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.