Chitinase and beta-N-acetylglycosaminidase in the digestive juice of Helix pomatia.

A beta-N-acetylglucosaminidase from Helix pomatia digestive juice was separated and partly purified by gel chromatography. The optimal pH for the degradation of p-nitrophenyl-N-acetyl-beta-D-glucosaminide was 3.4. The molecular weight was around 160 000 and the pI = 4.95. In the same gel chromatography run two chitinase active peaks were also obtained. These chitinase active peaks were also obtained. These chitinases, with molecular weights around 26 000 and 13 000, had somewhat different pH activity curves with optima at 4.2 and 4.3. By isoelectric focusing the first peak with molecular weight around 26 000 was divided in two chitinase active regions with pI at 5.7 and 3.5. The second peak with molecular weight around 13 000 had a pI at 7.3.     

Affinity capillary electrophoresis study of the linkage existing between proton and zinc ion binding to bacitracin A1

Measurements by capillary electrophoresis (CE) of bacitracin A(1) effective mobility at different pH values permitted to estimate the five acidic dissociation constants and the Stokes radii at different protonation stages of the macrocyclic dodecapeptide. The pK(a) values were 3.6 and 4.4 for the two carboxylic groups of the lateral chains of D-Asp-11 and D-Glu-4, respectively, 6.4 for the aza-atom of the imidazole ring of His-10, 7.6 for the amino group of N-terminal Ile-1 and 9.7 for the delta-amino group of D-Orn-7, very close to the values obtained by other researchers by titration experiments. In agreement with a rigid macrocyclic structure the Stokes radii of different protonated forms ranged only between 14.3 and 14.8 A. Best fitting procedures performed on experimental mobility measured at two different pH values (5.50 and 6.72) in the presence of increasing Zn(+2) concentration allowed confirming the model that assumes the binding of Zn(+2) to P(0) peptide form with a 1.5 x 10(3) M(-1) intrinsic association constant. Following to Zn(+2) binding, the pK(a) of the amino group of N-terminal Ile-1 is shifted from 7.6 to 5.9 and the Stokes radius is reduced of about 3 A. The mean charge of the bacitracin A(1)-Zn(+2) complex resulted +1.67 and +1.12 at pH 5.50 and 6.72, respectively. These results suggest that the amino group of N-terminal Ile-1 is not essential for Zn(+2) binding.     

Micellar Electrokinetic Chromatography Using a Cationic Surfactant for Rapid Separation and Determination of Bisbenzylisoquinoline Alkaloids from Embryo of the Seed of Nelumbo nucifera Gaertn

Liensinine (LIE), isoliensinine (ISO) and neferine (NEF) from embryo of the seed of Nelumbo nucifera Gaertn were first separated and quantitatively analyzed by micellar electrokinetic chromatography with internal standard (IS) using a cationic surfactant, tetradecyltrimethyl ammonium bromide (TTAB). Complete separation of the analytes was optimally achieved within 15 min using 40 mM sodium dihydrogenphosphate (pH 6.40) containing 10 mM TTAB and 15% (v/v) methanol as buffer. Tetrahydropalmatine was used as IS to improve the repeatability and linearity relativity. The IS method resulted in excellent linearity, with correlation coefficients of regression equations of 0.9997, 0.9997 and 0.9992 for LIE, ISO and NEF, respectively. Finally, two sample extraction methods were used to investigate the contents of different parts of the embryos.     

Kinetic studies on the addition of phenol to S-phenyl-S-vinyl-N-p-tosylsulfilimine derivatives

The rate constants of nucleophilic addition of phenyl to vinylsulfilimine(VSI) and its derivatives were determined by an ultraviolet spectrophotometer at 25°C, and the rate equations which can be applied over a wide pH range were obtained. On the basis of the pH-rate profile, product analysis, general base catalysis, and the substituent effect, a plausible mechanism of this addition reaction was proposed: Below pH 3.0, the reaction proceeded via the addition of a neutral molecule to the carbon-carbon double bond after protonation at the nitrogen atom of the sulfilimine. Above pH 10.0, the addition of a phenoxide ion to the carbon-carbon double bond was rate controlling. However, in the pH range of 3.0 to 10.0, these two reactions occurred competitively.     

