The correct choice is (b) They have an integral repeat of seven residues
The explanation is: Coiled coils have an integral repeat of seven residues (heptads) which assume a side-chain packing geometry at facing residues. For every seven residues, the first and fourth are hydrophobic, facing the helical interface; the others are hydrophilic and exposed to the solvent. The sequence periodicity forms the basis for designing algorithms to predict this important structural domain.
