Investigation of the interactions of the luminescent probe cis-[Ru(phen)2(4ImAC)]+, where 4ImAC is 4-imidazole carboxylic acid, with phospholipid cell membrane models (neutral and charged)

Abstract

Our laboratory has been working on the development of luminescent Ru(II) polypyridine complexes for application as molecular probes in the biological environment. The complexes prepared so far show stability at physiological pH, intense visible emission with significant Stokes shifts, and a long emission lifetime that allows real-time monitoring of biomolecules. For example, the cis-[Ru(phen)2(3,4-Apy)2]2+, 3,4-aminopyridinecomplex is taken up by neuronal cells (Neuro2A, PC12, SH-S5Y5) with accumulation in the cytoplasmic region without apparent toxicity. The emission of this probe in red allows the monitoring of the aggregation process of amyloid proteins (beta-amyloid and insulin)by the change in luminescent responses. Recent interaction studies with negatively charged phospholipids have shown that this luminescent probe is also very sensitive to the lipid environment. Biological membranes are electrostatically charged entities and coexist with electrolyte solutions. Therefore, interactions with ions and/or molecules that are also charged can influence various physiological processes. It is noteworthy that changes in the coordination sphere and oxidation state of the metallic center can change both the photophysical properties of the probe and its interactions with biomolecules. These results motivated us to (1) synthesize and characterize the luminescent probe cis-[Ru(phen)2(4ImAC)]+, (RuImAC), ImAC= 4-imidazole carboxylic acid; (2) explore how the sphere changes of coordination and oxidation state alter the photophysical properties of the probe; (3) determine how these alterations affect the luminescent probe-lipid membrane interactions (LUVs, GUVs) and the integrity of the lipid membrane. The RuImAC luminescent probe is similar in size to the RuApy but differs in charge, and does not contribute to hydrogen bonding, thus showing significant differences in interactions with phospholipids. Therefore, we will explore the effects of zwitterionic phospholipids(POPC) as well as negatively charged phospholipids (DOPG) and if possible with additional membrane components such as gangliosides and cholesterol. The studies involve luminescence spectroscopy (steady-state and time-resolved), fluorescent imaging microscopy. With the advancement of studies, it is intended in the future to expand the investigations into infrared reflection techniques with modulated polarization (PMIRRAS) applied to Langmuir monolayers, in addition to isothermal titration calorimetry (ITC). The studies proposed in this project are included in the regular FAPESP project (2019/21143-0) by Professor Rose Maria Carlos. (AU)