Exploring the thermodynamics of metal-peptide complexes: unveiling promising perspectives for antimicrobial innovation

The global identification of antibiotics as crucial tools in fighting infectious diseases is undeniable. However, the rise of antimicrobial resistance among various microorganisms demands a reevaluation of their utility. Antimicrobial peptides have emerged as promising alternatives to conventional antibiotics. These peptides exhibit bioactivity through diverse mechanisms, including the interaction with metal micronutrients. One such example is the human antimicrobial peptide calcitermin, which has been previously investigated by our research team. Our aim was to assess its ability to chelate metals and its effectiveness against a range of pathogens in vitro [1,2]. From this perspective, we conducted a systematic study of calcitermin derivatives. By introducing chemical modifications to the native sequence, we aimed at understanding the influence of divalent Cu(II) and Zn(II) ions on the stability, coordination, and antimicrobial activity of the resulting complexes. Among various derivative, we investigated the calcitermin mutants A7R and A8R, wherein the alanine residues at positions 7 and 8 were substituted with arginines, known to enhance antibacterial activity. Additionally, the A7H analogue has been considered, to obtain a chelating sequence with four histidines in alternate position. Notably, the affinity for metal binding not only increases with the number of histidines but is also higher for histidines separated by a single amino acid rather than separated by several amino acids or in consecutive order [3]. Through a comprehensive approach involving potentiometric titrations, mass spectrometry, UV-Vis spectrophotometry, circular dichroism, and electronic paramagnetic resonance, we delved into the formation equilibria and coordination chemistry of these complexes. Antimicrobial assays were also performed to assess the bioactivity of the compounds against a broad spectrum of microorganisms, revealing the pivotal role of metal ions. This work highlights the significance of characterizing the thermodynamic properties of metal-peptide complexes in solution. The obtained results serve as a foundational step toward the development of novel metal-based antimicrobial agents. Financial support of the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.1 – NextGenerationEU (PRIN PNRR 2022 - P2022EMY52) and of the Polish National Science Centre (UMO-2020/37/N/ST4/03165) is gratefully acknowledged.