1,721,067 research outputs found
Conformational investigation of dibenzo-p-dioxin by proton magnetic resonance in a liquid crystalline solvent
No full textThe structure of dibenzo-p-dioxin has been studied by the nematic liquid crystal n.m.r. method. This molecule is solution adopts a folder conformation about the axis connecting the two heteroatoms with an angle of folding of 165.6 ± 2°
Association Constants for Metal Hexacyanide Binding to Cytochrome c
No full textThe binding of [Co(CN)6]3-, and that of [Fe(CN)6]3- and [Ru(CN)6]4- using a competitive method, to horse cytochrome c has been studied by 59Co NMR spectroscopy. At I = 0.07 M, without added salt and in 2H2O at pH* 7.3 (measured in 2H2O) and 25 degrees C, there are at least two binding sites on ferricytochrome c and ferrocytochrome c for [Co(CN)6]3-. Association constants were determined to be 2.0 +/- 0.6 X 10(3) M-1 and 1.5 +/- 0.5 X 10(2) M-1, respectively, with no effect of the oxidation state of the cytochrome. At higher ionic strength (I = 0.12 M) adjusted with KCl the binding markedly decreased, and, although it was not possible to determine the precise binding stoichiometry and magnitude of association constants, it is clear that the association constants are less than or equal to 1.5 X 10(2) M-1. The binding of [Ru(CN)6]4- at I = 0.07, without added salt and in 2H2O at pH* 7.3 and 23 degrees C, was not precisely defined, but its binding strength relative to that of [Fe(CN)6]3- was determined. Extrapolating this to I = 0.12 (KCl) suggests that under these conditions the association constant for [Ru(CN)6]4- binding to ferricytochrome c is less than or equal to 3 X 10(2) M-1
Conformational analysis of N-acetyldaunomycin in solution. A transient 1H nuclear Overhauser effect study of the glycosidic linkage geometry
No full textThe conformational properties of daunomycin (1) and several analogues (2)–(16) have been investigated by 1H n.m.r. in different solvents, CDCI3, D2O, DMSO, dioxane, and pyridine. From H,H three-bond and long-range coupling constants, the geometry of ring A in each solvent has been determined, through a Karplus–Altona equation, which includes a correction for the substiuent electronegativity. Conformers 9H8, 8H9, 9S, S9, and S8 have been found in solution, and in several cases, especially in DMSO, more than one conformer is present at equilibrium. The equilibrium is always fast compared with n.m.r. times and the relative populations of each conformer have been calculated from experimental and model coupling constants by using a least-squares procedure. Two factors have been recognized to be responsible for the shape of ring A: the intramolecular hydrogen-bond 9–OH O(7) and steric interactions between peri substituents on the A and B rings. Evidence for this hydrogen-bonding has been obtained by dilution experiments, and, for the peri interactions, by evaluating the substituent effects on the conformational preference. Other intramolecular hydrogen bonds have been proved not to exist in solution. The influence of the solvents on the shape of ring A has also been studied. All the results are discussed and compared with those obtained in the solid phase. Daunosamine moiety has also been analysed, and the conformation of the sugar ring is always 1′C4. (L) in all the solvents examined
Conformation of thianthren and 2,7-dichlorothianthren partially oriented in a nematic phase
No full textThe 1H n.m.r. spectra of thianthren and 2,7-dichlorothianthren dissolved in the Merck Nematic Phase IV were analysed and the structural parameters obtained. These molecules in solution adopt a folded conformation about the axis connecting the two sulphur heteroatoms. The angle of folding is 140.6° for thianthren and 139.8° for 2,7-dichlorothianthren. The correspondent standard deviations are ±0.4 and ±0.7°, respectively
Dynamic aspects of the stereochemistry of phenothiazines in solution : Part 2. Segmental motion and conformational analysis of the side-chain in promazines
No full textSome promazines and the model compound NN-dimethyl-3-phenylpropylamine have been studied, as protonated and unprotonated species, by 1H and 13C n.m.r. spectroscopy. 13C T1 relaxation times have shown that the mobility of the dimethylaminopropyl chain in chloroform solution varies depending on whether the chain is protonated or unprotonated, and whether it is attached to a phenyl or to a tricyclic system. Segmental motions were also detected in two compounds. The conformation of the side-chain has been deduced for different solvents. Remarkably, for protonated promazines the preferred conformations are gauche for the Cα-Cβ fragment and trans for the Cβ-Cγ in chloroform as well as in water solution, whereas the model compound exists preferentially in the fully extended trans–trans-conformation. In dioxan and in DMSO the population of the αβ-gauche forms decreases. The presence and the nature of the 2-substituent do not appear to have any effect, whereas the tricyclic system is predominant in the stabilization of the αβ-gauche form. This has been related to the ability of these molecules to aggregate in solution with a vertical stacking-type association
A Quantitative Approach in Conformational Analysis by 2D-Inversion Recovery Difference Spectroscopy
