Bio-organometallic Peptide Conjugates: Recent Advances in Their Synthesis and Prospects for Biomedical Application

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.

Bioactive half-sandwich Rh and Ir bipyridyl complexes containing artemisinin

Reaction of dihydroartemisinin (DHA) with 4-methyl-4'-carboxy-2,2'-bipyridine yielded the new ester derivative L1. Six novel organometallic half-sandwich chlorido Rh(III) and Ir(III) complexes (1-6) containing pentamethylcyclopentadienyl, (Cp*), tetramethylphenylcyclopentadienyl (Cp^xph), or tetramethylbiphenylcyclopentadienyl (Cp^xbiph), and N,N-chelated bipyridyl group of L1, have been synthesized and characterized. The complexes were screened for inhibitory activity against the Plasmodium falciparum 3D7 (sensitive), Dd2 (multi-drug resistant) and NF54 late stage gametocytes (LSGNF54), the parasite strain Trichomonas vaginalis G3, as well as A2780 (human ovarian carcinoma), A549 (human alveolar adenocarcinoma), HCT116 (human colorectal carcinoma), MCF7 (human breast cancer) and PC3 (human prostate cancer) cancer cell lines. They show nanomolar antiplasmodial activity, outperforming chloroquine and artemisinin. Their activities were also comparable to dihydroartemisinin. As anticancer agents, several of the complexes showed high inhibitory effects, with Ir(III) complex 3, containing the tetramethylbiphenylcyclopentadienyl ligand, having similar IC₅₀ values (concentration for 50% of maximum inhibition of cell growth) as the clinical drug cisplatin (1.06-9.23 μM versus 0.24-7.2 μM, respectively). Overall, the iridium complexes (1-3) are more potent compared to the rhodium derivatives (4-6), and complex 3 emerges as the most promising candidate for future studies.