AbstractConformational energy computations on a derivative and a homo‐dipeptide of Cα,α‐diethylglycine were performed. In both cases the N‐ and C‐terminal groups are blocked as acetamido and methylamido moieties, respectively. It was found that the Cα,α‐diethylglycine residues are conformationally restricted and that the minimum energy conformation corresponds to the fully extended C5 structure when the NCαC′ bond angle is smaller than 108° (as experimentally observed). The results of the theoretical analysis are in agreement with the crystal‐state structural propensity of the complete series of N‐trifluoroacetylated homo‐peptides of this Cα,α‐dialkylated residue from monomer to pentamer, determined by x‐ray diffraction and also described in this work. Interestingly, for the first time, a crystallographically planar peptide backbone was observed (in the protected tripeptide). A comparison with peptides of Cα,α‐dimethylglycine, Cα‐methyl, Cα‐ethylglycine, and Cα,α‐di‐n‐propylglycine indicates that the fully extended conformation becomes more stable than the helical structures when both amino acid side‐chain Cβ atoms are substituted.