Unraveling Multicopper Cu3 and Cu6 Clusters with Rare μ3-Sulfato and Linear μ2-Oxido-Bridges as Potent Antibiofilm Agents against Multidrug-Resistant Staphylococcus aureus

In this research article, two multicopper Cu3 and Cu6 clusters, Cu3(cpdp)(μ3-SO4)(Cl)(H2O)2·3H2O (1) and Cu6(cpdp)2(μ2-O)(Cl)2(H2O)4·2Cl (2) (H3cpdp = N,N′-bis2-carboxybenzomethyl-N,N′-bis2-pyridylmethyl-1,3-diaminopropan-2-ol), have been explored as potent antibacterial and antibiofilm agents. Their molecular structures have been determined by a single-crystal X-ray diffraction study, and the compositions have been established by thermal and elemental analyses, including electrospray ionization mass spectrometry. Structural analysis shows that the metallic core of 1 is composed of a trinuclear Cu3 assembly encapsulating a μ3-SO42– group, whereas the structure of 2 represents a hexanuclear Cu6 assembly in which two trinuclear Cu3 motifs are exclusively bridged by a linear μ2-O2– group. The most striking feature of the structure of 2 is the occurrence of an unusual linear oxido-bridge, with the Cu3–O6–Cu3′ bridging angle being 180.00°. Whereas 1 can be viewed as an example of a copper(II)-based compound displaying a rare μ3:η1:η1:η1 bridging mode of the SO42– group, 2 is the first example of any copper(II)-based compound showing an unsupported linear Cu–O–Cu oxido-bridge. Employing variable-temperature SQUID magnetometry, the magnetic susceptibility data were measured and analyzed exemplarily for 1 in the temperature range of 2–300 K, revealing the occurrence of antiferromagnetic interactions among the paramagnetic copper centers. Both 1 and 2 exhibited potent antibacterial and antibiofilm activities against methicillin-resistant Staphylococcus aureus (MRSA BAA1717) and the clinically isolated culture of methicillin-resistant S. aureus (MRSA CI1). The mechanism of antibacterial and antibiofilm activities of these multicopper clusters was investigated by analyzing and determining the intracellular reactive oxygen species (ROS) generation, lipid peroxidation, microscopic observation of cell membrane disruption, membrane potential, and leakage of cellular components. Additionally, 1 and 2 showed a synergistic effect with commercially available antibiotics such as vancomycin with enhanced antibacterial activity. However, 1 possesses higher antibacterial, antibiofilm, and antivirulence actions, making it a potent therapeutic agent against both MRSA BAA1717 and MRSA CI1 strains.