Thermal behaviour of strontium tartrate crystals grown with the aid of sodium metasilicate gel is investigated using thermogravimetry (TG) and differential thermal analysis (DTA). Effect of magnetic field and dopant (Pb) 2+ on the crystal stability is also studied using thermal analysis. This study reveals that water molecules are locked up in the lattice with different strengths in the grown crystals. Keywords. Strontium tartrate; dopant; effect of magnetic field; thermal behaviour.
SynopsisThe synthesis of the tetrapeptide benzyloxycarbonyl(a-aminoisobutyryl-L-prolyl)zmethyl ester (Z-(Aib-Pro)z-OMe) and an analysis of its conformation in solution and the solid state are reported. Stepwise synthesis using dicyclohexylcarbodiimide leads to racemization a t Pro(2). Evidence for the presence of diastereomeric tetrapeptides is obtained from 270-MHz'H-nmr and 67.89-MHz T C -n m r . The all-L tetrapeptide is obtained by fractional crystallization from ethyl acetate. The NH of Aib (3) Peptides in which the imino acid proline occurs at every alternate position are stereochemically interesting, since conventional a-helical and 0-structures are precluded because the presence of proline interrupts a regular sequence of intra-or interchain hydrogen bonds. The possibility that X-Pro tertiary amide bonds may exist in both cis and trans conformations, in solution, also needs to be considered.' The importance of understanding the conformations accessible to (X-Pro), peptides is emphasized by the observation of such sequences in rabbit skeletal muscle alkali light chain (residues 13-30)2 and in bovine P-casein A2 (residues 60-67).3 Interestingly, the isolated heptapeptide sequence Tyr-ProPhe-Pro-Gly-Pro-Ile has been shown to have opioid a~t i v i t y .~ A program to synthesize well-defined (X-Pro), oligopeptides-where X = Gly, Sar, Ala, and a-aminoisobutyric acid (Aib)-and to study their solution and solid-state conformations by spectroscopic and x-ray methods has been initiated in this laboratory. The systematic variation of the bulk of the X residue should allow an evaluation of the influence of the C" substituent on backbone conformation in this sequence. We describe in this report
The suitability of calcium phosphate crystals for thermoluminescence dosimetry (TLD) applications is investigated, owing to their equivalence to bone mineral. The α and β phases of tricalcium phosphate (TCP) were synthesized through wet precipitation and high temperature solid state routes and doped with Dy and Eu. The photoluminescence and thermoluminescence studies of the phosphors have been carried out.The TL properties were found to be highly dependent on the method of preparation of the material. Eu doping gave good PL emission, whereas Dy doping was more efficient in TL emission. β-TCP was found to be less TL sensitive than α-TCP, yet it was identified as a better phosphor material owing to its better fading characteristics.The dependence of TL of β-TCP : Dy on the energy and dose of radiation, and on the doping concentration were studied. The TL intensity was observed to fade exponentially during a storage period of 20 days, but it stabilized at 70% of the initial value after 30 days. The optimum doping concentration was found to be 0⋅5 mol%.
Knowledge of the molecular mechanisms involved in ionophore-mediated cation transport would be valuable for understanding many essential functions of biological membranes. Cations are transported in several stages, such as formation of the ionophore-cation complex, diffusion across the cell membrane and subsequent release of the cation. Several conformational rearrangements are involved in this process, and so a detailed understanding of all the conformational possibilities of the ionophore seems to be essential for elucidating the molecular mechanism of ion transport. We are carrying out spectroscopic and crystallographic studies to explore the possible conformational stages of ionophores by complexing them, in different solvents, with cations of various sizes and charges. We report here a novel conformation of the ionophore valinomycin in its barium complex. It can be described as an extended depsipeptide chain, without interval hydrogen bonds, wound in the form of an ellipse with the two barium ions located at the foci.
C 16H20N204, monoclinic, P2, a = 6.270 (1), b= 11.119(3),c= ll.640(4)A, fl= 100.7 (2)°,Dm = 1-27 (flotation), D c = 1-26 Mg m -3, Z = 2. The structure has been refined to a final R value of 0.041 for 1584 independent counter-measured reflections. The oxazolone ring in the molecule is nearly planar. The exocyclic O atom is 0.065 A out of the plane defined by the other four atoms in the ring belonging to the lactone group. The difference in length between the two adjacent C-O bonds in the ring is small, but significant. The crystal structure is stabilized by van der Waals interactions and a N--H... N hydrogen bond.
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