Final answer:
The distance between the R groups in an alpha helix is defined by the structured spacing of amino acid residues in the helical structure. Each turn of the helix spans 3.6 amino acids with a pitch of 5.4 Å, with the R groups protruding outward from the spiral coil.
Step-by-step explanation:
The student has asked about the distance between the R groups in an alpha helix. An alpha helix is a common structural element of proteins where the polypeptide chain is coiled in a right-handed spiral. Each turn of this helix includes 3.6 amino acid residues and is spaced by a distance of 5.4 Å (angstroms). Along the axis of the helix, the -NH group of each amino acid forms a hydrogen bond with the -C=O group of the amino acid four residues ahead of it in the sequence. This consistent spatial arrangement causes the R groups of the amino acids to protrude outward from the helix at regular intervals.