Showing papers on "Reagent published in 1969"

Location of Energy Barriers. I. Effect on the Dynamics of Reactions A + BC

Abstract: The dynamics of exchange reactions A+BC→AB+C have been examined on two types of potential‐energy hypersurfaces that differed in the location of the energy barrier along the reaction coordinate. On “surface I” the barrier was in the entry valley of the energy surface, along the approach coordinate. On “surface II” the barrier was in the exit valley of the energy surface, along the retreat coordinate. The classical barrier height was Ec = 7.0 kcal mole−1 on both surfaces, and was displaced from the corner of the energy surface by the same amount; on surface I, r1‡ = 1.20 A, r2‡ = 0.80 A; on surface II, r1‡ = 0.80 A, r2‡ = 1.20 A (r1 ≡ rAB, r2 ≡ rBC, and the superscript ‡ refers to the location of the crest of the barrier). Three‐dimensional (3D) classical trajectory calculations were performed for the mass combination mA = mB = mC at several reagent energies. The reagent energy took the form of translation, vibration or an equilibrium distribution of the two. The main findings were that translation was markedly more effective than vibration in promoting reaction on surface I, and vibration markedly more effective than translation in promoting reaction on surface II. The total reactive cross section with the entire reagent energy vested in translation was symbolized ST, with the reagent energy (but for 1.5 kcal) in vibration, SV, and with an equilibrium distribution over reagent translation and vibration, Seq. On surface I ST ≫ SV: on surface II SV ≫ ST. Close to the threshold for ST on surface I, ST / Seq ∼ 10; close to the threshold for SV, on surface II, SV / Seq ∼ 10. At high reagent energies (2 × threshold) on surface I ST / Seq fell to 2, whereas on surface II SV / Seq increased to extremely large values. Product energy and angular distributions were recorded for two reagent energies. On surface I with low translational energy in the reagents a major part of the available energy appeared as vibration in the molecular product. At higher collision energy this fraction decreased. On surface II with low vibrational energy in the reagents only a small part of the available energy appeared as vibration in the product. At higher vibrational energy this fraction increased. The product angular distribution at low reagent translational energy on surfaces I and II corresponded to backward‐peaked scattering of the molecular product. At increased reagent energy on both surfaces the distribution shifted forward (this is a novel phenomenon in the case of increased reagent vibration; surface II).

The reaction of polypropylene with maleic anhydride

Abstract: An addition reaction of maleic anhydride with polypropylene takes place in the presence of radical reagents or sunlight. The initial rate of the reaction was proportional to the concentration of polypropylene and maleic anhydride, and one-half power of the concentration of the radical reagents. The increase in the temperature from 80 to 120°C increased the rate of the reaction and di-cumyl peroxide was effective as a radical reagent for this reaction. Ionic crosslinked rubber-like polymers were obtained from the reaction of maleic polypropylene with some alkali metal compounds. They showed the characteristic absorption band due to COO⊖ in their infrared spectra.

Some characteristics of the cold hydrolysis technique for staining plant tissues by the feulgen reaction

Abstract: Hydrolysis of Vicia faba primary root meristems in 5 NHC1 at 20°C, followed by reaction with the Feulgen reagent, was investigated using microdensitometry. Some differences in response were observed with prophase nuclei fixed in different solutions, but in general this type of hydrolysis leads to a plateau for stainability. After plateau hydrolysis most dye binding is completed within 15 min. For 3:1 alcohol-acetic fixatives, fixation times between 1 day and 13 weeks do not affect plateau stainability but shorter (½ hr) and longer (21 weeks) times give slightly lower values for dye content. Alcohol-acetic fixatives for short periods of time give the least variable results. A comparison of 5 N HCl hydrolysis at 20°C and 30°C shows that at the higher temperature the plateau is brief but not different in dye binding. However, the peak values for 1 N HCl hydrolysis at 60°C are significantly lower by 20%.

Serum Cholesterol Assay Using a Stable Liebermann-Burchard Reagent

Abstract: A single stable Liebermann-Burchard reagent was prepared and studied for the rapid and accurate determination of total cholesterol in serum. The preparation, stability, and developed spectral character of the proposed reagent were assessed. Reproducibility, recovery, and interference studies on pure standard and patient sera were performed using the Abell-Kendall method as reference. The influence of water, protein, bilirubin, hemoglobin, and other serum constituents was evaluated.

An automated procedure for the determination of boron in plant tissue

Abstract: A routine procedure in which the Technicon Auto Analyzer is used for the determination of boron in plant tissue with azomethine H is described. The preparation of the reagent as the condensation product of H-acid and salicylaldehyde is given. The reagent is highly sensitive to boron, and only copper, iron and aluminium interfere. These interference effects are eliminated with an EDTA solution. The procedure is performed in aqueous medium thus simplifying its adaptation to an automated system and its use for routine determination.

