Hydrophobic side chain interdigitation

The four-helix bundle is a common motif in which (usually) antiparallel a helices are packed side by side. It is found in myohemerythrin, various cytochromes, and a number of other proteins. A viral example is the coat protein of tobacco mosaic virus (TMV) (Bloomer et al, 1978). TMV represents the most common type, in which the helix axes are nearly antiparallel, off by 18°, coiled with a left-handed superhelical twist (Fig. 5 see Color Insert). The slight misalignment of the individual helix axes allows the side chains to interdigitate efficiently, burying internal hydrophobic side chains. 

Hydrophobic homopolypeptides are stable in their a-helical conformation at the air/water surface. a-Helices are adsorbed with their long axis parallel to the water surface, the long axis being predominantly perpendicular to the direction of compression. Poly-L-alanine and poly-L-leucine pack together in the form of a-helices on the water surfaces. Inflection points appeared at 12.8 A and 17.0 A per residue. The a-helices are organized in such a way that their side-chains are interdigitated (Figure 6.16) . ... 

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