Peptide Chain Structure: Backbone, Direction and Geometry

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

A peptide chain has three structural features worth knowing before anything else: it has a direction, it is built from a repeating three-atom unit, and its shape is described by three torsion angles per residue. Everything else about chain architecture, from helix geometry to how long a chain is in nanometres, follows from those three facts. This page explains each one and then connects residue count to physical size.

Direction matters because the two ends of a chain are chemically different, and because the conventions for writing a sequence and for building one on a resin run in opposite directions. Getting them straight prevents most of the errors beginners make when reading a sequence. The full context sits in our peptide chemistry and structure reference.

Direction: N-Terminus to C-Terminus

One end of a chain carries a free alpha-amine group, or an acylated version of it, and is called the N-terminus. The other carries a free alpha-carboxyl group, or an amidated or esterified version of it, and is called the C-terminus. By universal convention a sequence is written from the N-terminus on the left to the C-terminus on the right, so the residue named first is the one with the free amine and the residue named last is the one with the free carboxyl.

Ribosomal synthesis runs in the same direction: the first residue translated retains its amine group and each new residue is added to the carboxyl end of the growing chain. Chemical synthesis on a solid support runs the other way. The first residue is anchored to the resin through its carboxyl group and each cycle adds a residue to the free amine, so the chain is assembled from the C-terminus towards the N-terminus. Both routes give the same final connectivity; only the order of operations differs.

Residue numbering also follows the sequence direction, starting at 1 for the N-terminal residue. Insertions in related sequences are labelled with letters after the preceding number, which is how numbering schemes such as the chymotrypsin numbering convention stay comparable across families. The residue at the far end is described in the structure and charge of the C-terminal end, and the motifs available once a chain is long enough are set out in helices, sheets and turns.

The Repeat Unit and the Three Torsion Angles

The backbone repeats as nitrogen, alpha carbon, carbonyl carbon, with the carbonyl of residue i joined to the nitrogen of residue i plus one. Three torsion angles describe rotation at each residue. Phi is the angle about the bond between the amide nitrogen and the alpha carbon. Psi is the angle about the bond between the alpha carbon and the carbonyl carbon. Omega is the angle about the peptide bond itself.

Omega behaves differently from the other two. Because the amide nitrogen lone pair is delocalised into the carbonyl, the C-N bond has partial double-bond character, measures about 1.32 angstroms rather than the 1.45 angstroms of a single bond, and holds six atoms in a plane. Omega is therefore essentially fixed at 180 degrees in the trans arrangement or at 0 degrees in the cis arrangement, leaving phi and psi to carry all the conformational freedom. Proline is the exception worth remembering: its ring constrains phi to roughly minus 65 degrees, and the energy difference between cis and trans before proline is small enough that about 5 to 10 percent of X-Pro bonds are cis, compared with roughly 0.1 percent elsewhere.

Because each residue contributes one phi and one psi, the number of conformations grows explosively with chain length, yet observed combinations cluster into a few allowed regions. Proline and glycine sit at the extremes: proline occupies a narrow band because of its ring, while glycine, having only a hydrogen as its side chain, can adopt angles forbidden to every other residue.

Chain Length, Mass and Physical Extent

Two length scales are useful. A fully extended chain advances about 3.6 angstroms per residue, which is the beta strand spacing. A chain wound into an alpha helix advances 1.5 angstroms per residue along the helix axis, because 3.6 residues make one turn of 5.4 angstroms pitch. Real chains fall between these limits and, once folded, are far more compact than either.

Mass scales linearly. Using the average residue mass of 110 daltons plus 18.02 daltons for the terminal water, a 20-residue chain comes to about 2.2 kilodaltons and a 300-residue chain to about 33 kilodaltons. the table combines both scales so you can read off approximate size from a residue count.

Both figures are idealisations, and knowing which way they err matters. An unfolded chain in solution behaves closer to a random coil, whose end-to-end distance grows with the square root of the residue count rather than linearly, so the extended column is best read as an upper bound. A folded chain is far more compact than either column: a 300-residue globular structure typically measures only 4 to 6 nanometres across, against the 108 nanometres the same chain would reach if fully extended. Measured masses are what confirm these estimates, which is the subject of how HPLC purity and MS identity are reported.

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