A substrate-induced conformation change in the reaction of alkaline phosphatase from Escherichia coli

Abstract: 1. Benzyl phosphonates were prepared and their potentialities as chromophoric reagents for the exploration of the substrate-binding site of Escherichia coli alkaline phosphatase were investigated. 4-Nitrobenzylphosphonate is a competitive inhibitor of the enzyme. 2-Hydroxy-5-nitrobenzylphosphonate changes its spectrum on binding to the enzyme. This spectral change is reversed when the phosphonate is displaced from the enzyme by substrate. 2. The kinetics of the reaction of 2-hydroxy-5-nitrophenylphosphonate were studied by the stopped-flow and the temperature-jump techniques. It was found that the combination of the phosphonate with the enzyme occurred in two successive and reversible steps: enzyme-phosphonate complex-formation followed by rearrangement of the complex. The spectral change is associated with the rearrangement. At pH8 in 1m-sodium chloride at 22 degrees the rate constant is 167sec.(-1) for the rearrangement of the initially formed binary complex and is 18sec.(-1) for the reverse process. 3. It has previously been proposed that the reactions of phosphatase with its substrates include a distinct step between enzyme-substrate combination and chemical catalysis. The rate constant involved could be predicted but not measured from experiments with substrates. The value for the rate constant measured from the rate of the enzyme-phosphonate rearrangement is in excellent agreement with the predicted value. A model for the reaction mechanism is proposed that includes a conformation change in response to phosphate ester binding before phosphate transfer from substrate to enzyme.

σ-Alkyl and -Aryl Derivatives of Transition Metals

Abstract: Publisher Summary This chapter describes σ-alkyl and -aryl derivatives of transition metals. By far the most widely applied method for the formation of metal-carbon σ-bonds is the reaction of a metal-X compound where X is a halide or some other leaving group, with an anionic alkylating agent. The degree of alkylation often depends on the activity of the alkylating reagent. By varying the metal complex anion and RX, a wide variety of metal compounds containing transition metal-carbon σ-bonds have been prepared directly by this reaction. In addition, aryl metal derivatives prepared by this method may be decarbonylated to give alkyl and aryl metal complexes. A semiquantitative estimation of the relative nucleophilicities of a series of transition metal anions has been made by measuring the rates of their reactions with alkyl halides. Transition metal-carbon a-bond formation by this method most commonly involves the insertion of some carbon-containing species into a metal-hydrogen bond.

The chemical modification of nucleic acids.

Abstract: Publisher Summary This chapter describes chemical modification of nucleic acids. Modification of the units of polymer chains by reaction with chemical agents is one of the promising methods for studying the structure and function of biopolymers, particularly polynucleotides. The method can be applied at all stages of nucleic acid investigation—isolation of individual polynucleotide species, studies of primary and secondary structure, studies of nucleoprotein complexes, and studies of polynucleotide functional specificity. Highly selective single-stage reactions leading to stable modified units of known structure and properties are the most convenient for chemical modification. Usually the action of a chemical reagent results in simultaneous; modifications of several types of nucleoside units may proceed at different rates. The reactions are usually multistage and yield mixtures of products even with monomer models. Even more complicated are the reactions with polynucleotides, which must be regarded as parallel-consecutive ones. Therefore, elaboration of a method of chemical modification should always involve a kinetic study of the reactivity of different nucleosides under the conditions (pH, temperature, and concentration) used subsequently for polymer modification.

Catalysis and Inhibition in Solutions of Synthetic Polymers and in Micellar Solutions

Abstract: Publisher Summary This chapter describes some characteristics of enzymic catalysis, reaction rates in solutions of long-chain macromolecules, catalysis and inhibition in micellar solutions, and critique of the use of synthetic polymers and micelles as enzyme models. The catalysis of various reactions in the solutions exhibits a number of characteristics normally associated with heterogeneous catalysis. An inhibition of the catalytic effect may result, when the reagent has to compete with an inert species for the available catalytic sites. The analogy between the high reactivity of the enzyme-substrate complex and the reactivity of substrates bound to polymers or micelles is even less satisfactory. Kinetic effects based on long-range electrostatic effects, particularly the concentration of catalytic hydrogen or hydroxyl ions in the vicinity of a polyion or the micellar surface, have no counterpart in enzyme chemistry. An inhibition of the catalytic effect may result, when the reagent has to compete with an inert species for the available catalytic sites.

Fluorometric Determination of Histidine

Abstract: An accurate and precise fluorometric procedure was developed for the determination of histidine based on the observation that o-phthalaldehyde forms a fluorophor with the analogous compound, histamine. The precision of the method was improved, and the fluorophor was stabilized for days by the addition of a phosphoric acid reagent.