Balancing skin permeation and antioxidant activity in C₁₆-conjugated antioxidant peptides

Antioxidant peptides are promising candidates for mitigating oxidative skin damage, but their topical application is often limited by poor skin permeability and an incomplete understanding of how permeability-enhancing modifications affect biological activity. In this study, we designed a focused series of antioxidant peptides by rational sequence engineering and N-terminal C₁₆ conjugation to investigate how this modification influences skin delivery, supramolecular assembly, antioxidant performance, and cellular compatibility. Four designed antioxidant peptides and their C₁₆-modified derivatives were synthesized and systematically evaluated. N-terminal C₁₆ conjugation markedly increased peptide hydrophobicity and promoted self-assembly, as evidenced by reduced critical aggregation concentrations (CACs) and the formation of distinct supramolecular structures, including liquid-like coacervates and micelle-like assemblies. In vitro Franz diffusion studies using porcine skin showed that C₁₆ modification substantially enhanced skin permeation and retention, with the improvement mainly associated with increased skin partitioning rather than accelerated diffusion. However, the biological consequences of lipidation were strongly sequence- and dose-dependent. Although selected C₁₆-conjugated peptides improved intracellular ROS suppression, cytoprotection against H₂O₂-induced oxidative injury, and fibroblast migration, N-terminal C₁₆ conjugation did not universally enhance chemical radical-scavenging activity or endogenous antioxidant responses. In several cases, unmodified peptides showed stronger SOD- and GSH-related regulation, whereas highly hydrophobic derivatives exhibited reduced biocompatibility at elevated concentrations. These findings reveal a permeability-bioactivity trade-off in N-terminal C₁₆-conjugated antioxidant peptides and demonstrate that optimizing transdermal peptide antioxidants requires balancing hydrophobicity, self-assembly, skin partitioning, and cellular safety. This work provides a systematic framework for designing lipid-modified antioxidant peptides for topical skin protection and repair.