How Do Radionuclides Accumulate in Marine Organisms? A Case Study of Europium with Aplysina cavernicola.

In the ocean, complex interactions between natural and anthropogenic radionuclides, seawater, and diverse marine biota provide a unique window through which to examine ecosystem and trophic transfer mechanisms in cases of accidental dissemination. The nature of interaction between radionuclides, the marine environment, and marine species is therefore essential for better understanding transfer mechanisms from the hydrosphere to the biosphere. Although data pertaining to the rate of global transfer are often available, little is known regarding the mechanism of environmental transport and uptake of heavy radionuclides by marine species. Among marine species, sponges are immobile active filter feeders and have been identified as hyperaccumulators of several heavy metals. We have selected the Mediterranean sponge Aplysina cavernicola as a model species for this study. Actinide elements are not the only source of radioactive release in cases of civilian nuclear events; however, their physicochemical transfer mechanisms to marine species remain largely unknown. We have targeted europium(III) as a representative of the trivalent actinides such as americium or curium. To unravel biological uptake mechanisms of europium in A. cavernicola, we have combined radiometric (γ) measurements with spectroscopic (time-resolved laser-induced fluorescence spectroscopy, TRLIFS, and X-ray absorption near-edge structure, XANES) and imaging (transmission electron microscopy, TEM, and scanning transmission X-ray microscopy, STXM) techniques. We have observed that the colloids of NaEu(CO3)2·nH2O formed in seawater are taken up by A. cavernicola with no evidence that lethal dose has been reached in our working conditions. Spectroscopic results suggest that there is no change of speciation during uptake. Finally, TEM and STXM images recorded at different locations across a sponge cross section, together with differential cell separation, indicate the presence of europium particles (around 200 nm) mainly located in the skeleton and toward the outer surface of the sponge.

Evidence for 5d-σ and 5d-π covalency in lanthanide sesquioxides from oxygen K-edge X-ray absorption spectroscopy.

The electronic structure in the complete series of stable lanthanide sesquioxides, Ln2O3 (Ln = La to Lu, except radioactive Pm), has been evaluated using oxygen K-edge X-ray absorption spectroscopy (XAS) with a scanning transmission X-ray microscope (STXM). The experimental results agree with recent synthetic, spectroscopic and theoretical investigations that provided evidence for 5d orbital involvement in lanthanide bonding, while confirming the traditional viewpoint that there is little Ln 4f and O 2p orbital mixing. However, the results also showed that changes in the energy and occupancy of the 4f orbitals can impact Ln 5d and O 2p mixing, leading to several different bonding modes for seemingly identical Ln2O3 structures. On moving from left to right in the periodic table, abrupt changes were observed for the energy and intensity of transitions associated with Ln 5d and O 2p antibonding states. These changes in peak intensity, which were directly related to the amounts of O 2p and Ln 5d mixing, were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by the O K-edge XAS is discussed in the context of several recent theoretical and physical studies on trivalent lanthanide compounds.

Effects of Ligand Geometry on the Photophysical Properties of Photoluminescent Eu(III) and Sm(III) 1-Hydroxypyridin-2-one Complexes in Aqueous Solution.

A series of 10 tetradentate 1-hydroxy-pyridin-2-one (1,2-HOPO) ligands and corresponding eight-coordinated photoluminescent Eu(III) and Sm(III) complexes were prepared. Generally, the ligands differ by the linear (nLI) aliphatic linker length, from 2 to 8 methylene units between the bidentate 1,2-HOPO chelator units. The photoluminescent quantum yields (Φtot) were found to vary with the linker length, and the same trend was observed for the Eu(III) and Sm(III) complexes. The 2LI and 5LI bridged complexes are the brightest (Φtotxε). The change in ligand wrapping pattern between 2LI and 5LI complexes observed by X-ray diffraction (XRD) is further supported by density functional theory (DFT) calculations. The bimodal Φtot trends of the Eu(III) and Sm(III) complexes are rationalized by the change in ligand wrapping pattern as the bridge (nLI) is increased in length.

Direct observation of 4f intrashell excitation in luminescent Eu complexes by time-resolved X-ray absorption near edge spectroscopy.

We report time-resolved X-ray absorption near edge structure (TR-XANES) measurements at the Eu L3 edge upon photoexcitation of several Eu(III)-based luminescent lanthanide complexes. We find an unambiguous signature of the 4f intrashell excitation that occurs upon energy transfer from the photoactive organic antennas to the lanthanide species. Phenomenologically, this observation provides the basis for direct investigation of a crucial step in the energy transfer pathways that lead to sensitized luminescence in lanthanide-based dyes. Interestingly, the details of the TR-XANES feature suggest that the degree of 4f-5d hybridization may itself vary depending on the excited state of the Eu(III) ion.

Fast Detection Allows Analysis of the Electronic Structure of Metalloprotein by X-ray Emission Spectroscopy at Room Temperature.

The paradigm of "detection-before-destruction" was tested for a metalloprotein complex exposed at room temperature to the high x-ray flux typical of third generation synchrotron sources. Following the progression of the x-ray induced damage by Mn Kß x-ray emission spectroscopy, we demonstrated the feasibility of collecting room temperature data on the electronic structure of native Photosystem II, a trans-membrane metalloprotein complex containing a Mn(4)Ca cluster. The determined non-damaging observation timeframe (about 100 milliseconds using continuous monochromatic beam, deposited dose 1*10(7) photons/µm(2) or 1.3*10(4) Gy, and 66 microseconds in pulsed mode using pink beam, deposited dose 4*10(7) photons/µm(2) or 4.2*10(4) Gy) is sufficient for the analysis of this protein's electron dynamics and catalytic mechanism at room temperature. Reported time frames are expected to be representative for other metalloproteins. The described instrumentation, based on the short working distance dispersive spectrometer, and experimental methodology is broadly applicable to time-resolved x-ray emission analysis at synchrotron and x-ray free-electron laser light sources.

The p110 delta structure: mechanisms for selectivity and potency of new PI(3)K inhibitors.

Deregulation of the phosphoinositide-3-OH kinase (PI(3)K) pathway has been implicated in numerous pathologies including cancer, diabetes, thrombosis, rheumatoid arthritis and asthma. Recently, small-molecule and ATP-competitive PI(3)K inhibitors with a wide range of selectivities have entered clinical development. In order to understand the mechanisms underlying the isoform selectivity of these inhibitors, we developed a new expression strategy that enabled us to determine to our knowledge the first crystal structure of the catalytic subunit of the class IA PI(3)K p110 delta. Structures of this enzyme in complex with a broad panel of isoform- and pan-selective class I PI(3)K inhibitors reveal that selectivity toward p110 delta can be achieved by exploiting its conformational flexibility and the sequence diversity of active site residues that do not contact ATP. We have used these observations to rationalize and synthesize highly selective inhibitors for p110 delta with greatly improved potencies.