Pyridine-based buffers

Pyridine was used in the beginning of the development of the method. The reaction was slow and the endpoint unstable because of weak basicity of pyridine. The pyridine system buffers at about pH 4. A stronger base, imidazole, has been used to replace pyridine since it gives a faster response and has the advantages of lower toxicity and decreased odor. The optimal pH range for the SO2 imidazole buffer is at pH 6. It is important that the pH of the Karl Fisher reaction be maintained within the range 5 to 7. Outside this recommended pH range, the endpoint may not be reached. 

A series of Sephadex columns have been widely used in separation of intact or truncated LPS. The selection of the column is based on the repeating constitution of O-antigen in LPS. The intact LPS can be separated by Sephadex G-200 and eluted with detergent containing materials (Morrison and Leive, 1975 Peterson and McGroarty, 1985 Rivera et al., 1988). Sephadex G-200 was also used to separate the alkaline hydrolyzed LPS which can be eluted with pyri-dine/0.05 M acetate buffer (Chester and Meadow, 1975). Sephadex G-25, 50, and 75 have been applied to fractionate acetic-acid hydrolyzed LPS with distilled water or pyridine acetate buffer elution (Byrd and Kadis, 1989 Carlson, 1984 Koval and Meadow, 1977 Kropinski et al., 1982 Lacroix et al., 1993 Prehm et al., 1975 Temple et al., 1986). Biogel P6 has been used to fractionate the partially degraded LPS with 1% acetic acid, eluted with pyridine/0.05 M acetate buffer (Koval and Meadow, 1977). Sepharose 4B has been used to fractionate LPS from E. coli 0111 B4, eluted with 0.12 M Tris buffer (Morrison and Leive, 1975). 

Weak bases should be adsorbed on a strong cation exchanger. Separation from strong bases is achieved by elution with a weak base, buffered one or two pH units above the compound s pKa. Aqueous pyridine is often used for this purpose. If the compound is unstable to alkaline conditions, it may not be possible to separate it from strong bases by an ion-exchange process. 

Similar observations of base catalysis have been used to invoke the ElcB mechanism for elimination from 4,4-dicyano-3-p-nitrophenyl-1 -phenylbutan-1 -one in neutral and acidic methanol (34) , and l,l,l,3-tetranitro-2-phenylpro-pane in methanol in the presence of hydrochloric acid and pyridine-pyridine hydrochloride buffers (36) . In the former reaction, an example of a reverse Michael addition, the carbanion intermediate with the electron pair alpha to the carbonyl rather than in the gamma position is favoured, as the methyl isomer (35) eliminates more rapidly than the parent compound. 

Assays for melanocyte-stimulating hormone were carried out by Mr. S. Kulovich using the in vitro frog skin assay (1). Subcutaneous assay for adrenocorticotropic activity (2), based on ascorbic acid depletion in hypophysectomized rats, was performed by Dr, J.D. Fisher of the Armour Laboratories. Acid hydrolysates (constant boiling HCl, deaerated, 22 hours, 110°) of these fractions were characterized by automatic amino acid analysis with a Spinco model 120B analyzer. The number of tryptophan residues in the intact peptide was estimated from the ultraviolet absorption curves made with a Cary model 15 spectrophotometer. Electrophoresis was carried out on Whatman paper No. 1 with pyridine-acetate buffer, pH 6.5, and 4 molar urea for 3 hours at 26 volts per cm. Peptides were detected with bromphenol blue(3). 

These rate constants are for the hydrolysis of cinnamic anhydride in carbonate buffer, pH 8.45, total buffer concentration 0.024 M, in the presence of the catalysts pyridine, A -methylimidazole (NMIM), or 4-dimethylaminopyridine (DMAP). In the absence of added catalyst, but the presence of buffer, the rate constant was 0.005 24 s . You may assume that only the conjugate base form of each catalyst is catalytically effective. Calculate the catalytic rate constant for the three catalysts. What is the catalytic power of NMIM and of DMAP relative to pyridine ... 

