Tuning chemical properties on ruthenium(II) complexes through PEGylation of N-heterocyclic carbene ligands

A series of ruthenium complexes containing a N-heterocyclic carbene (NHC) ligand with three types of PEG substituents were synthesized, and their molecular modeling and electrochemical characterization were performed. Their interaction with BSA was also investigated using fluorescence quenching, circular dichroism, DLS, and zeta potential measurements. The PEG fragments strongly influenced reactivity, modulating both biomolecular interactions and chemical kinetics. Although all complexes exhibited identical reduction potentials, larger PEG chains imposed steric hindrance around the metal center, leading to slower mass transport, hindered heterogeneous electron transfer, and reduced hydride formation. Interaction studies with BSA indicated predominantly static quenching, consistent with stable adduct formation, while larger PEG units also introduced dynamic contributions. This behavior suggests that BSA can act as a carrier, facilitating plasma transport, or support their application as artificial nanozymes. Importantly, the Ru-PEG systems were generally non-toxic and displayed catalytic activity in hydrogen transfer reactions, including ketone and NAD⁺ reduction. This activity was further enhanced when cationic micelles or BSA were employed as carriers. Overall, the non-toxic nature, catalytic potential, and ability to interact with proteins highlight Ru-PEG complexes as promising candidates for intracellular catalytic applications.