Photophysics of a xanthenic derivative dye useful as an "on/off" fluorescence probe

The photophysical behavior of a new fluorescein derivative has been explored by using absorption and steady-state and time-resolved fluorescence measurements. The influence of ionic strength, as well as total buffer concentration, on both the absorbance and fluorescence has been investigated. The apparent acidity constant of the dye determined by absorbance is almost independent of the added buffer and salt concentrations. A semiempirical model is proposed to rationalize the variations in the apparent pKa values. The excited-state proton-exchange reaction around the physiological pH becomes reversible upon addition of phosphate buffer, inducing a pH-dependent change of the steady-state fluorescence and decay times. Fluorescence decay traces, collected as a function of total buffer concentration and pH, were analyzed by global compartmental analysis, yielding the following values of the rate constants describing excited-state dynamics: k01 = 1.29 x 10(10) s(-1), k02 = 4.21 x 10(8) s(-1), k21 approximately 3 x 10(6) M(-1) s(-1), k12B= 6.40 x 10(8) M(-1) s(-1), and k21B = 2.61 x 10(7) M(-1) s(-1). The decay rate constant values of k01, k21, k21B, along with the low molar absorption coefficient of the neutral form, mean that coupled decays are practically monoexponential at buffer concentrations higher than 0.02 M and any pH. Thus, the pH and buffer concentration can modulate the main lifetime of the dye.     

Size control of gold nanocrystals in citrate reduction: the third role of citrate

Growth kinetics and temporal size/shape evolution of gold nanocrystals by citrate reduction in boiling water were studied systematically and quantitatively. Results reveal that the size variation and overall reaction mechanism were mostly determined by the solution pH that was in turn controlled by the concentration of sodium citrate (Na3Ct) in the traditional Frens's synthesis. This conclusion was further confirmed by the reactions with variable pH but fixed concentrations of the two reactants, HAuCl4 and Na3Ct. Two substantially different reaction pathways were identified, with the switching point at pH = 6.2-6.5. The first pathway is for the low pH range and consists of three overlapping steps: nucleation, random attachment to polycrystalline nanowires, and smoothing of the nanowires via intra-particle ripening to dots. The second pathway that occurred above the pH switching point is consistent with the commonly known nucleation-growth route. Using the second pathway, we demonstrated a new synthetic route for the synthesis of nearly monodisperse gold nanocrystals in the size range from 20 to 40 nm by simply varying the solution pH with fixed concentrations of HAuCl4 and Na3Ct. The switching of the reaction pathways is likely due to the integration nature of water as a reaction medium. In the citrate reduction, the solution pH was varied by changing the initial HAuCl4/Na3Ct ratio. Consequently, when pH was higher than about 6.2, the very reactive [AuCl3(OH)]- would be converted to less reactive [AuCl2(OH)2]- and [AuCl(OH)3]-.     

A solvolytic C-C cleavage reaction of 6-acetoxycyclohexa-2,4-dienones: mechanistic implications for the intradiol catechol dioxygenases

6-Acetoxycyclohexa-2,4-dienones are found to undergo a rapid reaction in methanol/water under mildly basic conditions to give an acyclic ketoester as the major product for 6-phenyl and 6-methyl substrates. Reaction monitoring by UV spectroscopy indicates the formation of an unsaturated ketone reaction intermediate (lambda(max) 275 nm, R = Ph) and the transient appearance of a highly conjugated species. Reaction of the 6-phenyl substrate (4.95 x 10(-6) s(-1)) is 2-fold faster than the 6-methyl substrate (2.47 x 10(-6) s(-1)). The reaction rate is first order with respect to substrate concentration, and the final step in the reaction is pH-dependent. No cleavage was observed for a substrate lacking an acetyl substituent. A reaction mechanism for C-C cleavage is proposed involving a benzene oxide-oxepin interconversion. The possible relevance to the catalytic mechanism of the intradiol catechol dioxygenases is discussed.     

