Hydrophobic residues usually appear at the 1st and 4th 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 this sequence Trp-Gln-Glu-Tyr-Glu-Arg-Asp to appear in a coiled form?