Investigation of Aquo and Chloro Complexes of UO(2)(2+), NpO(2)(+), Np(4+), and Pu(3+) by X-ray Absorption Fine Structure Spectroscopy.

U, Np, and Pu L(II,III)-edge X-ray absorption fine structure (XAFS) spectra were collected for the UO(2)(2+), NpO(2)(+), Np(4+), and Pu(3+) ions as a function of chloride concentration in aqueous solution. At low chloride concentration, the hydration numbers and corresponding bond lengths for the different ions are as follows: UO(2)(2+), N= 5.3, R = 2.41 Å; NpO(2)(+), N = 5.0, R = 2.50 Å; Np(4+), N = 11.2, R = 2.40 Å; Pu(3+), N = 10.2, R = 2.51 Å. As the Cl(-) concentration increases, inner-sphere Cl(-) complexation occurs, resulting in a decrease in the hydration numbers and an expansion of the actinide-oxygen (water) bond lengths. The Pu(3+) ion shows only a decrease in hydration number (40%) and no inner-sphere Cl(-) complexation for [Cl(-)] < 14 M. For concentrations up to 10-14 M Cl(-), the average Cl(-) coordination numbers and bond lengths are as follows: UO(2)(2+), N = 2.6, R = 2.73 Å; NpO(2)(+), N = 1.0, R = 2.84 Å; Np(4+), N = 2.0, R = 2.61 Å. Structural changes are observed in the near-edge spectral region as shown by significant changes in the white line intensities upon Cl(-) complexation. For ions with similar structures, i.e. Pu(3+) and Np(4+) or the actinyl ions NpO(2)(+) and UO(2)(2+), positive energy shifts are observed with increasing oxidation state. The ability to use XAFS speciation results to calculate equilibrium constants and the relationship of these results to previous studies are discussed.

237Np: oxidation state in vivo and chelation by multidentate catecholate and hydroxypyridinonate ligands.

Chemically, 237Np(V) is as toxic as U(VI), and radiologically, about as toxic as 239Pu. Depending on redox conditions in vivo, 237Np exists as weakly complexing Np(V) (NpO2+) or as Np(IV), which forms complexes as stable as those of Pu(IV). Ten multidentate catecholate (CAM) and hydroxypyridinonate (HOPO) ligands with great affinity for Pu(IV) were compared with CaNa3-DTPA for in vivo chelation of 237Np. Mice were injected intravenously with 237NpO2Cl: those in a kinetic study were killed 1 to 2880 min; in ligand studies, fed mice were injected intraperitoneally with a ligand 5, 60, or 1440 min after 237Np(V) (molar ratio 5.6 to 73), mice fasted for 16 h were gastrically intubated with a ligand 3 min after 237Np(V) (molar ratio 5.6 to 274), and all were killed 24 h after ligand administration; tissues and excreta were radioanalyzed. Rapid plasma clearance and urinary excretion of 237Np(V) resemble U(VI); deposition and early retention in skeleton and liver resemble Pu(IV). The x-ray absorption near edge structure spectroscopy (XANES) spectra of femora of 237Np(V)-injected mice, compared with spectra of Np(V) and Np(IV) from reference solids, showed predominantly Np(IV). Significant in vivo 237Np chelation was obtained with all of the HOPO and CAM ligands injected at molar ratio 22; the HOPO ligands reduced 237Np in skeleton, liver, and other soft tissue, on average, to 72, 25, and 25% of control, respectively, while CaNa3-DTPA was ineffective. Two HOPO ligands injected 60 min after 237Np (molar ratio 5.6) significantly reduced body and liver 237Np, and three HOPO ligands given orally (molar ratio > or = 73) significantly reduced body and liver 237Np, compared with controls. Combined with earlier work, these results indicate that: the dominant neptunium species circulating and excreted in urine is Np(V), while that in bone and liver deposits is Np(IV); Np(V) must be reduced to Np(IV) before it can be stably chelated; efficient decorporation of neptunium requires multidentate ligands that form exceptionally stable actinide(IV) chelates and facilitate Np(V) reduction.

Identification of a 33-kilodalton cytoskeletal protein with high affinity for the sodium channel.

The voltage-sensitive sodium channel is an intrinsic membrane protein that is nonrandomly distributed in neurons, suggesting a possible interaction with other cellular constituents. In this study, we have directly tested the hypothesis that components of the cytoskeleton interact with sodium channels. Utilizing the methods of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and blot overlay, we have identified a 33-kilodalton cytoskeletal protein (p33) that binds 32P-labeled sodium channel purified from rat brain. This binding is a high-affinity (KD less than 1 nM) protein-protein interaction that is blocked by low concentrations of unlabeled sodium channels but is not blocked by monosaccharides, the complex glycoprotein fetuin, the transmembrane protein Na+-K+-ATPase, or bovine serum albumin. Levels of p33 are highest in lung and spleen while lower levels are found in brain, peripheral nerve, skeletal muscle, liver, and testes. This tissue distribution implies that the sodium channel may not be the only ligand for p33.