Mechanism of the hydrolysis of N-aryliminotriphenylphosphoranes

Hydrolysis reactions of N-aryliminotriphenylphosphoranes (I) at varying pH have been investigated kinetically. The reactions produced aniline derivatives and triphenylphosphine oxide as the products. The reactions are first-order, and the Hammett values are -0.29 and -0.63 at pH 3.0 and 8.0, respectively. The reaction rate increased linearly with acetate ion concentration at [AcO^–] < 0.05 M and approached a limiting value at higher base concentration. The plot of logk _t versus pH shows that there are two different regions in the rate profile; one part in which logk _t increases with hydronium ion concentration and the other where k _t is a constant regardless of pH. On the basis of these results, a plausible hydrolysis mechanism is proposed. At pH < 8.0, the reaction proceeds by protonation of the iminophosphorane nitrogen atom, followed by the addition of a water molecule. At pH > 8.0, the proton transfer from water to -nitrogen of the iminophosphorane becomes the rate limiting step.From Kinetika i Kataliz, Vol. 46, No. 1, 2005, pp. 26–34.Original English Text Copyright © 2005 by Pyun, Lee, Kim.This article was submitted by the authors in English.     

Fluorimetric determination of peroxynitrite based on an enzymatic reaction.

A novel fluorimetric method for the determination of peroxynitrite (ONOO-) using hemoglobin (Hb) as a catalyst is described. The method employs the reaction of ONOO with thiamine (TM), a colorless, non-fluorescent reagent in a glycine-NaCl-NaOH buffer solution (pH 12.7), to generate a highly fluorescent product, thiochrome (TC). The fluorescent product was monitored by fluorimetry. A linear calibration graph was obtained over an ONOO- concentration range from 4.95 x 10(-7) mol L(-1) to 2.97 x 10(-5) mol L(-1), with a detection limit of 9.78 x 10(-9) mol L(-1) ONOO-. The relative standard deviation at an ONOO- concentration of 2.11 x 10(-6) mol L(-1) was 4.15% (n = 9).     

Mechanistic investigation on 2-aza-spiro[4,5]decan-3-one formation from 1-(aminomethyl)cyclohexylacetic acid (gabapentin)

The intramolecular cyclization of the amino acid gabapentin has been studied in the pH range 2.24-11.15 at 80 °C in buffered solutions and constant ionic strength, and monitoring the progress of the process by fluorimetric method and proton NMR spectroscopy. From the profile of log k₀ versus pH two different acid-base equilibria are involved. The maximum rate is observed above pH 9.80 and the minimum rate has been measured between pH 5.15 and 6.21. The pK_a1 and pK_a2 have been determined by potentiometric titration to be 3.72 and 9.37, respectively. The buffer effect and the solvent kinetic isotopic effect suggest that the reaction is subject to general acid and general base catalysis. The process is sensitive to the gabapentin concentration (pH 10.45) and the pseudo-first order rate constant decrease with increasing the reagent concentration above 5.50×10⁻² M.     

Interaction of colloidal particles of NH(4)(+)-montmorillonite with activated carbon

The interaction between negative colloidal particles of NH(4)(+)-montmorillonite and particles of activated carbon was studied as a function of particle concentration, pH, and time of contact. The results show that carbon particles act as a support/bridge for clay particles, the type of resulting clay/carbon/clay associations depending on the pH and the clay/carbon ratio in the system. The relation between clay and carbon particles can be described by equations of the Langmuir type. For the same carbon particle concentration in the system, the relation varies from 6820 to 36,100 and is dependent on pH. The interaction coefficients at pH 6.5 correspond to reaction of pseudo-first (k(1)=4.14 x 10(-3) to 1.93 x 10(-3) s(-1)) and pseudo-second order (k(2)=9.36 x 10(-14) to 2.47 x 10(-14) cm(3)s(-1)) for different clay/carbon ratios. By using the Dubinin-Radushkevich equation, the interaction energy (+/-22.42, +/-685.0, and +/-14.63 J mol(-1)) was obtained for three different pH values (5, 6.7, and 7.6), demonstrating that the reaction is mainly physical.     

One-step capillary isoelectric focusing for the separation of the recombinant human immunodeficiency virus envelope glycoprotein glycoforms

