Synthesis, characterisation and antimicrobial activity of supramolecular cobalt-peptide conjugates.

Herein, we describe the synthesis and characterisation of four new supramolecular cobalt conjugates of antimicrobial peptides functionalised with terpyridine ligands (L). Peptides were chosen based on the well-established arginine-tryptophan (RW) motif, with terpyridine-derivatized lysine (Lys(tpy)) added to the sequence, or replacing tryptophan residues. Self-assembly of the antimicrobial peptides with Co(BF)·6HO formed exclusively CoL dimers (for peptides with one tpy ligand each) and CoL metallo-macrocycles (for peptides with two tpy ligands for each peptide), which could be 'locked' by oxidation of Co(+II) to Co(+III) with ammonium ceric nitrate. The Co-peptide complexes were characterised by mass spectrometry and in solution by NMR spectroscopy, including 2D diffusion ordered NMR spectroscopy (DOSY) which confirmed the proposed stoichiometries. The antimicrobial activity of the novel peptides and their metallo-supramolecular assemblies was investigated by determination of their minimal inhibitory concentration (MIC) against a panel of Gram-positive and Gram-negative bacteria. Complexation with cobalt increases the activity of the peptides in almost every case. Most of the new metal-peptide conjugates showed good activity against Gram-positive bacteria, including a multi-resistant strain and the opportunistic pathogenic yeast (down to 7 μmol l for the most active CoL derivate), a value that is increased five-fold compared to the lysine-derivatized peptide ligand alone. Interestingly, conjugates of the CoL type also showed decent activity against Gram-negative bacteria including the WHO-flagged problematic strain (down to 18 μmol l for the most active derivative).