Overexpression of wild-type and dominant negative mutant vimentin subunits in developing Xenopus embryos.

Vimentin belongs to the diverse multigene family of intermediate filament proteins, each member of which is expressed in a tissue-specific and developmentally regulated pattern. The existence of vimentin filaments has been documented in oocytes, eggs, and early embryos of Xenopus laevis, but the role of these cytoskeletal components remains unknown. To investigate the functions of vimentin during early development in Xenopus, we induced the overexpression of wild-type and deletion mutant subunits in most of the cells of embryos by injecting synthetic RNA into fertilized eggs. Wild-type vimentin subunits, as well as subunits lacking most of the amino-terminal head piece, assembled into normal appearing filaments in vivo. Deletion mutants of the fourth alpha-helical rod domain were assembly incompetent and dominantly inhibited the polymerization of wild-type subunits when both types of subunit were co-expressed in cells. Expression of at least a tenfold excess of wild-type or mutant subunits within cells of embryos did not lead to any detectable morphological or developmental abnormalities, suggesting that the presence and proper regulation of vimentin expression is not essential during the initial stages of embryogenesis in Xenopus.

Molecular genetic analysis of cytoskeletal proteins in development and implications for the membrane skeleton.

The membrane skeleton of nonerythroid cells may be involved in a variety of processes, including the formation and maintenance of specific membrane-cytoskeletal domains. Although much has been learned about the ultrastructure and protein chemistry of the membrane skeleton, there are few direct tests of the in vivo functions of the constituent proteins of the membrane skeleton. Recent advances in molecular genetic analysis provide techniques for studying the membrane skeleton and its components in vivo. Considered here in brief detail are a variety of genetic techniques that have already been used to study cytoskeletal proteins. These techniques should also prove useful for future study of the membrane skeleton.

Coordination chemistry of trivalent lanthanide and actinide ions in dilute and concentrated chloride solutions.

We have used EXAFS spectroscopy to investigate the inner sphere coordination of trivalent lanthanide (Ln) and actinide (An) ions in aqueous solutions as a function of increasing chloride concentration. At low chloride concentration, the hydration numbers and corresponding Ln,An-O bond lengths are as follows: La3+, N = 9.2, R = 2.54 A; Ce3+, N = 9.3, R = 2.52 A; Nd3+, N = 9.5, R = 2.49 A; Eu3+, N = 9.3, R = 2.43 A; Yb3+, N = 8.7, R = 2.32 A; Y3+, N = 9.7, R = 2.36 A; Am3+, N = 10.3, R = 2.48 A; Cm3+, N = 10.2, R = 2.45 A. In ca. 14 M LiCl, the early Ln3+ ions (La, Ce, Nd, and Eu) show inner sphere Cl- complexation along with a loss of H2O. The average chloride coordination numbers and Ln-Cl bond lengths are as follows: La3+, N = 2.1, R = 2.92 A; Ce3+, N = 1.8, R = 2.89 A; Nd3+, N = 1.9, R = 2.85 A; Eu3+, N = 1.1, R = 2.81 A. The extent of Cl- ion complexation decreases going across the Ln3+ series to the point where Yb3+ shows no Cl- complexation and no loss of coordinated water molecules. The actinide ions, Am3+ and Cm3+, show the same structural effects as the early Ln3+ ions, i.e., Cl- ion replacement of the H2O at high chloride thermodynamic activities. The Clion coordination numbers and An-Cl bond lengths are: Am3+, N = 1.8, R = 2.81 A; Cm3+, N = 2.4, R = 2.76 A. When combined with results reported previously for Pu3+ which showed no significant chloride complexation in 12 M LiCl, these results suggest that the extent of chloride complexation is increasing across the An3+ series. The origin of the differences in chloride complex formation between the Ln3+ and An3+ ions and the relevance to earlier work is discussed.

EXAFS studies of americium (III)-benzimidazole complex in ethanol.

The local structures of Am, Nd and Er-Benzimidazole (Biz) in solution were determined by EXAFS. The BIZ molecule coordinated to Am and Nd through two nitrogen atoms in a bidentate fashion. Two nitrogen atoms of BIZ ligated to Am and Nd with the bond distances R(Am-n) N=2.63A and R(Nd-N) = 2.65 A, respectively. The total coordination number of the Am BIZ complexes (at a molar ratio of metal ion to ligand of 1:20) was approximately 10 but that of Nd BIZ complex was approximately 9.