One-step capillary isoelectric focusing was investigated as a rapid method to resolve the glycoforms of the heterogeneous recombinant human immunodeficiency virus (HIV) envelope glycoprotein (rgp 160sMN/LAI). The separation was performed in a poly(vinyl alcohol) (PVA) coated capillary using a mixture of ampholyte of narrow and wide pH range. A combination of saccaharose and 3-(cyclohexylamino)-1-propanesulfonic acid was shown to be the most efficient additive to avoid protein precipitation which occurs at a pH close to its pI. Although the calibration curve [isoelectric point (pI) vs. migration times] showed a non-linear relationship, an adequate linearity could be yielded for short pI ranges permitting to exhibit the acidic character of the different glycoforms of the rgp 160s MN/LAI (pI from 4.00 to 4.95). Reproducibility evaluated by comparing the performance of a polyacrylamide and a PVA coated capillary showed that low RSD values were obtained for intra-day (0.5 to 1.9%) and inter-day (1.6 to 7.6%) measurements using the PVA capillary. Moreover, the long term stability of the PVA capillary was demonstrated by measuring the variation of migration times of the protein markers for a long period of use. Finally, this method was able to differentiate the glycoform pattern of two close glycoproteins such as the rgp 160 of two sub-populations of the virus HIV-1.     

{[(n-C_4H_9)_4N]_2Ag_(4.95)Cu(Ⅰ)_(1.05)I_8}_n聚合物的合成和晶体结构

标题化合物由AgI、CuI和(n-C4H9)4NI在DMF溶剂中直接反应而得。晶体结.构通过单晶X-射线衍射法确定,其晶体属于单斜晶系,空间群P21/c,a=9.4342(4),b=15.7638(7),c=18.877(1)??=100.65(1)?V=2758.9(2)3,化学式C32H72N2Ag4.95Cu1.05I8,Mr=2100.79,Dc=2.529g/cm3,Z=2,(MoK?=6.618cm-1,F(000)=1930。在1.70≤?≤27.46范围内收集到22473个衍射点,独立衍射点6175(Rint=0.0753),其中可观测衍射点4158个(I>2(I))。晶体结构用直接法和差傅立叶合成方法找出初步结构模型,再分别对Ag和Cu的坐标进行修正,最后用全矩阵最小二乘法对266个变量进行精修,最终偏离因子R=0.0476,wR=0.1315。标题化合物的结构由[(M6I8)-]阴离子链和链间的n-Bu4N+阳离子组成。     

二元酸溶解度对二元醇与一些芳族二元酸的聚酯化反应动力学的影响

一些芳族二元酸与二元醇的聚酯化反应过程包含非均相和均相两个反应阶段,针对这一反应特性,提出了二元酸在反应混合物中溶解度及反应清晰点的两个计算方程,并据此将Ｃｈｅｎ和Ｗｕ提出的自催化聚酯化反应的动力学模型修改为两个分别适用于非均相和均相反应阶段的速率方程．将这两个速率方程应用于双（对 羧苯基）苯基氧化膦与乙二醇的恒温自催化聚酯化反应,计算值与实验数据相一致,并计算得到了该反应的速率常数和活化能     

Mechanistic studies on oxidation of L-ascorbic acid by an oxo-bridged diiron complex in aqueous acidic media

[Fe2(micro-O)(phen)4(H2O)2]4+ (1) (Fig. 1, phen = 1,10-phenanthroline) equilibrates with [Fe2(micro-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(micro-O)(phen)4(OH)2]2+ (3) in aqueous solution in the presence of excess phen, where no phen-releasing equilibria from 1, 2 and 3 exist. 1 quantitatively oxidizes ascorbic acid (H2A) to dehydroascorbic acid (A) in the pH range 3.00-5.50 in the presence of excess phen, which buffers the reaction within 0.05 pH units and ensures complete formation of end iron product ferroin, [Fe(phen)3]2+. The reactive species are 1, 2 and HA- and the reaction proceeds through an initial 1 : 1 inner-sphere adduct formation between 1 and 2 with HA-, followed by a rate limiting outer-sphere one electron one proton (electroprotic) transfer from a second HA- to the ascorbate-unbound iron(III).     

Kinetics and mechanism of formation of the platinum-thallium bond: the [(CN)(5)Pt-Tl(CN)(3)](3)(-) complex

