Synthesis, Photophysical Properties and Biological Evaluation of New Conjugates BODIPY: Dinuclear Trithiolato-Bridged Ruthenium(II)-Arene Complexes

The synthesis, photophysical properties and antiparasitic efficacy against Toxoplasma gondii β-gal (RH strain tachyzoites expressing β-galactosidase) grown in human foreskin fibroblast monolayers (HFF) of a series of 15 new conjugates BODIPY-trithiolato-bridged dinuclear ruthenium(II)-arene complexes are reported (BODIPY=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, derivatives used as fluorescent markers). The influence of the bond type (amide vs. ester), as well as that of the length and nature (alkyl vs. aryl) of the spacer between the dye and the diruthenium(II) complex moiety, on fluorescence and biological activity were evaluated. The assessed photophysical properties revealed that despite an important fluorescence quenching effect observed after conjugating the BODIPY to the diruthenium unit, the hybrids could nevertheless be used as fluorescent tracers. Although the antiparasitic activity of this series of conjugates appears limited, the compounds demonstrate potential as fluorescent probes for investigating the intracellular trafficking of trithiolato-bridged dinuclear Ru(II)-arene complexes in vitro.

Intracellular Physical Properties with Small Organic Fluorescent Probes: Recent Advances and Future Perspectives

The intracellular physical parameters i. e., polarity, viscosity, fluidity, tension, potential, and temperature of a live cell are the hallmark of cellular health and have garnered immense interest over the past decade. In this context, small molecule organic fluorophores exhibit prominent useful properties including easy functionalizability, environmental sensitivity, biocompatibility, and fast yet efficient cellular uptakability which has made them a popular tool to understand intra-cellular micro-environmental properties. Throughout this discussion, we have outlined the basic design strategies of small molecules for specific organelle targeting and quantification of physical properties. The values of these parameters are indicative of cellular homeostasis and subtle alteration may be considered as the onset of disease. We believe this comprehensive review will facilitate the development of potential future probes for superior insight into the physical parameters that are yet to be quantified.

Single molecule probes of membrane structure: orientation of BODIPY probes in DPPC as a function of probe structure

Single molecule fluorescence measurements have recently been used to probe the orientation of fluorescent lipid analogs doped into lipid films at trace levels. Using defocused polarized total internal reflection fluorescence microscopy (PTIRF-M), these studies have shown that fluorophore orientation responds to changes in membrane surface pressure and composition, providing a molecular level marker of membrane structure. Here we extend those studies by characterizing the single molecule orientations of six related BODIPY probes doped into monolayers of DPPC. Langmuir-Blodgett monolayers transferred at various surface pressures are used to compare the response from fluorescent lipid analogs in which the location of the BODIPY probe is varied along the length of the acyl chain. For each BODIPY probe location along the chain, comparisons are made between analogs containing phosphocholine and smaller fatty acid headgroups. Together these studies show a general propensity of the BODIPY analogs to insert into membranes with the BODIPY probe aligned along the acyl chains or looped back to interact with the headgroups. For all BODIPY probes studied, a bimodal orientation distribution is observed which is sensitive to surface pressure, with the population of BODIPY probes aligned along the acyl chains increasing with elevated surface pressure. Trends in the single molecule orientations for the six analogs reveal a configuration where optimal placement of the BODIPY probe within the acyl chain maximizes its sensitivity to the surrounding membrane structure. These results are discussed in terms of balancing the effects of headgroup association with acyl chain length in designing the optimal placement of the BODIPY probe.

Hydration effects on membrane structure probed by single molecule orientations

Single molecule fluorescence measurements are used to probe the structural changes in glass-supported DPPC bilayers as a function of relative humidity (RH). Defocused polarized total internal reflection fluorescence microscopy is employed to determine the three-dimensional orientation of the fluorescent lipid analogue BODIPY-PC, doped into DPPC membranes in trace amounts. Supported DPPC bilayers formed using vesicle fusion and Langmuir-Blodgett/Langmuir-Schäfer (LB/LS) transfer are compared and show similar trends as a function of relative humidity. Population histograms of the emission dipole tilt angle reveal bimodal distributions as observed previously for BODIPY-PC in DPPC. These distributions are dominated by large populations of BODIPY-PC molecules with emission dipoles oriented parallel (≥81°) and normal (≤10°) to the membrane plane, with less than 25% oriented at intermediate tilts. As the relative humidity is increased from 13% to 95%, the population of molecules oriented normal to the surface decreases with a concomitant increase in those oriented parallel to the surface. The close agreement in trends observed for bilayers formed from vesicle fusion and LB/LS transfer supports the assignment of an equivalent surface pressure of 23 mN/m for bilayers formed from vesicle fusion. At each RH condition, a small population of BODIPY-PC dye molecules are laterally mobile in both bilayer preparations. This population exponentially increases with RH but never exceeds 6% of the total population. Interestingly, even under conditions where there is little lateral diffusion, fluctuations in the single molecule orientations can be observed which suggests there is appreciable freedom in the acyl chain region. Dynamic measurements of single molecule orientation changes, therefore, provide a new view into membrane properties at the single molecule level.