bit of wire and coil it into a spring shape. The lead on your computer mouse is fine for doing this! The next diagram shows how the alpha-helix is held together by hydrogen bonds. This is a very simplified diagram, missing out lots of atoms. We'll talk it through in some detail after you have had a look at it. What's wrong with the diagram? Two things: First of all, only the atoms on the parts of the coils facing you are shown. If you try to show all the atoms, the whole thing gets so complicated that it is virtually impossible to understand what is going on. Secondly, I have made no attempt whatsoever to get the bond angles right. I have deliberately drawn all of the bonds in the backbone of the chain as if they lie along the spiral. In truth they stick out all over the place. Again, if you draw it properly it is virtually impossible to see the spiral. So, what do you need to notice? Notice that all the "R" groups are sticking out sideways from the main helix. Notice the regular arrangement of the hydrogen bonds. All the N-H groups are pointing upwards, and all the C=O groups pointing downwards. Each of them is involved in a hydrogen bond. And finally, although you can't see it from this incomplete diagram, each complete turn of the spiral has 3.6 (approximately) amino acid residues in it. If you had a whole number of amino