We describe the X-ray diffraction structure of the terminally protected dipeptide amide Boc-Aib-L-Mag-NHBzl (Aib is alpha-aminoisobutyric acid and Mag is C-alpha-methyl, C-a-allyl-glycine) and the results of conformational energy computations on Ac-L-Mag-NHMe, Ac-Aib-L-Mag-NHMe, and Ac-L-Mag-Aib-NHMe. On the basis of these data, we performed conformational energy computations on the sequence -Aib-Xxx-(Aib)(2)-Xxxx-Aib- (Xxx = L-Mag) to check the feasibility of ring-closing metathesis, a currently extensively investigated reaction useful to enhance peptide helicity and metabolic stability, on this 3(10)-helix forming model hexamer with the two olefinic amino acids at the i, i+3 relative positions. Computations were extended to peptides based on olefinic residues with the side chains elongated by one (L-hMag) or two (L-hhMag) carbon atoms. A comparison was also made with peptides characterized by the related, non-C-alpha-methylated (C-alpha-trisubstituted) L-Agl (C-alpha-allyl-L-glycine), L-hAgl, and L-hhAgl residues. We conclude that, to achieve ring-closing metathesis with an unperturbed 3(10)-helical conformation and a symmetrical all-hydrocarbon tether, the side-chain length for each of the two i, i+3 olefinic amino acids requires at least five carbon atoms (hhMag or hhAgl), thereby producing an 18-atom macrocycle.