See chapter 3 of Branden and Tooze, Introduction to Protein Structure.
Briefly, a repetitive motif of leucine or isoleucine every seven amino
acids is a common feature of hydrophobic coiled-coiled interactions
between two alpha helices. You find them in dimerization domains of
proteins, and also on the "outward facing" side of transmembrane
helices, among other common places. Search for the pattern LLxxLLxxLL
in your protein to locate simple ones.