No full textConformational analysis of 9-deoxydaunorubicin in solution : The application of a quantitative transient 1H nuclear Overhauser effect
No full textThe preferred conformation of 9-deoxy-N-trifluoroacetyldaunorubicin has been determined by 1H and 13C n.m.r. in CDCl3 solution. The conformation of ring A was easily defined as a pure half-chair 9H8, from the values of proton coupling constants, and on the basis of a preceding study of daunomycin derivatives. The orientation of the sugar with respect to the aglycone moiety has been obtained by quantitative transient nuclear Overhauser experiments (n.O.e.). The interproton distances have been deduced from the cross-relaxation rates determined by measuring the time development of n.O.e.s, after selective inversion of single resonances. The experimental points were fitted to the theoretical curves through a non-linear least-squares procedure. The overall isotropic motion was proved by 13C T1 measurements of all protonated carbons; the interproton reference distance r4′–5′, deduced from the correlation time value, is in satisfactory agreement with those measured by X-ray analyses of daunomycin and carminomycin. The glycoside linkage geometry has also been expressed in terms of the rotational angles φ= H(1′)–C(1′)–O(7)–C(7) and ψ= C(1′)–O(7)–C(7)–H(7). The results have been compared with those obtained through molecular mechanics calculations, performed with the Allinger MMP2 program
Estratti di trifoglio per la preparazione di agenti antiafide
No full textSi descrivono estratti di semi di specie di Trifoglio come insetticida ad azione aficida
Structure and dynamics of intercalation complexes of anthracyclines with d(CGATCG)2 and d(CGTACG)2. 2D-1H and 31P NMR investigations
No full textThe interaction of (2S )-2-methoxymorpholinodoxorubicin 4 and morpholinodoxorubicin 5 with the hexanucleotides d(CGATCG)2 and d(CGTACG)2 has been studied by a combined use of 2D-1H and 31P NMR techniques and molecular dynamics (MD) calculations, in comparison with doxorubicin 1, daunorubicin 2 and idarubicin 3. Both 1H and 31P chemical shifts of imino protons and phosphates respectively have been shown to be a sensitive probe for the intercalation sites (two anthracycline molecules at the CpG sites). A relevant number of NOE interactions allowed the structure of the complexes in solution to be derived through restrained MD calculations, which were compared with those obtained by X-ray analysis. In all the complexes the aglycone was shown to be located in the middle of the double helix, orthogonally oriented with respect to the base pairs, with ring D extending out of the helix on the major groove and ring A, with 9H8 conformation, between guanines G2 and G12. The daunosamine and morpholino moieties lie in the center of the minor groove, with slightly different positions than in the X-ray structures. In all the complexes the daunosamine ring is located at the A3 (T3) level, while the morpholino ring displays NOE interactions with the fourth unit T4 (A4). The deformations of the double helix are revealed by an increased distance between protons of the C5 and G6 units and by conformational changes at the level of the α, ζ, Îμ and δ angles of the phosphoribose backbone. The variation of the 31P chemical shifts is interpreted in terms of conformational equilibria leading to different populations of conformers. This is clearly shown from the values of the α and ζ torsion angles, monitored during the MD, which indicate a relevant population of trans forms for ζ and also for α angles of C5pG6 and G2pT3 (G2pA3) units, while the other phosphates exist entirely in the α, ζ, gauche,gauche conformation. The dissociation rate constants were measured by 2D 31P NOESY-exchange experiments for 1 and 4. The decrease of koff for 4, corresponding to a ten-fold increase of the residence time of the drug in the intercalation sites, is in line with the higher activity found for methoxymorpholinodoxorubicin 4
Response to Anoxia in Rice and Wheat Seedlings: Changes in the pH of Intracellular Compartments, Glucose-6-Phosphate Level, and Metabolic Rate
No full text31P nuclear magnetic resonance spectroscopy was used to measure intracellular pH in living tissues. Oxygen deprivation caused fast cytoplasmic acidification from pH 7.4 to 7.0 in shoots of rice, Oryza sativa L. var arborio, a species highly resistant to anoxia. Acidification was complete after 10 minutes of anoxia. Alkalinization of both cytosplasm and vacuole followed thereafter. In the anoxia intolerant wheat shoots, Triticum aestivum L. var MEK, the same treatment caused a sharper cytoplasmic acidification, from pH 7.4 to 6.6, which occurred during a period of 2 hours. Cytoplasmic acidification continued with progress of anoxia and there was no vacuolar alkalinization comparable to the one observed in rice. In wheat oxyen, withdrawal also caused the reduction of both glucose-6-phosphate level and of metabolic rate. It also induced heavy losses of inorganic phosphate from tissues. Conversely, in rice, glucose-6-phosphate level and metabolic rate were increased and inorganic phosphate leakage from tissues was completely absent. These results are discussed in relation to the mechanisms of plant resistance to anoxia
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