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Biomedical subjects

K A Connors

Publications and source records attributed to K A Connors.

At least 37 records · Page 2Linked to original sources
PubMed1984

Improved competitive indicator methods for the study of alpha-cyclodextrin complexes.

The competitive indicator method for studying molecular complexes is extended to systems forming 1:1 (SL) and 1:2 (SL2) complexes of substrate (S) and ligand (L). A modification is described for slightly soluble substrates, in which the presence of solid substrate establishes a constant concentration of uncomplexed substrate. These methods are applied to complexes of alpha-cyclodextrin with some aromatic substrates, with methyl orange as the indicator in acid solution; nitrazine yellow is introduced as an indicator for these studies in basic solution.

Chemical Phenomena↗
PubMed1983

Kinetic study and analytical application of the hexadecyltrimethylammonium bromide-catalyzed reaction of 1-fluoro-2,4-dinitrobenzene with amines.

Arylation of amines by reaction with 1-fluoro-2,4-dinitrobenzene is catalyzed by micelles of cetrimonium bromide. This catalysis has been exploited to reduce the analysis time in the spectrophotometric determination of amines as their dinitrophenyl derivatives. The kinetics of the catalysis were studied for the five amines: alanine, phenylalanine, aniline, 4-methylaniline, and 4-methoxyaniline. The dependence of rate constant on surfactant concentration can be quantitatively accounted for by Berezin's model, in which uptake of the amine and the 1-fluoro-2,4-dinitrobenzene by the micelle is described as a partitioning phenomenon for both species. An alternative model is developed in which one reactant partitions into the micellar phase and the other binds to the micelle with 1:1 stoichiometry; the two models are formally equivalent. Intrinsic catalytic rate constants and binding constants were evaluated. About one-third to one-half of the maximum observed micellar acceleration is attributed to a true micellar catalysis, the remainder being ascribed to an increase in local reactant concentrations in the micelle.

Amines↗
PubMed1983

Kinetics and mechanism of hydroxy compound cinnamoylation in acetonitrile catalyzed by N-methylimidazole and 4-dimethylaminopyridine.

The kinetics of reaction of the acylating agents trans-cinnamic anhydride and trans-cinnamoyl chloride with the hydroxy compounds n-propyl alcohol and water in the presence of N-methylimidazole and 4-dimethylaminopyridine were studied spectrophotometrically in acetonitrile solution at 25 degrees. The acid chloride reacted via the intermediate formation of the N-acyl catalyst, which underwent general base-catalyzed reaction with the hydroxy compound. The anhydride did not form the N-acyl intermediate, but instead underwent direct general base catalysis. In the presence of water, all systems formed the N-acyl intermediate. The mechanistic route followed by the system was determined by the nucleophilicity of the catalyst, the ability of the leaving group, and the polarity of the solvent.

1-Propanol↗
PubMed1983

Solvent effects on the cinnamoylation of n-propyl alcohol catalyzed by N-methylimidazole and 4-dimethylaminopyridine.

The kinetics of reaction of trans-cinnamic anhydride or trans-cinnamoyl chloride with n-propyl alcohol, catalyzed by N-methylimidazole or 4-dimethylaminopyridine, were studied spectrophotometrically at 25 degrees in methyl ethyl ketone, ethylene dichloride, methylene chloride, and toluene. The acid chloride reacted in all solvents via the intermediate formation of the N-acyl catalyst, which underwent reaction with the alcohol catalyzed by another molecule of the base. The anhydride did not form the intermediate in any of the solvents, but underwent direct general base catalysis. The rate of the anhydride reactions was not sensitive to solvent polarity, whereas the rate of the chloride reactions tended to increase as the solvent polarity decreased. A kinetic analysis is given of the effect of ion-pair formation on the kinetics of acyl transfer in systems where the charged N-acyl catalyst intermediate is formed.

1-Propanol↗
PubMed1983

Stability constants for complex formation between alpha-cyclodextrin and some amines.

