Index Chemicus

Viscosity imaging at a microscopic scale can provide important information about biosystems, including the development of serious illnesses. Microviscosity imaging is achievable with viscosity-sensitive fluorophores, the most popular of which are based on the BODIPY group. However, most of the BODIPY probes fluoresce green light, whereas the red luminescence is desired for the imaging of biological samples. Designing a new viscosity probe with suitable spectroscopic properties is a challenging task because it is difficult to preserve viscosity sensitivity after modifying the molecular structure. Here we describe how we developed a new red-emitting, viscosity-sensitive, BODIPY fluorophore BP-PH-2M-NO2 that is suitable for reliable intracellular viscosity imaging of lipid droplets in MCF-7 breast cancer cells. The design of BP-PH-2M-NO2 was aided by DFT calculations that allowed a successful prediction of the viscosity sensitivity of fluorophores before synthesis. In summary, we report a new red viscosity probe possessing monoexponential fluorescence decay that makes it attractive for lifetime-based viscosity imaging.

We recently discovered a [Fe-S]-containing protein with in vivo thiouracil desulfidase activity, dubbed TudS. The crystal structure of TudS refined at 1.5 resolution is reported; it harbors a [4Fe-4S] cluster bound by three cysteines only. Incubation of TudS crystals with 4-thiouracil trapped the cluster with a hydrosulfide ligand bound to the fourth nonprotein- bonded iron, as established by the sulfur anomalous signal. This indicates that a [4Fe-5S] state of the cluster is a catalytic intermediate in the desulfuration reaction. Structural data and site-directed mutagenesis indicate that a water molecule is located next to the hydrosulfide ligand and to two catalytically important residues, Ser101 and Glu45. This information, together with modeling studies allow us to propose a mechanism for the unprecedented non-redox enzymatic desulfuration of thiouracil, in which a [4Fe-4S] cluster binds and activates the sulfur atom of the substrate.