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PubMed2001

Superheating and supercooling of vortex matter in a Nb single crystal: direct evidence for a phase transition at the peak effect from neutron diffraction.

We report the first observation of a striking history dependence of the structure function of vortex matter in the peak effect regime in a Nb single crystal by using small angle neutron scattering combined with in situ magnetic susceptibility measurements. Metastable phases of vortex matter, supercooled vortex liquid and superheated vortex solid, have been identified. We interpret our results as direct structural evidence for a first-order vortex solid-liquid transition at the peak effect.

Journal Article↗
PubMed1980

Interparticle effects in low-angle x-ray and neutron diffraction from chromatin.

Published diffraction data are critically reviewed, and replotted in a new way to show the variation with concentration of the 8- to 25- nm diffraction maximum. Most of the early data are found to be consistent with a single model for a liquid-type array of mutually repulsive particles, whose molecular weight is calculated to be that of a nucleosome or possibly a dimer. The data for all but the highest concentrations, where distortion due to dehydration is possible, support no particular model for the higher-order coiling of chains of nucleosomes, and cannot be used to support models for "native" chromatin. Only in the presence of excess salts or after isolation with polyamines is there aggregation in solution of nucleosomes, which then give peaks at 11 and 5.5 nm that do not change much with concentration. Recent work by the authors confirms that under some conditions nucleosome undergo a transition to a state whose diffraction is consistent with hexagonal packing of extended DNA to which histones are still attached. This state is probably responsible for much of the strong 2.7-nm peak previously obtained from certain samples, which was in some cases assigned to nucleosome structure. Only the peak at 3.7 nm is clearly attributable to the form factor of the isolated native nucleosome.

Animals↗
PubMed1998

Syntheses of the Uranium Complexes [U{N(SiMe(3))(2)}(2){N(SiMe(3))(SiMe(2)CH(2)B(C(6)F(5))(3))}] and [U{C(Ph)(NSiMe(3))(2)}(2){&mgr;(3)-BH(4)}(2)]. Determination of Hydrogen Positions by Single-Crystal X-ray and Neutron Diffraction.

The complex [U{N(SiMe(3))(2)}(2){N(SiMe(3))(SiMe(2)CH(2)B(C(6)F(5))(3))}] (1) is formed in the reaction between the hydride complex [U{N(SiMe(3))(2)}(3)(H)] and B(C(6)F(5))(3), and H(2) is evolved. The X-ray [C(36)H(53)BF(15)N(3)Si(6)U.3.5C(6)D(6), triclinic, space group P&onemacr;, Z = 2, 90 K, a = 14.065(1) Å, b = 14.496(1) Å, c = 18.759(1) Å, alpha = 82.898(1) degrees, beta = 74.415(1) degrees, gamma = 62.919(1) degrees ] and neutron structure [C(36)H(53)BF(15)N(3)Si(6)U.3.5C(6)D(6), triclinic, space group P&onemacr;, Z = 2, 20 K, a = 13.993(1) Å, b = 14.484(1) Å, c = 18.720(1) Å, alpha = 82.810(1) degrees, beta = 74.200(1) degrees, gamma = 63.054(1)E] of compound 1, which crystallizes with 3.5 molecules of C(6)D(6) per asymmetric unit, show the electron deficiency of the uranium atom to be effectively compensated by the formation of multicenter bonds between U and three Si-CH(2) units of the amido ligands. The reaction of the uranium complex [U{C(Ph)(NSiMe(3))(2)}(2)(Cl)(2)] with [Na(BH(4))] gives the complex [U{C(Ph)(NSiMe(3))(2)}(2){&mgr;(3)-BH(4)}(2)] (2). The X-ray structure of 2 [C(26)H(54)B(2)N(4)Si(4)U, monoclinic, space group C2/c, Z = 4, 90 K, a = 21.613(1) Å, b = 9.233(1) Å, c = 18.132(1) Å, beta = 98.804(1) degrees ] proves unequivocally the &mgr;(3) coordination of the BH(4) moieties. In both single-crystal X-ray structure determinations, all hydrogen and deuterium atoms could be located and isotropically refined, including those which are directly coordinated to the uranium. The reliability of the refined hydrogen and deuterium positions for compound 1 is confirmed by comparison of the X-ray and neutron structure determinations. The ability to locate the hydrogen and deuterium positions in these uranium compounds by single-crystal X-ray diffraction is due to good crystal quality, the measurement of data at low temperature, and the use of image plate technology for data collection.

Journal Article↗