Heptad Repeat, (abcdefg)n:

Positions (a) and (d) represent hydrophobic residues, while the other postions display mainly polar residues. Within the heptad repeat, the leucines at positions (d) and the amino acids in the alternate hydrophobic position (a) are surrounded by four residues of the neighboring helix. Each leucine at postitioin (d) packs against the leucine (d'), and the amino acid (e') of the neighboring helix. In adjacent layers, the amino acid at position (a) packs against (a') and (g'). These two amino acid layers alternate throughout the structure and form the extensive hydrophobic interface. This layering maximizes the amount of buried surface and contributes to dimer stability.

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