Factors influencing glabridin stability

Glabridin, a polyphenolic isoflavan of Glycyrrhiza glabra, has shown a variety of pharmaceutical properties. We have previously studied the isolation of glabridin using macroporous resin and found that it is partially degraded, giving a dark color. To illustrate the degradation of glabridin, the present work studied the stability of glabridin under various conditions. Licorice extract containing about 20% glabridin, obtained from G. glabra by silica gel column chromatography, was used in the stability study. Seven different factors (temperature, illumination, humidity, pH, solvent, oxygen, and oxidant) were studied and content changes were determined through HPLC analysis. Except for oxygen, all the above factors had an effect on the stability of glabridin, with illumination being the main one. Moreover, the interactions between temperature and pH, temperature and humidity, and illumination and pH can promote the degradation of glabridin. In conclusion, we suggest that a dark, dry and airtight environment provides the optimized condition for the long-term storage of glabridin.     

Factors influencing the thermal stability of azo and bisazo compounds

Journal of Thermal Analysis and Calorimetry - The present study investigates the effects of substituents, structural rigidity, tautomerism, hydrogen bonding interaction, and spatial hindrance on...     

Factors influencing deformation stability of binary glasses

A possible mechanism of strain accommodation in large deformation of glasses is crystallization; deformation stability is a measure of the resistance of glasses to crystallization. We study the effect of atomic size ratio and atomic stiffness parameter (related to the curvature of the interatomic potential) on deformation stability of binary glasses using molecular static simulations. The deformation stability of a glass is found to increase with increasing atomic size ratio and magnitude of the atomic stiffness, which is proportional to the bulk modulus of the pure crystalline system, as well as the ratio of atomic stiffnesses of constituent atoms. To understand the role of the above parameters on deformation stability, misfit energies of randomly substituted solid solution fcc crystals and glasses are compared for various atomic size ratios and atomic stiffness values. Unlike in fcc solid solution, the misfit energy of binary glasses is found to be insensitive to the atomic size ratio. It is also found that the packing fraction of glasses is insensitive to the atomic size ratio, consistent with the above result. Beyond a critical atomic size ratio, the misfit energy of fcc solid solution exceeds the energy of the glass, thus making the amorphous state completely stable to deformation induced crystallization. Our analysis shows that critical atomic size ratio decreases with increasing atomic stiffness which leads to an increase in the deformation stability of glasses.     

ZSM-5分子筛催化剂稳定性的研究

为观察C₄烯烃转化制丙烯过程中影响催化剂稳定性的因素，对经历不同反应时间（即新鲜、反应48h、反应650h、反应650h后再生和再生后再进行反应48h）的系列ZSM-5分子筛催化剂进行了表征。结果表明，经历长期反应后，催化剂微孔体积和BJH吸附累积孔体积明显降低；再生后，孔体积可以恢复到新鲜催化剂的初始值。与此同时，NH₃-TPD酸量不仅降低，而且再生后也不能得到恢复。与高温水蒸气处理的催化剂相比较，²7Al MAS NMR表征结果说明，积炭因素对催化剂孔体积的影响可通过再生消除，而分子筛骨架铝丢失和分子筛晶胞参数的降低对催化剂酸量的持续减少起主要作用。长期在高温水蒸气条件下运行后，催化剂酸量可维持在一定水平，使催化剂长期稳定运行成为可能。     

Factors influencing retention and resolution of substituted alkylbenzenes in reversed-phase liquid chromatography

Summary The retention data of isomeric xylenes, ethyltoluenes and diethylbenzenes, and of mesitylene, benzene, toluene and ethylbenzene were obtained on a reversedphase column using methanol-water and ethanol-water mobile phases at four different temperatures. This database was used to relate the dependence of solute retention and resolution on the polarity of the mobile phase, solute dipole moment, and column temperature. The additivity of the free energy of the transfer of solute molecules or solute segments from the stationary phase to the mobile phase, was examined for the isomeric compounds. For this, the logarithm of the net retention volume was substituted for the free energy. Deviations from the additivity of free energies indicate that the separation of isomeric substituted alkylbenzenes is governed by their differential interactions with both the polar mobile phase and nonpolar stationary phase. Among the disubstituted alkylbenzenes,ortho-isomers favor the mobile phase more andpara-isomers tend to prefer the stationary phase more. Themeta-isomers are found to follow the additivity rule closely. These trends are amplified as the polarity of the mobile phase is increased indicating that these isomers are resolved better in water-rich mobile phases. These findings are substantiated by analogous results from gas-liquid chromatographic retention data, estimation of dipole moment effects, and examination of the entropic and enthalpic contributions to the net retention volume.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Factors influencing Al(3+)-dimer speciation and stability from density functional theory calculations

