The helical conformation of each chain is dependent on the fact that every third residue is a Gly, and that the sequence is rich in Pro. A larger side chain than Gly would prevent the close contact of the three chains. Here is a model collagen structure. Hydrogen bonds between the backbone atoms of the 3 alpha-chains are indicated. To obtain a similar rendition with RasMol, colour the structure by chain, and use the space-filling option.
Collagen results from the cleavage of the extension peptide domains.
Collagen type I forms collagen fibrils. All the molecules point in the same direction. The -OH groups of hydroxyproline are involved in hydrogen bonds between chains, while interactions between other side chains are thought to be important in formation of fibrils from a number of individual molecules. This suggests that the collagen molecules are aligned such that the maximum electrostatic and hydrophobic interactions occur between different molecules. The diagram here indicates interactions between alpha-chains in two different molecules.