NobelTimeline · a mode
HelixStrip
In 1948, ill in bed, Linus Pauling drew a protein chain on a strip of paper and folded it until the hydrogen bonds could line up. The question he was asking: how many amino acids make one turn?
Your guess first
How many residues per turn let every backbone N–H reach a C=O along the coil?
Measured on the AlphaFold model of human haemoglobin α (UniProt P69905, its longest helix, residues 54–72): each residue turns 100.6° around the axis, so one turn takes 3.58 residues, and each residue rises 1.49 Å along it (one turn: 5.34 Å). The textbook values are 3.6 residues, 1.5 Å and 5.4 Å.
Pauling's step forward was to stop expecting a whole number. If every peptide group stays flat and every N–H bonds to the C=O four residues earlier, the coil has to come out at about 3.6.
Open haemoglobin α in the viewer and count along a helix yourself.
Prediction Computed by a model, with how sure it is. Not an experiment.
Print it, roll it
Print this page, cut out the strip along the outer line, and roll it so the two long edges meet, dots outward, then tape it. The strip is one turn wide, so each dot lands 101° round from the last. Look down the side: dot n sits almost above dot n + 4, which is the pair a hydrogen bond joins.
Dashed lines join each residue to the one four along: on the rolled strip they run almost straight up the side. Roll it the other way round and you get the mirror-image coil; the helices in proteins made of ordinary (L-) amino acids turn the right-handed way.
The other half: a pleat
Take a second strip and fold it like a fan, back and forth. Each fold is one residue, and the side chains point up, down, up, down. Lay two pleated strips side by side and you have the start of a β-sheet, the other shape Pauling and Corey described in 1951.
Honest notes
- The bedside story is Pauling's own later account. The helix was published in 1951 by Pauling, Robert Corey and Herman Branson.
- The measurement above is on a prediction, not an experiment: an AlphaFold model, very confident here. X-ray structures of haemoglobin give the same geometry.
- More paper models: PDB-101's printable helix and strand templates.