We have investigated aqueous Al-dimer complexes using density functional theory methods (e.g. the B3LYP exchange-correlation functional and the 6-311++G(d,p) basis set). In these calculations interactions between the Al(3+) cations and the H(2)O or OH(-) coordinating ligands are considered explicitly while the second hydration shell and remaining solvent are treated as a continuum under the IEF-PCM formalism. The Al-dimer chemical reactivity is discussed by analysis of changes in geometry, electronic structure and Gibbs free energy of formation, relative to two independent Al(H(2)O) monomers, as a function of water and hydroxide coordination. Our results indicate that the mechanism of cooperativity, i.e. decreased Al-water bond stability with increasing OH(-) coordination and increased water ligand hydrolysis as complex CN decreases, is operating on the dimer species and that, therefore, a wide variety of dimer species are available. While the stability of these species is observed to be dependent on the number of water and hydroxide ligands, the hydroxide bridging structure (singly, doubly and triply bridged species are considered) does not appear to correlate with dimer stability. Interestingly, intra-molecular H-bonds (in the form of the well known H(3)O bridge as well as two bridging structures, H(4)O(2) and H(2)O, that have not, to our knowledge, been previously considered) are observed to influence dimer stability. The evaluation of the equilibrium mole fraction of the dimer species in equilibrium with the aqueous Al(3+) monomer species of our previous study displays the qualitatively correct trend of solution composition as pH increases, namely monomeric aqueous Al(3+) and Al(OH) complexes dominate at low and high pH, respectively, and all remaining monomer and dimer species exist at intermediate pH. Further refinement of our data set by eliminating dimer complexes with OH/Al ratios greater than 2.6 brings our predicted equilibrium mole fraction distributions into excellent agreement with experimental observations. The triply bridged dimer is observed in low amounts while the singly and doubly bridged dimers dominate our model system at pH = ～4-7.     

Factors influencing the stability of H-bonds between guanidinium ions and counter ions or neutral ligands

Guanidinium ions can form H-bonds either with counter ions with proton acceptor sites or with neutral ligands. In solvents as nitrobenzene their formation constants K _h depend a priori on the pK _a of the guanidinium ion in water. Nevertheless the sequence of these constants is perturbed by two other factors: the number of equivalent N–H sites and the steric hindrance of the substituents of the guanidinium ion. In this work the stability constants K _h of guanidinium and methyl or phenyl substituted guanidinium ions with counter ions and neutral ligands were determined from the conductimetric data in nitrobenzene at 25°C. The constants are first divided by the number of available proton donor sites yielding K _h ^/* . Considering only the cases where steric hindrance is not important, correlations are established between log K _h ^/* and the pK _a of the guanidinium ion in water, also corrected for the number of available sites. The effect of steric hindrance in the other systems can be estimated from the comparison of the actual value of K _h ^/* with that of K _h ^/** predicted by the correlations. To a rough approximation, the effect of the steric hindrance can be described by factors _i characterizing the guanidinium ion and _j characterizing the partner as expressed by the relation log₁₀K _h ^** /K _h ^* = _ij.     

Factors influencing the acidity of nitro-compounds

Conclusions The factors influencing the acidity of aliphatic nitro-compounds were discussed, and an attempt was made to estimate quantitatively the contributions of the inductive and mesomeric effects to the values of the ionization constants.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1048–1054, March, 1968.     

Factors influencing proton positions in biomolecules

Results of quantum mechanical calculations are presented that suggest a number of mechanisms whereby protons may be shifted from one group to another along an H bond. The first factor to be considered is a stretching of the bond that drastically raises the energy barrier to transfer. It is possible to predict barriers for an arbitrary system based only on results for a simple system and knowledge of the relevant bond length in the isolated subsystems. Factors that increase the intrinsic basicity of the B group in A-H-B lead not only to a lowering of the energy of the A-HB state relative to AH-B but also to a reduction in the barrier to transfer of the proton from A to B. Ions in the vicinity of the H bond exert a powerful influence and can shift the proton to the less basic group across a gradient of several pK units. Rather than shielding the proton from the external ion, the H bond acts instead to amplify the effects of the electric field. Reorientation of the A and B groups relative to one another, i.e., bends of the H bond, also produce surprisingly large changes in the relative energies of the AH-B and A-HB states. Such bends are capable of pushing the proton across to the normally less basic group, providing a mechanism of coupling conformational changes to proton ‘pumping’ activity. It is found that the high and low pH states of a given H bond can have dramatically differnt relative populations of the AH-B and A-HB configurations. These observations are explained in terms of fundamental concepts involving electrostatic interaction energies.     

