What Is a Polypeptide Made Of?

By What Peptides Editorial Team · Updated 2026-09-14 · Part of What Peptides Are: Structure, Bonds and How Chains Are Built

A polypeptide is made of amino acid residues joined end to end by peptide bonds, and nothing else. Every chain, whatever its length or sequence, is assembled from the same three-part inventory: a repeating backbone, one variable side chain per position, and the two terminal groups left over at the ends. Understanding that inventory makes sequence notation, molecular mass and elemental composition all fall into place.

The word residue is the part people trip over. Once an amino acid is incorporated into a chain it has lost the elements of water, so a residue is slightly lighter than the free amino acid it came from. This page breaks the chain down into its components, gives average masses for each part, and shows how to estimate the mass of a whole chain from its residue count. The structural consequences are covered in our peptide chemistry and structure reference.

Residues, Not Free Amino Acids

When the carboxyl group of one amino acid reacts with the amine group of another, a molecule of water is eliminated: the hydroxyl from the carboxyl and one hydrogen from the amine. What remains in the chain is that amino acid minus H2O, and that remainder is the residue. For a chain of n residues there are n minus 1 peptide bonds, so the molecular mass of the whole chain is the sum of the free amino acid masses minus 18.02 daltons for every bond formed.

Elemental composition follows from the same logic. The standard residues contain carbon, hydrogen, nitrogen, oxygen and, in the cases of cysteine and methionine, sulfur. Many proteins also carry non-amino-acid components such as metal ions, heme groups or attached carbohydrates, but those are cofactors and modifications rather than parts of the polypeptide itself. When a specification sheet quotes an elemental analysis, it is describing the peptide plus any counterions present, usually trifluoroacetate or acetate from purification.

Because the peptide bond is an amide, the nitrogen in the backbone is much less basic than the nitrogen of a free amino group. That is why a long chain has far fewer protonatable nitrogens than its residue count suggests: only the N-terminal amine and the side chains of lysine, arginine and histidine take up protons in the usual pH range. This is one of the details that separates a polymer of amino acids from a simple mixture of them.

The Repeating Backbone and the Variable Side Chains

The backbone is the constant part. It repeats as nitrogen, alpha carbon, carbonyl carbon, written N-C-alpha-C', and every residue in every protein contributes exactly the same atoms to it. Roughly 56 daltons of every residue is backbone, and this figure does not change whether the chain is two residues long or thirty thousand. The side chain is the variable part, and it is the only thing that distinguishes one position from another in a sequence.

The table below splits a residue into its contributions. The average side chain mass of about 54 daltons is an equal-weighted mean over the twenty standard residues, so any individual chain can deviate substantially from it. Glycine, with a single hydrogen as its side chain, sits at the bottom of the range, while tryptophan, whose indole group alone weighs 130.19 daltons, sits at the top.

The split between backbone and side chain also explains why composition data cannot simply be added up. Amino acid analysis measures the free amino acids released by hydrolysis, and each of those is about 18 daltons heavier than the residue it came from, so summing the analysis directly overshoots the intact mass. Reporting the result as residues per molecule sidesteps the problem and makes the numbers comparable with a sequence.

Estimating Molecular Mass From Residue Count

With the 110 dalton average in hand, the arithmetic is straightforward. Multiply the residue count by 110 and add 18.02 for the water that is restored at the two termini, because the free chain carries an extra hydrogen at the N-terminus and an extra hydroxyl at the C-terminus. A 30-residue chain therefore lands near 3,318 daltons, or about 3.3 kilodaltons, and a 100-residue chain lands near 11 kilodaltons.

Two caveats keep the estimate honest. First, the 110 figure is an average, so a chain rich in tryptophan and tyrosine will run heavier and a chain rich in glycine and alanine will run lighter; composition tables from amino acid analysis exist precisely to correct for this. Second, the number quoted on a certificate may be a monoisotopic mass, calculated from the lightest isotope of each element, or an average mass, calculated from natural isotopic abundances, and the two differ by roughly 0.05 percent at 5 kilodaltons and more above that.

This is also the reason composition data is reported as residues per molecule rather than as a percentage by weight. Hydrolyzed collagen is a useful illustration: its Gly-X-Y repeating pattern makes it roughly one-third glycine by residue count, which is a statement about sequence, not about mass. Our page on the composition of hydrolyzed collagen fragments works through that example, and backbone geometry and chain direction shows what the repeating unit looks like in three dimensions. Confirming a calculated mass against a measured one is standard practice in identity testing by mass spectrometry.

This page is part of the What Peptides Are: Structure, Bonds and How Chains Are Built guide.