Complex formation of alpha-cyclodextrin with 15 amines (including seven 4-substituted anilines) was studied by the potentiometric method, supplemented by direct UV spectrophotometry and a competitive indicator spectrophotometric method. The data were analyzed in terms of 1:1 and 1:2 complexes (amine-cyclodextrin ratios) and the stability constants K11a, K12a, K11b, and K12b were evaluated; the subscripts indicate the stoichiometry and conjugate acid-base form. For all amines K11b was greater than K11a and K12a was 0. On the basis of the relationship of complex stability to amine structure, it was concluded that the primary binding site in anilines is the 4-substituent.

Amines↗
PubMed1983

Complex formation between alpha-cyclodextrin and 4-substituted phenols studied by potentiometric and competitive spectrophotometric methods.

Stability constants for complex formation between alpha-cyclodextrin and the conjugate acid and base forms of nine phenols were measured in aqueous solution at 25 degrees. The potentiometric method, in which the apparent acid dissociation constant of the phenol is measured as a function of cyclodextrin concentration, was supplemented by a modified version of a competitive spectrophotometric methyl orange method. For all phenols, the 1:1 stability constant for the conjugate base form (K11b) was larger than K11a for the conjugate acid form. Finite K12b values were found for phenols whose 4-substituents could tolerate a positive charge by electron delocalization. Complex stability, as measured by K11a and K11b, increases with electron density and polarizability at the 4-substituent. It is concluded that the 4-substituent is the sole or predominant site of binding for both the conjugate acid and base forms of the phenols. The general result that K11b is greater than K11a for any phenol is accounted for by relative delocalization of charge in the anion and neutral species.

Binding Sites↗
PubMed1982

Potentiometric study of molecular complexes of weak acids and bases applied to complexes of alpha-cyclodextrin with para-substituted benzoic acids.

The theory of the potentiometric methods of studying complexes of ionizable substrates was developed, nd graphical techniques are described for obtaining stability constant estimations from the data. The method described is for a system in which the conjugate acid and base forms of the substrate (S), are capable of forming 1:1 (SL) and 1:2 (SL2) complexes with the ligand (L). It was applied to complexes of alpha-cyclodextrin (cyclohexaamylose) with 10 para-substituted benzoic acid derivatives. Letting K11a and K12a be stability constants for the conjugate acid forms of the substrates, and K11b, K12b for the conjugate base forms, it was found that K12b is zero for all substrates, K12a is zero for seven of the substrates, and K11a greater than K11b in every case. Hammett plots yielded p11a and p11b values of -0.31 and 0.77, respectively, which was interpreted to mean that K11a mainly represents binding at the carboxylic acid site, and K11b describes binding at the site of the para-substituent. This model of the complexing suggests that K12a represents binding at the para-substituent, and therefore K12a should vary roughly with substituent as K11b does; this trend was observed.

Acids↗
PubMed1982

Kinetics and mechanism of hydroxy group acetylations catalyzed by N-methylimidazole.

The kinetics of acetylation of alcohols by acetyl chloride and acetic anhydride, with N-methylimidazole as the catalyst, were studied in acetonitrile solution at 25 degrees; some measurements were also made with 4-dimethylaminopyridine as the catalyst. The acetic anhydride-N-methylimidazole system proceeds entirely by a general base catalysis, whereas the acetyl chloride-N-methylimidazole system reacts entirely via a nucleophilic route, with the intermediate formation of the N-acylated catalyst. The reaction of this intermediate with the alcohol is general base catalyzed. The acetyl chloride-4-dimethylaminopyridine system also reacts via the nucleophilic route. In the acetic anhydride-4-dimethylaminopyridine system a small fraction of the intermediate was detected. The acetic anhydride-N-methylimidazole system was studied in n-propanol-acetonitrile solvent mixtures; no spectral evidence for intermediate formation was seen. However, the hydrolysis reaction in acetic anhydride-N-methylimidazole, studied over a wide range of water-acetonitrile mixtures, revealed a change in mechanism from general base in dry acetonitrile to a solely nucleophilic route at high water concentrations.

Acetic Anhydrides↗
PubMed1981

Kinetics of trans-cinnamic anhydride reactions catalyzed by pyridine, 4-dimethylaminopyridine, and N-methylimidazole.

