Calcium-binding proteins afford calibration of dihedral-angle dependence of 3J(NC(gamma)) coupling constant in aspartate and asparagine residues

Calibration of the 3J(NC(gamma)) couplings across the N-C(alpha)-C(beta)-C(gamma) fragment of aspartate and asparagine residues is afforded by two interactions that produce fixed conformations of the side chains in solution. One is the binding of these side chains to calcium ions; the other is the H-bond interaction of these side chains with a backbone amide.     

Aromatic interactions in unusual backbone nitrogen-coordinated zinc peptide complexes: a crystallographic and spectroscopic study

A series of zinc complexes with dipeptide ligands of the type Dpg-Xaa was synthesized, where Dpg is dipicolylglycine and Xaa is phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), 2-naphthylalanine (Nal), or glycine (Gly). It was shown that aromatic interactions promote the unusual coordination of an anionic peptide backbone nitrogen atom to zinc. This binding mode was, for the first time, characterized by X-ray structure analyses of the electrically neutral complexes [(Dpg-Phe)(-H)Zn], [(Dpg-Tyr)(-H)Zn], [(Dpg-Trp)(-H)Zn], and [(Dpg-Nal)(-H)Zn]. The pKa values for amide nitrogen deprotonation were determined by 1H NMR titrations {[(Dpg-Phe)Zn], 7.17; [(Dpg-Tyr)Zn], 6.85; [(Dpg-Trp)Zn], 6.85; [(Dpg-Nal)Zn], 6.64; [(Dpg-Gly)Zn], 8.54}. It was calculated that aromatic interactions contribute ca. -8 to -11 kJ/mol of stabilizing free enthalpy changes in the derivatives with aromatic amino acid side chains. These are the first quantitative data obtained for crystallographically characterized metal complexes. A comparison with the literature shows that it is difficult to distinguish between pi-cation attraction and pi-pi stacking. However, it is evident that modification of small peptides with synthetic pyridine ligands enhances their ability to stabilize secondary structures by noncovalent interactions. This is an important consideration for the design of biomimetic metallopeptides.     

Discharge studies of the lithium dimer diffuse bands

We report the observation of lithium dimer diffuse bands in the region from 410 nm to 460 nm emitted from a low-power discharge sustained inside a heat-pipe oven. The identification of the lithium diffuse band as a specific feature originating from the transitions from higher excited levels of the Li₂ molecule was supported by the construction of the emission profile in terms of recently calculated potential energy curves, relevant transition dipole moments and application of the modified quasi-static theory of the satellite band shape.     

On the shape of the yellow diffuse band of potassium

The absorption profile of the diffuse band of K₂ observed in the 575-nm region compares quite favorably with that constructed in terms of recently calculated potential energy curves by the application of the Szudy-Baylis theory for the shape of the satellite bands. The influence of the Boltzmann factor and the transition dipole moment function on the diffuse band shape is discussed.     

Radiation effects on luminescent and structural properties of YPO4: Pr3+ nanophosphors

ABSTRACT

YPO₄ phosphors doped with trivalent ion Pr³⁺ were prepared by sol–gel method and treated with different doses of gamma radiation, from 0.25 MGy to 4 MGy. Effects of radiation on morphology, structure and luminescent properties were analyzed. Also, the influence of radiation on the change in the color of the samples was examined. The color efficiency of powders was evaluated by colorimetric analysis (CIE and L * a * b system). It has been observed that powders change color under the influence of radiation, i.e. they pass from white to pinkish red. Also, it has been determined that the radiation affects morphology change, as the particle size increases with increasing of the radiation dose. With the increase in the radiation dose, the emission intensity of samples decreases. The structure remains almost unchanged after irradiation, and the intensity constantly decreases with increasing of dose.     

Nonlinear Schrödinger equation and DNA dynamics

We study a possible solitary wave solution of the nonlinear Schrödinger equation (NLSE). It is shown that the wave can be both modulated and nonmodulated depending on a ratio of the envelope and the carrier wave velocities. We also study the same type of the soliton solution in DNA dynamics. We show that the ratio of these two velocities is a measure of modulation and we conclude that the modulated wave is more stable than the nonmodulated one. Finally, we solved the problem concerning three parameters arising from the applied procedure for the solution of the NLSE.     

Influence of the average content of sulfur in polysulfide polymers on the depolymerization of polysulfide polymers by dithionite ions

The results of our studies show that it is possible to depolymerize polysulfide polymers with a nominal sulfur content in excess of 2 per segment of polymer, without previous desulfurization, in order to obtain disulfide polymers. For this purpose it is necessary to take larger amounts of sodium dithionite per segment of polymer, than with a disulfide polymer, to obtain approximately the same of the average molecular weight of the depolymerized product. It is also possible to complete depolymerization with an amount below 2 moles of sodium hydroxide per mole of sodium dithionite, though the upper limit of the ratio of Na₂S₂O₄ to NaOH, at which depolymerization occurs, remains unchanged, i.e., 1:4. During the depolymerization reaction the occurrence of larger or smaller amounts of hydrosulfide, depending on the labile sulfur content, as well as the change of the color of the reaction dispersion, first white, then bright yellow or gray, thereafter white again and, finally, dingy pink, was observed. The reaction, representing in the best way the optimum conditions of depolymerization and the actual relations in the reaction system, is proposed.     

Splitting of the disulfide bonds in polysulfide polymers by hydrazine

The action of hydrazine on desulfurized tetrasulfide polymer obtained from bis-2-chloroethyl formal leads to S? S bond splitting and depolymerization. Our studies show that successful depolymerization can be initiated only in a strong alkali medium at 95–98°C. During this process cleavage of the disulfide bonds and the formation of polymers of lower molecular weight with ? SH end groups takes place. From the results of the experiments performed it can be seen that the degree of depolymerization increases with an increase in the amount of hydrazine, in the mole ratio of sodium hydroxide, and in the time of reaction. When 0.7–1.0 mole of hydrazine per segment of polymer and 4 mole of sodium hydroxide per mole of hydrazine are used, a polymer of low average molecular weight in the form of water-soluble sodium salt (fraction I) and a polymer in dispersion form (fraction II) are obtained. Similar results could be attained if the amount ot hydrazine were decreased and the mole ratio of sodium hydroxide were simultaneously increased to more than 4 mole per segment of hydrazine used. In a constant amount of sodium hydroxide with a smaller quantity of hydrazine or a shorter reaction time the amount of water-soluble polymer (as a sodium salt) decreases and the amount of polymer in dispersion form increases. If a portion of the polymer in dispersion is coagulated with a portion of the polymer in the mother liquid, the yield of the depolymerized product is higher, but the average molecular weight is also higher than those of fraction I. The influence of the average contents of sulfur on the efficiency of the depolymerization is tested in polymers that contain 2.5 (i.e., 3) atoms of sulfur per segment of polymer. The result is that the depolymerization of the polysulfide polymer could be carried out without previous desulfurization, but the molecular weights of the products obtained with an equivalent quantity of hydrazine are much higher than those obtained by depolymerization of the disulfide polymer.     

Total synthesis of five thapsigargins: guaianolide natural products exhibiting sub-nanomolar SERCA inhibition

Herein we describe the total synthesis of five guaianolide natural products: thapsigargin, thapsivillosin C, thapsivillosin F, trilobolide and nortrilobolide. Prodrug derivatives of thapsigargin have shown selective in vivo cytotoxicity against prostate tumours and the need for further investigation of this phenomenon highlights the importance of these total syntheses. The first absolute stereochemical assignment of thapsivillosin C is also delineated.