Solid-phase synthesis and evaluation of linear and cyclic ferrocenoyl/ruthenocenoyl water-soluble hexapeptides as potential antibacterial compounds.

A series of novel water-soluble short peptide-bioconjugates containing a ferrocenoyl (Fc) or ruthenocenoyl (Rc) unit was synthesized and characterized to combine the unique activity of ferrocene and the isoelectronic ruthenocene with precisely designed peptide structures. We aim at evaluating these bioconjugates as a new class of OrganoMetallic Short AntiMicrobial Peptides (OM-SAMPs). The series of OM-SAMPs was designed with a set of linear and "head-to-tail" cyclic metallocene-based hexapeptides derived from the homo-sequence H-KKKKKK-NH2 by substitution of lysine (K) by tryptophan (W) and by orthogonal derivatization of the ε-N-amine group of lysine by a metallocene moiety. Peptide conjugates were characterized by RP-HPLC, mass spectrometry (ESI and MALDI-TOF) and circular dichroism (CD) spectroscopy. Gram-positive and Gram-negative antibacterial activity testings were carried out to explore the role of insertion of the metallocene fragment into the peptide, and the effect of the modification of the cationic charge and aromatic residues on the physiochemical properties of these OM-SAMPs. These results show that the insertion of two tryptophan residues and ferrocenoyl/ruthenocenoyl moieties into a linear homo-sequence peptides increase significantly their antibacterial activity with minimum inhibitory concentration values as low as 5 μM for the most active compounds. However, "head-to-tail" cyclic metallocene-based hexapeptides were not active against Gram-negative bacteria up to concentrations of 50 μM. These studies provide a better understanding of the role of structural modifications to enhance antibacterial peptide activity, which is promising for their therapeutic application.