The kinetics of hydrolysis of trans-cinnamic anhydride and of its reactions with hydroxy compounds were studied in the presence of pyridine, 4-dimethylaminopyridine, and N-methylimidazole as catalysts. The absolute rates of the catalyzed hydrolysis decreased with increasing acetonitrile content (decreasing solvent polarity), but the catalytic efficiency of N-methylimidazole and 4-dimethylaminopyridine relative to pyridine increased as the solvent polarity decreased. The relative catalytic rates for the cinnamoylation of n-propanol in acetonitrile were 1:259:16,000 for pyridine, N-methylimidazole, and 4-dimethylaminopyridine, respectively.

Catalysis↗
PubMed1980

trans-Cinnamic acid--alpha-cyclodextrin system as studied by solubility, spectral, and potentiometric techniques.

Complex formation in aqueous solutions of trans-cinnamic acid or trans-cinnamate ion (the substrate, S) and alpha-cyclodextrin (the ligand, L) can be described quantiatively as the 1:1 and 1:2 complexes, SL and SL2. The solubility, spectral, and potentiometric data over a wide range of ligand concentrations yielded consistent estimates of the complex association constants. For cinnamic acid at 25 degrees K11 = 2260 M-1, delta H degree 11 = 9.3 kcal/mole, and delta S degree 11 = -8 e.u.; and K12 = 60 M-1, delta H degree 12 = -12 kcal/mole, and delta S degree 12 = -26 e.u. For cinnamate ion at 25 degrees, K11 = 110 M-1, delta H degree 11 = -1.9 kcal/mole, and delta S degree 11 = +11 e.u.; and K12 = 15 M-1, delta H degree 12 = 9 kcal/mole, and delta S degree 12 = -15 e.u. (all entrophy changes are unitary quantities). Thermodynamic cycles for the complexes, using solubility data, reveal that complex formation in the solid phase is thermodynamically spontaneous but that complex stability is greater in ageous solution than in the solid phase.

Chemical Phenomena↗
PubMed1980

Stoichiometric model of alpha-cyclodextrin complex formation.

The solubility, spectral, and kinetic methods were used to study complexing between alpha-cyclodextrin (ligand, L) and 3,5-dimethoxycinnamic acid, benzalacetone, and methyl cinnamate (substrates, S). In aqueous solution at 25 degrees and with an ionic strength of 0.01 M, the following stability constants were found (K11 for SL and K12 for SL2): 3,5-dimethoxycinnamic acid, K11=1965 M-1 and K12=0; benzalacetone, K11=105 M-1 and K12=15 M-1; and methyl cinnamate, K11=1200 M-1 and K12=50 M-1. A model of complex formation is proposed that can account for the observed stoichiometry and that yields stability estimates for two isomeric 1:1 complexes in the systems in which a 1:2 complex forms. For cinnamic acid, benzalacetone, and methyl cinnamate, stability constants are inversely correlated with the substrate dipole moment.

Butanones↗
PubMed1976

Effect of cycloamyloses on apparent dissociation constants of carboxylic acids and phenols: equilibrium analytical selectivity induced by complex formation.

Apparent dissociation constants of organic acids were determined by potentiometric titration in the presence of cyclohexaamylose or cycloheptaamylose. The quantity deltapKa' = pKa' (cycloamylose)-pKa (water) was positive or zero for all carboxylic acids studied and negative or zero for all phenols. The term delta pKa' can be related to the cycloamylose concentration, K11alpha, and K11b, where K11a and K11b are 1:1 stability constants for complexes of the acid and the anion, respectively. From the dependen1a and K11b can be obtained. If deltapKa' not equal to 0, then K11a not equal to K11b; for carboxylic acids, K11a greater than or equal to K11b; for phenols, K11b greater than or equal to K11a. Because of variable pKa' shifts, it is possible to carry out differentiating titrations of some acid mixtures in cycloamylose solutions, whereas the same acids cannot be differentiated in water. If an acid is weakened by cycloamylose, its conjugate base is strengthened, and some carboxylate salts can be readily titrated in the presence of a cycloamylose.

Amylose↗