An alpha helix is a stretch of a protein chain coiled into a right-handed spiral, like a spiral staircase, and the most common of the regular shapes that protein chains fold into. Counting the side chains that stick out from the coil, it is about 1.2 nanometers across, and a nanometer is a billionth of a meter.

Size

A protein is a long chain of smaller molecules called amino acids. In an alpha helix, the chain's backbone winds around a central line, and each amino acid's side chain points outward, like the bristles of a bottle brush. The width of the whole rod, side chains included, is given in published research as about 1–1.2 nanometers. The 1.2 nanometers shown above is at the top of that range.

Along its length, the helix is very regular. Each turn holds 3.6 amino acids and rises 0.54 nanometers, so each amino acid adds 0.15 nanometers to the length. The coil is held in shape by hydrogen bonds, weak attractions that link each amino acid to the one four places farther along the chain. A helix can be short or long: one made of 20 amino acids is about 3 nanometers long.

Size comparisons

The DNA double helix, 2 nanometers wide, is about 1.7 times as wide as an alpha helix.
A helix of 20–25 amino acids, 3–3.75 nanometers long, is long enough to cross the water-repelling middle of a cell membrane's phospholipid bilayer. Proteins that pass through membranes often do it with alpha helices.
A typical protein is 3–6 nanometers across, or about 2.5 to 5 times the width of an alpha helix.
Between about 67,000 and 83,000 alpha helices lying side by side would span the width of a human hair, which is 80,000–100,000 nanometers wide.
Enlarged a billion times, an alpha helix would be 1.2 meters (4 feet) across, and each turn would rise 54 centimeters (21 inches). A human hair would be 80–100 kilometers (50–62 miles) wide.

How it was discovered

Linus Pauling, Robert Corey and Herman Branson proposed the alpha helix in 1951. Pauling had found the shape three years earlier, while in bed with a cold in Oxford, England. He sketched a protein chain on a sheet of paper and folded it until one turn of the spiral could form hydrogen bonds with the next. The drawing in the 1951 paper showed a left-handed helix, the mirror image of those found in natural proteins.

In the spring of 1951, Max Perutz read the paper and realized that the 0.15-nanometer step for each amino acid should show up when X-rays scatter off a protein. He tested the idea on a horse hair and found the signal exactly where the helix predicted it. In 1957 and 1959, John Kendrew's team mapped myoglobin, a protein that binds oxygen, and saw its helices as straight rods. About 75% of its chain is coiled into helices, 7–24 amino acids long, and all of them are right-handed.

Sources

α-helix. In Foundations of Protein Structure. EMBL-EBI Training.
Protein Structure. In Organic Chemistry. OpenStax, 2023.
Predicting Helical Topologies in RNA Junctions as Tree Graphs. Laing et al., PLoS ONE, 2013.
Type Three Secretion System in Attaching and Effacing Pathogens. Gaytán et al., Frontiers in Cellular and Infection Microbiology, 2016.
The Discovery of the α-Helix and β-Sheet, the Principal Structural Features of Proteins. Eisenberg, Proceedings of the National Academy of Sciences, 2003.
Myoglobin and the Structure of Proteins. John C. Kendrew, Nobel Lecture, 1962.
Components and Structure. In Biology 2e, 2nd edition. OpenStax, 2018.
How Big Is the "Average" Protein?. In Cell Biology by the Numbers. Milo and Phillips, Garland Science, 2015.
DNA Structure and Sequencing. In Biology 2e, 2nd edition. OpenStax, 2018.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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