Formation kinetics of the metal-metal bonded [(CN)(5)PtTl(CN)(3)](3)(-) complex from Pt(CN)(4)(2)(-) and Tl(CN)(4)(-) has been studied in the pH range of 5-10, using standard mix-and-measure spectrophotometric technique at pH 5-8 and stopped-flow method at pH > 8. The overall order of the reaction, Pt(CN)(4)(2)(-) + Tl(CN)(4)(-) right harpoon over left harpoon [(CN)(5)PtTl(CN)(3)](3)(-), is 2 in the slightly acidic region and 3 in the alkaline region, which means first order for the two reactants in both cases and also for CN(-) at high pH. The two-term rate law corresponds to two different pathways via the Tl(CN)(3) and Tl(CN)(4)(-) complexes in acidic and alkaline solution, respectively. The two complexes are in fast equilibrium, and their actual concentration ratio is controlled by the concentration of free cyanide ion. The following expression was derived for the pseudo-first-order rate constant of the overall reaction: k(obs) = (k(1)(a)[Tl(CN)(4)(-) + (k(1)(a)/K(f)))(1/(1 + K(p)[H(+)]))[CN(-)](free) + k(1)(b)[Tl(CN)(4)(-)] + (k(1)(b)/K(f)), where k(1)(a) and k(1)(b) are the forward rate constants for the alkaline and slightly acidic paths, K(f) is the stability constant of [(CN)(5)PtTl(CN)(3)](3)(-), and K(p) is the protonation constant of cyanide ion. k(1)(a) = 143 +/- 13 M(-)(2) s(-)(1), k(1)(b) = 0.056 +/- 0.004 M(-)(1) s(-)(1), K(f) = 250 +/- 54 M(-)(1), and log K(p) = 9.15 +/- 0.05 (I = 1 M NaClO(4), T = 298 K). Two possible mechanisms were postulated for the overall reaction in both pH regions, which include a metal-metal bond formation step and the coordination of the axial cyanide ion to the platinum center. The alternative mechanisms are different in the sequence of these steps.     

Interference in the kinetic determination of ascorbic acid by sulphide and sulphite

The reduction of 2,6-dichlorophenolindophenol (DCPI) by sulphides and sulphites has been studied kinetically by the stopped-flow technique. The reaction is first-order with respect to each of the reactants. From the distribution diagrams for the species DH(+)(2), DH and D(-) for DCPI and H(2)Q, HQ(-) and Q(2-) for sulphides or sulphites, a mechanism is proposed which suggests partial reactions of all possible combinations of the reacting species at any pH. An equation for calculation of the second-order reaction rate constants k at any pH is derived, which gives k as a function of [H(+)], the partial reaction rate constants and the dissociation constants of DCPI and H(2)S or H(2)SO(3). Values of the overall reaction rate constants over a wide pH-range have been determined, together with values of k for all possible partial reactions. For particular pH-values the second-order reaction rate constant was determined by four different methods. Mean values of k = 251 +/- 1 and 240 +/- 1 l.mole(-1).sec(-1) were obtained for pH 3.15 and 4.17, respectively, for the DCPI-Na(2)S reaction and k = 137 +/- 1, 127 +/- 1 and 136 +/- 1 l.mole(-1).sec(-1) for pH 2.02, 4.25 and 5.10, respectively, for the DCPI-Na(2)SO(3) reaction. From the slopes of the linear Arrhenius plots activation energies of 6.6 +/- 0.2 and 4.0 +/- 0.1 kcal/mole for the DCPI-Na(2)S and DCPI-Na(2)SO(3) reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.     

Acid dependence of the Pu(V) disproportionation reaction

The disproportionation of pentavalent plutonium is traditionally represented by two inconsistent equations. The inconsistency puts the widely-accepted, fourth-power hydrogen-ion dependence of the reaction into question. The balanced reaction equation suggests that both the stoichiometry and the acid-dependence of Pu(V) disproportionation depend on the pH of the plutonium solution.Los Alamos National Laboratory is operated for the U.S. Department of Energy by the University of California under Contract No. W-7405-ENG-36.     

New equations describing the combined effect of pH and organic modifier concentration on the retention in reversed-phase liquid chromatography

Six equations that express the combined effect of mobile phase pH and organic modifier content on sample retention in reversed-phase liquid chromatography (RPLC) are developed based on either the adsorption or the partition model for retention. The equations are tested against five retention data sets taken from literature. In the tests two pH scales are used, w(w)pH and s(s)pH. It is shown that a new seven-parameter equation works more satisfactorily, because it exhibits good numerical behavior, gives low values of the sum of squares of residuals and represents the experimental retention surfaces successfully. In addition, the danger of overfitting, which leads to the prediction of physically meaningless retention surfaces, is minimized by using the proposed new seven-parameter equation. Finally, the possibility of obtaining reliable pK values of weak acids or bases chromatographically by means of the derived equations is also considered and discussed.