Based on oligopeptide sequences, structural diversity elements from side chain substitutions and backbone amide modifications are used for the preparation of bioactive peptide mimetics, including δ-amino-b,γ-alkenyl, δ-amino-b,γ-cyclopropyl, and γ-amino-a,b-cyclopropyl dipeptide isosteres. The hybrid molecule XJB-5-131 is composed of an alkene dipeptide isostere, a regular tripeptide sequence extracted from the cyclodecapeptide antibiotic gramicidin S (GS), and the free radical scavenger 4-AT. This agent is highly enriched in mitochondria, and, among other positive functions in cells and animal models, it has demonstrated efficacy in improving neurocognitive outcome after traumatic brain injury in rats. It slows down the aging phenotype in Ercc1 progeria models and reduces oxidative damage to mitochondrial DNA, enhancing neuronal survival and improving mitochondrial function in a mouse model of Huntington’s disease. The shortened alkene dipeptide isostere JP4-039 also folds into a b-turn secondary structure in the solid state and mediates several desired physiological and cell-protective effects, including radiation damage prevention and mitigation.