Factors influencing the photostability of reactive dyes in polymers

This paper examines several important factors that are believed to affect the photostability of reactive dyes in polymer substrates. The light stability of the dye-polymer covalent bond was found to be influenced by several factors such as the nature of the dyeing process, the nature of the polymer and humidity. In the former the presence of residual washing powder played and important role, whereas in the latter two, although photohydrolysis occurred to a greater extent in nylon 6,6 than in cellulose, humidity had an important influence only in a cellulose environment. In agreement with these findings the presence of a hydroxy radical trap, potassium thiocyanate, inhibited photohydrolysis in cellulose, whereas the presence of an electron trap, cadmium sulphate, inhibited photohydrolysis in nylon 6,6.On the other hand, the influence of the covalent bond on photofading of the dyes was found to be variable. However, the presence of the reactive triazinyl group appeared to stabilise the dye.     

Factors influencing stereoselectivity in the photoreduction of some cyclohexanones

Photoreduction of cyclohexanones not α substituted, generally leads to diastereoisomeric cyclohexanols when the hydrogen-donor is an alcohol or an hydrocarbon. In ethers photoadducts are also isolated.The stereoselectivity of photoreduction depends on experimental conditions. In 2-propanol in particular, it varies with the cyclohexanone/2-propanol molar ratio, with the incident light intensity and with addition of radical scavengers (phenyl disulfide, 1-hexene).The present results can be interpreted at the radical step. They are discussed according to two assumptions of a strictly planar or pyramidal radical centre.     

Factors influencing the steroid profile in doping control analysis

Steroid profiling is one of the most versatile and informative screening tools for the detection of steroid abuse in sports drug testing. Concentrations and ratios of various endogenously produced steroidal hormones, their precursors and metabolites including testosterone (T), epitestosterone (E), dihydrotestosterone (DHT), androsterone (And), etiocholanolone (Etio), dehydroepiandrosterone (DHEA), 5alpha-androstane-3alpha,17beta-diol (Adiol), and 5beta-androstane-3alpha,17beta-diol (Bdiol) as well as androstenedione, 6alpha-OH-androstenedione, 5beta-androstane-3alpha,17alpha-diol (17-epi-Bdiol), 5alpha-androstane-3alpha,17alpha-diol (17-epi-Adiol), 3alpha,5-cyclo-5alpha-androstan-6beta-ol-17-one (3alpha,5-cyclo), 5alpha-androstanedione (Adion), and 5beta-androstanedione (Bdion) add up to a steroid profile that is highly sensitive to applications of endogenous as well as synthetic anabolic steroids, masking agents, and bacterial activity. Hence, the knowledge of factors that do influence the steroid profile pattern is a central aspect, and pharmaceutical (application of endogenous steroids and various pharmaceutical preparations), technical (hydrolysis, derivatization, matrix), and biological (bacterial activities, enzyme side activities) issues are reviewed.     

Factors influencing the sorption and migration behavior of uranium in shale

Journal of Radioanalytical and Nuclear Chemistry - The sorption of U(VI) on shale has been experimentally investigated as a function of pH, initial uranium concentration, rock particle size and...     

Kinetic resolution of tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates for the synthesis of homochiral 3-alkyl-cispentacin and 3-alkyl-transpentacin derivatives

High levels of stereocontrol are observed in the conjugate addition of lithium dibenzylamide to tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn), with addition occurring exclusively anti- to the 3-alkyl substituent. Treatment of a range of tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn, (i)Pr, (t)Bu) with lithium (RS)-N-benzyl-N-[small alpha]-methylbenzylamide indicates that good enantiorecognition is observed (E > 80) in their mutual kinetic resolution. In these reactions, conjugate addition of the lithium amide occurs exclusively anti- to the 3-alkyl substituent, with subsequent C(1)-protonation occurring preferably anti- to the 2-amino group in the 3-Et, 3-Bn and 3-(i)Pr cases, giving predominantly the corresponding 1,2-syn-2,3-anti-diastereoisomers. Conjugate addition to (RS)-3-tert-butyl cyclopentene-1-carboxylate results in exclusive 2,3-anti -addition and a reversal in C(1)-protonation selectivity, giving predominantly the 1,2-anti-2,3-anti-diastereoisomer. Furthermore, the kinetic resolution of the tert-butyl (RS)-3-alkylcyclopentene-1-carboxylates (alkyl = Et, Bn, (i)Pr, (t)Bu) with lithium (S)-N-benzyl-N-alpha-methylbenzylamide proceeds efficiently, giving, at between 47 and 51% conversion, the resolved 3-alkylcyclopentene-1-carboxylates in >85 to >98% ee and the beta-amino ester products of conjugate addition in high de, consistent with E > 80 in each case. Subsequent deprotection of the 1,2-syn-2,3-anti-3-alkyl-beta-amino esters (alkyl = Et, Bn, (i)Pr) by hydrogenolysis and ester hydrolysis gives the corresponding 1,2-syn-2,3-anti-3-alkylcispentacins in >98% de and 98 +/- 1% ee. Selective epimerisation of the 1,2-syn-2,3-anti-3-alkyl-beta-amino esters (alkyl = Et, Bn, (i)Pr, (t)Bu) by treatment with KO(t)Bu in (t)BuOH gives the corresponding 1,2-anti-2,3-anti-3-alkyl-beta-amino esters in quantitative yield and in >98% de, with subsequent deprotection by hydrogenolysis and ester hydrolysis giving the corresponding 1,2-anti-2,3-anti-3-alkylcispentacin hydrochlorides in >98% de.     

