For many proteins, proper function requires adoption of a specific tertiary structure. This study explores the effects of L-to-D amino acid substitutions on tertiary structure stability for two well-known miniproteins, a single-site variant of the chicken villin headpiece subdomain (VHP) and the human Pin1 WW domain (WW). For VHP, which features an α-helix-rich tertiary structure, substitutions led to significant destabilization, as detected by variable temperature circular dichroism (CD) measurements. For WW, which has a β-sheet-rich tertiary structure, most single L-to-D changes seemed to cause complete unfolding at room temperature, according to CD measurements. These findings suggest that amino acid residue configuration changes at a single site will often prove to be deleterious in terms of tertiary structure stability, and in some cases dramatically destabilizing.