An alpha helix is an organised coil of the peptide backbone. The spiral in a molecular illustration represents a particular repeating geometry, not an extra chemical strand wound around the peptide. Reading that geometry carefully helps explain residue spacing, exposed surfaces and the limits of a structural claim.
Translate the coil into residue positions
In the usual idealised alpha helix, a backbone carbonyl at residue i accepts a hydrogen bond from the backbone N–H at i+4. The geometry has approximately 3.6 residues per turn and a rise of 1.5 angstroms per residue. Side chains project outward from the helical axis.EMBL-EBI — Alpha-helix geometry (opens in a new tab)
The i+4 notation is an index difference. If the carbonyl belongs to residue 2, its corresponding donor is at residue 6. There are three residue positions between them. This is different from saying that a hydrogen bond occurs once every 3.6 residues: the latter number describes rotation around the axis, not an allowable fractional residue index.
| Carbonyl position | N–H partner | Index separation |
|---|---|---|
| 2 | 6 | 4 |
| 3 | 7 | 4 |
| 4 | 8 | 4 |
A schematic may leave these individual bonds out entirely. Their absence from a simplified ribbon picture is not evidence that the proposed helix lacks them; the drawing convention should be checked before interpreting the picture as an atom-by-atom record.
Residue spacing creates a molecular surface
Using the ideal repeat, the rotation per residue is approximately 360/3.6 = 100 degrees. A separation of four positions therefore corresponds to about 400 degrees, or 40 degrees beyond one complete turn. Positions i and i+4 can be on a similar face without lying on exactly the same radial line.
This arithmetic explains why a sequence and a side-on helix picture convey different information. Residues that are separated along the written chain can contribute to a neighbouring patch on the helical surface. A wheel diagram makes an assumed rotational arrangement easier to see, but does not experimentally establish that the peptide adopts it.
A bound helix describes a particular state
PDB entry 1YCR records a p53-derived peptide bound to MDM2. In that crystal structure, the peptide occupies a binding cleft as an amphipathic alpha helix. This is direct structural context: a named peptide segment, a named partner and a particular complex.RCSB PDB 1YCR — A p53 peptide bound to MDM2 (opens in a new tab)
The example should be described as a bound structure. It does not alone measure the fraction of isolated peptide that is helical in another solution. Nor does it establish that an altered sequence, terminal modification or labelling group preserves the same arrangement.
When comparing two images, first check whether both concern the free peptide or both concern equivalent complexes. Otherwise an apparent disagreement may reflect a different molecular state rather than conflicting measurements.
Read helix claims at the stated resolution
A result described as increased helicity may summarise an average signal. A residue-resolved model instead assigns particular positions. Neither phrase should silently become a claim that every molecule is a rigid, fully helical rod. Ask what was measured and how the authors translated it into their structural description.
- Identify the residue range assigned to the helix.
- Separate an ideal schematic from the experimental model.
- Record the solvent, partner and construct associated with the result.
- Keep structural assignment separate from any functional conclusion.
Sources and further detail
- EMBL-EBI — Alpha-helix geometry (opens in a new tab)
Idealised helix geometry and backbone hydrogen-bond pattern. End effects and real structural deviations are kept distinct from the ideal model.
- RCSB PDB 1YCR — A p53 peptide bound to MDM2 (opens in a new tab)
Kussie and colleagues, Science 274, 948–953 (1996), DOI 10.1126/science.274.5289.948. Crystal structure of a peptide–protein complex; not a structure of the isolated peptide in solution.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.