Hydrophobic residues usually appear at the first and fourth positions in the seven-residue repeats of polypeptides that

Question:

Hydrophobic residues usually appear at the first and fourth positions in the seven-residue repeats of polypeptides that form coiled coils.
(a) Why do polar or charged residues usually appear in the remaining five positions?
(b) Why is the sequence Ile-Gln-Glu-Val-Glu-Arg-Asp more likely than the sequence Trp-Gln-Glu-Tyr-Glu-Arg-Asp to appear in a coiled coil?