Protein chains are usually folded into one or several discrete globular units called domains (Schulz & Schirmer, 1979). Levitt & Chothia (1976) have shown that the structures of such domains frequently fall into one of the following three classes; α-proteins which are mainly α-helical, β-proteins which contain antiparallel β-strands and α/β proteins which have a central core consisting of a sheet of strands, most of which are parallel. The connexions between the parallel strands in αβ-proteins frequently contain helices which are packed on both sides of the sheet in a regular way (Chothia, Levitt & Richardson, 1977).