Unveiling Copper-Peptide Structures: Impact on Antioxidant Potential

Copper is an essential trace element, playing vital roles in numerous biological processes, but its dysregulation can lead to oxidative stress and contribute to various neurodegenerative diseases and inflammatory conditions. Peptides like GHK (Glycyl-L-Histidyl-L-Lysine) and DAHK (L-Aspartyl-L-Alanyl-L-Histidyl-L-Lysine) are naturally occurring human peptides known to form stable complexes with copper ions, influencing their biological activity, including antioxidant, wound-healing, and anti-inflammatory properties. However, a detailed understanding of how the precise three-dimensional structure of these copper-peptide complexes dictates their redox (reduction-oxidation) characteristics and potential therapeutic efficacy has remained elusive.

The study successfully elucidated the distinct coordination environments of Cu(II) in both GHK and DAHK complexes. X-ray crystallography revealed specific bond lengths and angles, indicating a square planar or distorted square pyramidal geometry around the copper ion, which differed between the two peptides. For instance, the average Cu-N bond length in Cu(II) GHK was determined to be approximately 1.98 Å, while in Cu(II) DAHK, it was slightly elongated to 2.02 Å, suggesting subtle differences in ligand field strength. Solution studies, utilizing EPR and UV-Vis spectroscopy, confirmed these structural motifs, showing subtle but significant differences in their dynamic behavior and electronic transitions, such as a 15 nm shift in the d-d absorption band maximum. The most critical finding was that the structural variations between Cu(II) GHK and Cu(II) DAHK complexes directly translated into measurable differences in their redox potentials, with Cu(II) DAHK exhibiting a more negative reduction potential compared to Cu(II) GHK, suggesting a potentially enhanced reducing capacity. Specifically, cyclic voltammetry measurements indicated that the Cu(II)/Cu(I) reduction potential for Cu(II) DAHK was approximately -0.15 V vs NHE, which was significantly more negative than the -0.05 V vs NHE observed for Cu(II) GHK, representing a 100 mV difference. These differences were attributed to the distinct amino acid side chains and their influence on the electronic environment of the copper center, particularly the aspartyl residue in DAHK providing an additional carboxylate coordination site.