To test the quality of some synthetic dyes according to standardized procedures, a screening is recommended based on TLC analysis on silica plates 60 F 254 using elutions with an ethyl acetate pyridine water 25 25 20 (v v v) mixture. To determine purity and secondary dyes (components or by-products of a dye that are not allowed to be present), successive TLC separations are recommended or, for more accurate answers, HPLC-DAD using RP-18 columns and eluents like acetonitrile and phosphate buffer."... 

All these electrolytes are neutral in Bronsted acid-base properties. Although rather exceptional, an acid, a base, or a pH buffer may be added to the supporting electrolyte of neutral salts. The acid-base system to be selected depends on the purpose of the measurement. We often use trifluoromethanesulfonic acid (CF3S03F1) as a strong acid acetic acid, benzoic acid, or phenol as a weak acid an amine or pyridine as a weak base and tetraalkylammonium hydroxide (ILtNOH) as a strong base. Examples of buffer systems are the mixtures of picric acid and its R4N-salt and amines and their PlCl04-salts. Here, we should note that the acid-base reactions in aprotic solvents considerably differ from those in water, as discussed in Chapter 3. 

Fiqure 2.4 Determination of the rate constants for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The first-order rate constants for the hydrolysis are plotted against various concentrations of the base. The slope of the linear plot is the second-order rate constant (k2). The intercept at zero buffer concentration is the "spontaneous hydrolysis rate constant for the particular pH. A plot of the spontaneous rate constants against pH gives the rate constants for the H+ and OH" catalysis. It is seen that pyridine is a more effective catalyst than the weaker base acetate ion. [From W. P. Jencks and J. Carriuolo, J. Am. Chem. Soc. 83,1743 (1961).]... 

This study has defined a few of the optimum conditions, but, the effects of other buffers and of the total concentrations of reactants need to be investigated. In addition, the influence of other base catalysts merits further study. The mechanisms by which pyridine enhances the yield is not understood, but the use of isotopically labeled pyridine might provide evidence as to whether or not pyridine is... 

Re has recently come to the forefront in liquid phase oxidation catalysis, mainly as a result of the discovery of the catalytic properties of the alkyl compound CH3Re03 [methyltrioxorhenium (MTO)]. MTO forms mono-and diperoxo adducts with H2O2 these species are capable of transferring an oxygen atom to almost any nucleophile, including olefins, allylic alcohols, sulfur compounds, amides, and halide ions (9). Moreover, MTO catalysis can be accelerated by coordination of N ligands such as pyridine (379-381). An additional effect of such bases is that they buffer the strong Lewis acidity of MTO in aqueous solutions and therefore protect epoxides, for example. 

HF pyridine complex in methanol or THF is commonly used for the removal of TBS ethers. With acid- and base-sensitive substrates, the reaction can be buffered with additional pyridine.65 At a late stage in the synthesis of Calyculin A, a potent protein phosphatase inhibitor, a primary TBS ether was cleaved selectively in the presence of three secondary TBS ethers by using HF pyridine complex in THF-pyridine [Scheme 4.43].66... 

In order to be able to predict the retention behavior of peptides of different composition, of peptides of the same composition but different sequence (positional isomers), and of diastereoisomeric peptides, a knowledge of the incremental contribution of each amino acid to the overall contact area term is required not only for each well-defined stationary phase but also for each mobile-phase condition. Group retention coefficient summation approaches based on the assumption that selectivity differences can be ascribed predominantly to amino acid sequence differences, have been developed by Meek (46a, 52b) and Su et al. (45a). These treatments have subsequently been applied to a number of different elution systems (52c-52e). A comparative analysis of the different amino acid group contribution coefficients derived for phosphate, perchlorate, pyridine/acetate, trifluoroacetate, and bicarbonate buffer systems has been reported (52f). 

Besides pyridine derivatives and bases u as buffers (mentioned ateve), some other comi unds catalyze azo coupling reactions. The mechanism of catalysis by urea was investigated by Gloor and Zollinger for the second substitution of a... 

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