Factors influencing the atomization of vanadium in graphite furnace AAS

We have made the determination of V conform to the requirements of the modern (stabilized temperature) furnace technology where the integrated absorbance (A·s) signals are used to quantitate analyte volatilized into a chamber whose temperature is relatively constant during the period when the analyte peak is measured. Graphite tubes with good pyrolytic coating and fast (maximum power) heating are required. We explored the advantages of specially designed tubes, of a cool-down procedure between the char and atomization steps and of very thin platforms. We found that Mg(NO₃)₂ was advantageous as a matrix modifier. With these conditions we found no problems with several matrices reported by earlier workers to be troublesome, for example HNO₃, phosphate, Fe, Mg and Ca. However metals that form very refractory carbides, such as La, Mo, W and Zr may remain troublesome for V, probably because mixed carbides result which include VC. A group of geological samples was analysed for V. Our recommendation is the use of wall atomization from tubes with good pyrolytic coatings, Mg(NO₃)₂ as a matrix modifier and the cool-down procedure to establish a nearly constant temperature along the tube.     

Factors influencing the distribution pattern of porphyrins in cell membranes

The mechanism of the sensitizer-membrane interactions has been studied by following the distribution properties of selected porphyrins, including haematoporphyrin (HP) and protoporphyrin (PP), into unilamellar liposomes of dipalmitoyl phosphatidylcholine (DPPC). The endomembrane distribution of HP and PP has been checked as a function of the membrane fluidity and composition by fluorescence polarization and quenching techniques. At porphyrin concentrations below 0.5 microM, HP and PP exclusively localize in the inner phospholipid monolayer; at higher concentrations, the outer monolayer also becomes populated. The porphyrin binding sites in liposomes, however, are different for HP and PP: HP preferentially distributes into water-accessible lipid regions, while PP localizes in the most hydrophobic loci of the lipid matrix. A porphyrin redistribution occurs when the fluidity properties of the liposomes are changed by addition of cholesterol or cardiolipin. In DPPC-cholesterol vesicles, all HP molecules dissolve in DPPC-rich regions while all PP molecules partition in cholesterol-rich environments. In DPPC-cardiolipin vesicles both porphyrins preferentially localize in regions accessible to the external medium. The effect of the nature of the carrier on porphyrin distribution in membranes has been studied by following the uptake and photosensitization properties of free and DPPC-incorporated PP and HP with rat liver mitochondria. The porphyrin photosensitizing efficiency has been checked by following the impairment of the respiratory function of mitochondria upon irradiation. Liposome-bound HP is less active than aqueous HP in determining membrane photodamage in mitochondria. On the contrary, aqueous PP is a very poor sensitizer as compared to a DPPC liposome-entrapped drug.(ABSTRACT TRUNCATED AT 250 WORDS)     

Factors influencing the free oxygen content in an electrothermal atomizer

A considerable decrease in the partial pressure of free oxygen in the gas phase within a graphite atomizer during atomization with the use of ascorbic acid as a matrix modifier was proved by using the method suggested by L'vov and Ryabchuk. A similar change in the gas-phase composition was observed in graphite tubes treated with compounds of refractory metals. A mechanism for this oxygen binding is suggested. It is shown that the sensitivity of the determination of tin is improved by decreasing the free oxygen content of the gas phase.     

A 3-D bonding perspective of the factors influencing the relative stability of the S1/S0 conical intersections of the penta-2,4-dieniminium cation (PSB3)

A balanced treatment of the covalent and ionic contributions to the ground and excited states originating from torsion about double bonds is known to be strongly dependent on the presence of dynamic electron correlation. We undertake an analysis of the minimum energy pathways corresponding to deactivation of the first excited singlet state of PSB3. In doing so we consider torsion about the three double bonds including other intramolecular degrees of freedom, such as the bond length alternation. The 3-D bond-path analysis provides a new ‘bond-localized orbital-like’ directional interpretation of bonding. Therefore, we present a more sophisticated method of determination of the degree of covalent and ionic contributions known to be responsible for altering the relative stability of the S₁/S₀ conical intersections. The results presented suggest that the commonly used simplified multi-reference methodologies that often result in incorrect predictions for the excited state deactivation reaction mechanism.