Biomolecules:
Proteins 2
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In this module:

Introduction
Secondary Structure
a Helices
ß Sheets
Motifs
Tertiary Structure
Disulfide Bonds, Domains
Cofactors, Quaternary Structure
Protein Folding
Alzheimer's and "Mad-Cow" Diseases

a Helices

What is most remarkable about Pauling's work that March morning is that he predicted very accurately the measurements of the a helix that have since been observed time after time in real proteins. Some important things to know about a helices are demonstrated below.

The screw sense of the helix is right-handed. This means that if you point the thumb of your right hand in the direction the strand is going (N- to C-terminus), your fingers will curl with the strand.
The distance betwen adjacent amino acids is 1.5 Å. Each turn rises through 5.4 Å, so there are 3.6 amino acids per complete turn of the helix.
The interior of an a helix is completely filled with atoms, so all the side chains project out from the helical axis. Adjacent side chains are on nearly opposite sides of the helix, while every third or fourth side chain is on the same side of the helix (side chains are numbered in the figure).
There is a regular pattern of hydrogen bonding between main-chain groups. The carbonyl group of amino acid n accepts a hydrogen bond from the amine group of amino acid n+4 (the a carbon atoms are labeled in the figure and the hydrogen bonds are shown as red lines).
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Alpha Helices