotopic mass

Every element in a peptide exists as a mixture of isotopes. Carbon is about 98.9 percent carbon-12 and 1.1 percent carbon-13; hydrogen, nitrogen, oxygen and sulfur each have their own minor heavy isotopes. The average mass uses the weighted mean of that mixture for each element, which is what the standard atomic weights are. The monoisotopic mass uses only the lightest stable isotope of each: carbon exactly 12, hydrogen 1.007825, nitrogen 14.003074, oxygen 15.994915, sulfur 31.972071, as tabulated by NIST. For BPC-157 the monoisotopic mass works out near 1418.7 against an average of 1419.6, and both are correct descriptions of the same molecule.

Which one a record should quote depends on what it will be compared with. A balance measures average mass, because a weighed sample contains the natural isotope mixture, so solution arithmetic and net-content figures use the average. A mass spectrometer resolves individual isotopologues, and for a peptide of this size the tallest peak in the cluster is the monoisotopic one, so the certificate's observed mass is read against the monoisotopic value. A certificate that reports 1418.7 observed against 1419.6 theoretical and calls it a match has compared two different quantities and been lucky that the gap is small.

The gap grows with the molecule. A peptide with sixty-two carbons has a modest chance of containing one carbon-13, so the monoisotopic peak is still the largest. Tirzepatide, with 225 carbons, is more likely than not to contain at least one heavy isotope, and the most abundant peak in its cluster sits two or three mass units above the monoisotopic one. For sequences above roughly two kilodaltons a certificate should say which peak it is reporting, and a record should carry both masses so a reader can tell which comparison is being made.

The two masses also differ in how they are rounded. Average mass is conventionally quoted to one or two decimal places, because the atomic weights it rests on are themselves intervals. Monoisotopic mass is quoted to four or more, because the isotopic masses behind it are known to that precision and a high-resolution instrument can resolve it. A record that quotes a monoisotopic mass to one decimal place has thrown away the precision that makes the comparison useful; one that quotes an average mass to four has invented precision the atomic weights do not have.

What the vial weighs that the formula does not

The formula describes the free peptide. The vial contains a salt of it, some water, and possibly other small components, and every one of those has mass that the balance sees and the formula does not. This is the distinction between gross mass and net peptide content, and it is the arithmetic that decides how much peptide is actually in a solution made from the vial.

Research peptides are almost always supplied as acetate or trifluoroacetate salts, because that is the form the purification buffer leaves them in. Each basic site on the peptide, the N-terminus and any lysine, arginine or histidine side chain, can hold a counterion.

Acetate as acetic acid weighs 60.05; trifluoroacetate as the acid weighs 114.02. BPC-157 has a free N-terminus and one lysine, so as a diacetate it carries two acetates, adding about 120 to a molecule of 1420, or roughly 8 percent of the salt's mass. A trifluoroacetate salt of the same peptide would carry about 228, or 14 percent. Neither figure appears in the molecular weight, and a laboratory that weighs 5 milligrams of the salt and treats it as 5 milligrams of peptide has overstated the peptide by the counterion fraction before touching the water.

Residual water is the other component. A lyophilized cake is hygroscopic and usually holds several percent water by mass, measured by Karl Fischer titration and reported on a full certificate. Net peptide content, where a certificate reports it, is the fraction of the gross mass that is peptide after counterion and water are subtracted, and it is the number that should feed the solution calculators. A record that gives a molecular weight but no net content has told you the mass of the molecule and not the mass of the molecule in the vial. The purity testing guide explains why the HPLC purity figure is blind to all of this.

Modified peptides: where the simple sum needs more

The residue table covers the twenty standard amino acids, and a growing share of research peptides are not made only of those. Semaglutide and tirzepatide carry a fatty diacid attached to a lysine side chain through a short linker, and tirzepatide also contains aminoisobutyric acid, a residue that is not in the standard table at all. Cyclic peptides close a ring and lose a further water, or a pair of hydrogens, depending on the chemistry of the closure. Peptides carrying a polyethylene glycol chain have a distribution of masses rather than one, and a single weight for them is a nominal figure.

None of this breaks the method, but each addition has to be written down before the sum can be checked. A record that states the sequence as a string of standard residue codes and omits the modification has given the reader a formula that cannot match the certificate, and the mismatch will look like a failed identity test when it is a missing line on the record. The IUPAC-IUBMB conventions provide a way to write every one of these cases: a non-standard residue by its own symbol, a side-chain attachment in parentheses after the residue that carries it, a cyclisation by naming the residues it joins. A record that uses the notation can be checked.

One that gives a trade name and a weight cannot, and the weight then has to be taken on trust, which is what this page exists to avoid.

Checking a record in four steps

The derivation above turns into a short routine that catches most errors on a product record before a vial is ordered, and every step can be done with the sequence, a residue table and a browser.

1. Confirm the sequence line. It should be written in IUPAC-IUBMB notation with the termini and any modification stated. A sequence with an unstated C-terminal amide gives a formula one nitrogen too low and a weight about one unit off.
2. Derive the formula. Sum the residue formulas, add H2O for free termini, apply modifications. Compare to the record. A mismatch is either a different sequence or a transcription error, and either way the record needs fixing.
3. Derive the average weight and compare. Agreement within a few hundredths is expected. A difference near one mass unit usually means the record is quoting monoisotopic mass, which is fine if labelled and confusing if not.
4. Read the certificate's observed mass against the right theoretical mass. Monoisotopic for a small peptide; the stated peak for a large one. An observed value that matches the theoretical to within the instrument's accuracy is the identity evidence. One that matches only after a counterion is added has measured the salt, and should say so.

The routine is the same whether the compound is a fifteen-residue sequence or a thirty-nine-residue one with a fatty-acid side chain. The arithmetic gets longer and the modifications need care, but nothing about it changes, and a record that survives it has cleared the check that most records in this category have never been put through. The CAS number guide covers the companion check on the identifier.

How these fields appear on our records

Each LabFirst product record states the sequence in standard notation where the entity is verified, and derives the formula and average molecular weight from that sequence rather than copying them from a supplier sheet. Where a certificate for the shipping lot reports an observed mass, the record links the certificate so the observed and theoretical values can be read together; where a certificate reports net peptide content, that figure is the one the calculators are meant to be fed. Fields that have not been verified are left showing as pending, because a blank is honest and a copied number is a claim nobody checked. The Verify a COA page explains how to confirm a certificate at the laboratory that issued it.

A formula and a weight identify a molecule. They say nothing about what it is for. FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. The derivation on this page exists so that a laboratory can check a record and read a certificate correctly, and for no other purpose.

Common questions

Sources

• IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), Standard atomic weights. The source of the average atomic weights used to compute average molecular mass.
• NIST, Atomic Weights and Isotopic Compositions for All Elements. The isotopic masses and abundances behind monoisotopic mass and the isotope cluster discussion.
• PubChem compound record: BPC-157. The formula C62H98N16O22 and the 1419.5 g/mol weight the worked example is checked against.
• PubChem compound record: tirzepatide. The large-peptide example used to explain why the most abundant isotope peak departs from the monoisotopic mass.
• IUPAC-IUBMB Joint Commission on Biochemical Nomenclature, Nomenclature and Symbolism for Amino Acids and Peptides. The notation rules for the sequence line the formula is derived from, including termini and modifications.
• FDA / ICH Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. The specification framework in which identity by mass, purity and quantity are distinct tests, which is why theoretical mass and net content are separate fields.