Coordination chemistry and antimicrobial activity of LL-37, its C-terminal fragment, and a peptidomimetic analogue.

Human cathelicidin LL-37 is an antimicrobial peptide naturally present in saliva that plays an important role in host defence against bacterial and fungal pathogens. Although LL-37 is known to bind Cu(II) and Zn(II) ions, the corresponding coordination modes have remained unresolved. Here, the metal-binding properties, conformational features and biological activity of LL-37, its C-terminal fragment DF-12, and the corresponding all-D analogue df-12 were investigated using potentiometric, spectroscopic, and microbiological methods. For the first time, experimentally supported coordination modes for both Cu(II)-LL-37 and Zn(II)-LL-37 complexes are proposed. At physiological pH, both metal ions bind through the N-terminal amino group and a carboxylate donor, whereas at higher pH, their coordination pathways diverge. Cu(II) progressively recruits backbone amide nitrogen donors, ultimately forming highly stable 4N complexes, while Zn(II) forms hydroxo-containing species consistent with its preferred tetrahedral coordination geometry. Peptide truncation decreases metal-binding affinity, whereas complete inversion of peptide chirality has virtually no effect on Cu(II) coordination or thermodynamic stability. In contrast, Zn(II) coordination is considerably more sensitive to stereochemical modification. LL-37 retains its α-helical conformation upon metal binding, whereas the shorter fragments exhibit only limited helical propensity and undergo significant conformational rearrangement only upon formation of Cu(II) 4N complexes. Biological studies revealed that LL-37 displays the highest antimicrobial and broad-spectrum antibiofilm activities, both of which are generally enhanced by Cu(II) and Zn(II) coordination. Although truncation reduces antibacterial potency, both DF-12 and df-12 retain antifungal and antibiofilm activities. Combined with the uniformly low cytotoxicity of all investigated peptides and their metal complexes, as well as the anticipated increase in proteolytic stability of the all-D analogue, these findings identify DF-12 and, particularly, its all-D analogue df-12 as promising scaffolds for the development of novel antibiofilm agents.