Many theoretical and experimental studies have been done on the interactions responsible for the three-dimensional structure of a protein. More specifically, the cation-Π interaction is now considered an important interaction involved in protein structure. Here we investigate the interaction of Trp/Arg (i, i+4) and Lys/Tyr (i, i+4) sidechains on 18-residue helical peptides WR4 and YK4 while using isomeric peptides WR5 (i, i+5) and YK5 (i, i+5) respectively as our control. The [Θ222] as well as relative free energies of stabilization for all peptides are reported. The data does not show that sidechain interactions in WR4 contribute to alpha helix stability, which contradicts published results. This inconsistency is oxidative damage to peptides, as confirmed by MALDI/TOF MS. The data does confirm that sidechains in YK4 do stabilize the helix.