Dynamics of the ammonium ion in the ferroelectric transition in sodium ammonium selenate, NaNH4SeO4·2H2O-an E.S.R. study

E.S.R. investigations of γ-irradiated ferroelectric Sodium ammonium selenate, NaNH4SeO4•2H2O and its deuteriated analogue in powder and single crystal forms have led to a deeper understanding of the nature of the ferroelectric transition of 180 K. A number of paramagnetic species formed due to γ-irradiation have been identified on the basis of their g-factors and hyperfine features from 77Se. The radical SeO4 has been used as a microprobe in studying the phase transition.

Load-Deflection Behavior of Restrained R/C Skew Slabs

A method is presented for determining the complete load-deflection behavior of reinforced concrete skew slabs restrained at the edges and subjected to uniformly-distributed loading. The analysis is considered in three stages. In the first stage the load-deflection behavior up to the cracking load is considered. The behavior between the cracking load and the yield line load is considered in the second stage. The load-deflection behavior beyond the yield line load, taking into account the effect of the membrane action, is considered in the third stage. Details of an experimental program of casting and testing 12 reinforced concrete skew slabs restrained at the edges are presented to verify the results of the analysis.

Infrared studies of the phase transitions of alkylammonium halides, RNH3X, and bis-(alkylammonium) tetrahalogenometallates(II), (RNH3)2MX4, (R = alkyl, M = metal, X = Cl or Br)

Internal vibration modes of bis-(alkylammonium) tetrachlorometallates(II) and the corresponding alkylammonium chlorides have been studied through their phase transitions using infrared spectroscopy. The studies show that the vibrational states of alkylammonium ions change markedly through the phase transitions. Spectra of the analogous tetrabromometallates and alkylammonium bromides also confirm this behaviour. There is appreciable motion of the alkylammonium ions in the high-temperature phases; thus, CH3NH+3 ions are essentially undistorted in these phases. The low-temperature, ordered phases show evidence of stronger hydrogen bonding of the cations and for the presence of C—N torsional modes.

Structure and magnetotransport properties of the layered manganites REl.2Srl.8Mn,07(R E = La, Pr, Nd)

We have examined the magnetotransport properties and the structure, by Rietveld refinement of powder X-ray data, of the phases RE(1.2)Sr(1.8)Mn(2)O(7) (RE = La, Pr, Nd). We find that on cooling, La1.2Sr1.8Mn2O7 undergoes a transition to a nearly perfect ferromagnet with 90% magnetization at 1.45 T, as reported by earlier workers, but the Pr and Nd phases show only a small magnetization that grows gradually as the temperature is decreased. There seems to be significant correlation between electrical transport and the Jahn-Teller elongation of the apical Mn-O bonds in these systems. The elongation of the apical Mn-O bonds forces the nine-coordinate rock-salt site to be occupied preferentially by the smaller rare-earth-metal cations. This preferential occupation is reliably obtained from the X-ray refinement. All three title phases show a magnetoresistance ratio of about 4(corresponding to a magnetoresistance, [R(0)-R(H)]/R(0), of about 75%) at a field of 7 T and temperatures around 100 K.

High-resolution refinement of orthorhombic bovine pancreatic phospholipase A2

The X-ray structure of recombinant bovine pancreatic phospholipase A(2) (PLA2), which specifically catalyzes the cleavage of the sn-2 acylester bond of phospholipids, has been refined at 1.5 Angstrom resolution. The crystal belongs to the space group P2(1)2(1)2(1) with unit-cell parameters a = 47.12, b = 64.59 and c = 38.14 Angstrom similar to the native enzyme reported previously by Dijkstra et nl. [J. Mel. Biol. (1981), 147, 97-123]. The refinement converged to an R value of 18.4% (R-free = 22.8%) for 16 374 reflections between 10.0 and 1.5 Angstrom resolution. The surface-loop residues (60-70) art: ordered in the present orthorhombic recombinant enzyme, but disordered in the trigonal recombinant enzyme. The active-site residues, His48, Asp99, and the catalytic water superimpose well with the trigonal form. Besides the catalytic water which is hydrogen bonded to His48, it is often seen that there is a second water attached to the same N atom of His48 and simultaneously hydrogen bonded to the O atom of Asp49. It is thought that the second water facilitates the tautomerism of His48 for enzyme catalysis, The catalytic water is also hydrogen bonded to the equatorial water coordinated to the calcium ion, In addition to the equatorial water, there is also an axial calcium water and the additional structural water. These five common water molecules are hydrogen bonded to the additional 16 water molecules in the present orthorhombic structure which may further enhance the structural integrity of the active site. Besides the protein and one calcium ion, a total of 134 water molecules were located in the present high-resolution refinement.

Determination of beta-turn conformation by laser Raman spectroscopy

To correlate the Raman frequencies of the amide I and III bands to beta-turn structures, three peptides shown to contain beta-turn structure by x-ray diffraction and NMR were examined. The compounds examined were tertiary (formula: see text). The amide I band of these compounds is seen at 1,668, 1,665, and 1,677 cm-1, and the amide III band appears at 1,267, 1,265, and 1,286 cm-1, respectively. Thus, it is concluded that the amide I band for type III beta-turn structure appears in the range between 1,665 and 1,677 cm-1 and the amide III band between 1,265 and 1,286 cm-1.

Structure of the Monosodium Salt of D-Glucose 6-Hydrogenphosphate

Na+.C6HI209 P-, Mr=282.1, monoclinic, e2~, a=5-762(1), b=7.163(2), c=12.313(1)A, fl= 99.97 (1) °, U= 500.5 A 3, Z= 2, D m = 1.86, D x = 1.87 Mg m -s, Cu Ka, 2 = 1.5418 A, /a = 3-3 mm -1, F(000) = 292, T= 300 K, final R for 922 observed reflections is 0-042. The phosphate ester bond, P-O(6), is 1.575 (5)A, slightly shorter than the P~O bond in monopotassium phosphoenolpyruvate [1.612 (6) A] [Hosur & Viswamitra (1981). Acta Cryst. B37, 839-843]. The pyranose sugar ring takes a 4C 1 chair conformation. The conformation about the exocyclic C(5)-C(6) bond is gauche-trans. The endocyclic C-O bonds in the glucose ring are nearly equal with C(5)-O(5) = 1.435 (8) and C(1)-O(5) = 1.436 (9) A. The sodium ion has seven near neighbours within a distance of 2.9 A. The crystal structure is stabilized by hydrogen bonds between the O atoms of symmetryrelated molecules.

Polarized Ethylenes: Structure of 2-Benzoyl-3-dimethylamino-3-methylthlo-2-propenenitrile

C13H14N2OS, M r = 246, is monoclinic, P21/c, with a = 7.214(1), b = 8.935(5), c = 20.243 (6) A, fl =99.42 (2) °, V = 1304.83 ,~3, Z = 4, D m = 1.23, D x =1.25 Mg m -3, p(Mo Ka, 2 = 0.7107 A) = 0.232 mm -~,F(000) = 520. The structure was solved by direct methods and refined to an R value of 0.042 using 1127 intensity measurements. The C=C and C-N bond distances differ considerably from their normal values. An appreciable rotation [38.3(4) °] about the C=C bond is observed, the bond length being 1.414(5)A.This is due to the combination of push-pull and steric effects.