Search results for "BOND"
showing 10 items of 3527 documents
Application of a weakly basic dimethylamino-modified silica ion exchanger to the separation of oligonucleotides
1979
Abstract LiChrosorb RP-8, RP-18 and Diol as well as a newly synthesized basic dimethylamino-modified silica ion-exchanger (DMA-silica) were applied for the separation of adenylic acid, cytidylic acid and uridylic acid oligoribonucleotides. On LiChrosorb RP-8 and RP-18, respectively, in aqueous buffered eluents (K 2 HPO 4 - H 3 PO 4 ), the retention of oligonucleotides was increased with decreasing number of nucleotide units in the solute, i.e., with increasing hydrophobic character. The elution behaviour of ologonucleotides on LiChrosorb Diol followed the same order but took place according to a size-exclusion mechanism. The retention of oligonucleotides on DMA-silica is assumed to be based…
Influence of the pH on the behavior of an imprinted polymeric stationary phase--supporting evidence for a binding site model.
2001
The equilibrium isotherms of the two enantiomers of phenylalanine anilide (PA) were measured by conventional frontal analysis at three different pH on a thermally-treated imprinted stationary phase selective for the L enantiomer. The first of these pH (buffer pH=3.0, pH(app)=4.0) is well below the apparent pKa (6.4) of the two solutes, the second (buffer pH=5.8, pH(app)=7.0) slightly below this pKa, and the third (buffer pH=7.0, pH(app)=8.3) well above it. The experimental data were fitted to several isotherm models. The best estimates of the parameters of these models are reported and discussed. The corresponding isotherms are compared with the experimental ones. The contributions of the e…
Chromatographic quantitation of the hydrophobicity of ionic compounds by the use of micellar mobile phases
1998
Abstract Many biologically active compounds of interest in structure–activity relationships are ionic at physiological pH. However, ionic organic compounds are only weakly or not retained in conventional RPLC which impedes the chromatographic estimation of their hydrophobicity and the development of quantitative retention–activity relationship studies. The use of micellar mobile phases allows the retention of ionic compounds. Hydrophobic and electrostatic forces govern the retention of ionic compounds in micellar liquid chromatography. In this paper three different retention models log k–log P for ionic compounds are tested (P=partition coefficient). The retention model (log k=a log P+bα+c)…
Elucidation of structure and microheterogeneity of the polypeptide antibiotics paracelsin and trichotoxin A-50 by fast atom bombardment mass spectrom…
1984
Specific and sensitive procedures have been developed which enabled the structure elucidation of the polypeptide antibiotics (peptaibols), paracelsin isolated from Trichoderma reesei, and of trichotoxin A-50 from Trichoderma viride, by fast atom bombardment and field desorption mass spectrometry. Both peptides were found to exhibit a pronounced microheterogeneity by single and multiple exchange of amino acids. Separation by analytical and semipreparative high-performance liquid chromatography (HPLC) on octadecylsilyl-bonded, reversed-phase columns afforded a series of sequence analogues for each polypeptide. Unequivocal molecular weight and sequence identifications were obtained by positive…
Isolation, structural and toxicological characterization of three new mycotoxins produced by the fungusAureobasidium pullulans.
1993
3 substances, B1, B2, and E1 were isolated from culture medium extracts ofAureobasidium pullulans by reversed phase liquid chromatography and subsequent liquid chromatographic purification steps on silica gel.The 3 compounds inhibited the metabolism ofSaccharomyces cerevisiae and showed toxic effects in the growth inhibition test toEscherichia coli andBacillus subtilis.Elementary analysis and mass spectroscopical methods revealed sum formulas of C23H22O6, C22H20O6 and C24H28O3 for B1 B2, and E1 and molecular weights of 394, 380, and 364, respectively. Mass spectroscopical, UV-, IR-,(13)C-NMR, and(1)H-NMR-spectroscopical investigations revealed polycyclic, non-aromatic compounds containing s…
Modeling of Amino Acid Electrodiffusion through Fixed Charge Membranes
2001
Abstract We study theoretically the electrodiffusion of amino acids through fixed charge membranes, calculating the ionic fractions of the amino acid in the membrane as well as its total flux as a function of the relevant experimental parameters (amino acid concentration, salt concentration, and pH of the external solution; membrane fixed charge concentration; and amino acid membrane/solution partition coefficients) under different experimental conditions (symport vs antiport transport, uphill transport, etc.). The theoretical approach employed is based on the Nernst–Planck flux equations in the (Goldman) constant electric field assumption and considers all the species present in the system…
Stereochemistry of terpene derivatives. Part 4:☆ Fragrant terpenoid derivatives with an unsaturated gem-dimethylbicyclo[3.1.0]hexane system
2005
Abstract Starting from (+)-3-carene 1 several chiral fragrant compounds with the bicyclo[3.1.0]hexane system 4–6 and 10–20 were synthesized. These compounds are structural analogues of naturally occurring fragrant compounds, such as ionones and damascones, and possess either an endo- or an exo-cyclic double bond in the bicyclo[3.1.0]hexane moiety. The absolute configuration of selected products was confirmed by X-ray crystallography and circular dichroism analysis.
Conformational characteristics of homo-oligopeptides of O-benzyl-L-tyrosine+
1983
Conformational studies of X[-L-Tyr(Bzl)-]n-series bound to polyethyleneglycol (X = H2, Nps; n = 3-8) in the solid state and in solvents of different polarities and capabilities of forming hydrogen bonds are reported. By using i.r. absorption, the occurrence of the beta-structure in the higher oligomers in the solid state was established. By means of i.r. absorption and CD the onset of that ordered conformation in solution was assessed as a function of chain length. The effects induced by the presence of the N-protecting group and added base, and by changing the nature of solvent on the conformational preferences of the [-L-Tyr(Bzl)-]n homo-peptides were also examined. The 2-nitrophenylsulph…
Conformational investigation of α,β-dehydropeptides VII*. Conformation of Ac-Pro-ΔAla-NHCH3 and Ac-Pro-(E)-ΔAbu-NHCH3: comparison with (Z)-substitute…
2009
The crystal structure and solution conformation of Ac-Pro-deltaAla-NHCH3 and the solution conformation of Ac-Pro-(E)-deltaAbu-NHCH3 were investigated by X-ray diffraction method and NMR, FTIR and CD spectroscopies. Ac-Pro-deltaAla-NHCH3 adopts an extended-coil conformation in the crystalline state, with all-trans peptide bonds and the deltaAla residue being in a C5 form, phi(1)=-71.4(4), psi(1)=-16.8(4), phi(2)= -178.4(3) and psi(2)= 172.4(3) degrees. In inert solvents the peptide also assumes the C5 conformation, but a gamma-turn on the Pro residue cannot be ruled out. In these solvents Ac-Pro-(E)-deltaAbu-NHCH3 accommodates a beta(II)-turn, but a minor conformer with a nearly planar dispo…
Inhibition of cyclodextrins on α-galactosidase.
2017
This work successfully investigated the effects of different influential factors and hydrophobic cavities of cyclodextrins (CDs) on α-galactosidase (α-Gal) by detecting α-Gal activity. The highest inhibitory concentration of three kinds of CDs (α-, β-, and γ-CD) on α-Gal was 10mM. Moreover, the highest inhibition of α-Gal was obtained under the following conditions: reaction time of 90min, temperature of 30°C, and pH 6.0. Compared with other CDs, β-CD showed more ability to interact with α-Gal due to its appropriate cavity geometric dimensions. From circular dichroism and nuclear magnetic resonance it was observed that β-CD changed the secondary structure of α-Gal and formed a hydrogen bond…