NMR Studies of electrostatic fields around charged monosaccharides and related molecules

A new and simple technique has been developed to experimentally measure electrostatic fields around charged molecules in solution. Using a series of three charged or neutral [4-hydroxy (or carboxy or amino)-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)] spin probes, the apparent rate constants (k₊, k_o, and k-) for a particular nucleus can be calculated as the slope of 1/T₁ (i.e., the spin-lattice relaxation rate) vs. increasing radical concentration. Protons residing within a particular charged region suffer a decrease in their apparent rate constants due to repulsive interactions when the nitroxide spin probe bears the same charge-type as that of the local electrostatic environment. Accordingly, electrostatic forces of attraction increase the apparent rate constants for these same protons when the nitroxide spin probe bears the opposite charge to that present in the local electrostatic field. The apparent rate constants of the neutral nitroxide spin probe are an indication of the solvent accessibility to that particular site in the molecule being investigated. Ratios of these apparent relaxation rate constants (k₊/k- > 1 or k-/k₊ > 1, etc.) thus provide a measurement of the sign and magnitude of the local electrostatic fields surrounding a particular proton in the target molecule. Three charged hexosamines [D-glucosamine·HCl ( 4 ), D-galactosamine·HCl ( 5 ), and D-mannosamine·HCl ( 6 )], a glucuronide metabolite of paracetamol [p-acetamidophenyl-ß-D-glucuronide sodium salt ( 7 )], and a mononucleotide [thymidine-3′-phosphate sodium salt ( 8 )], were investigated in this study. Positively charged environments around protons in the hexosamines are clearly seen by k_o/k₊ ratios greater than unity, while negatively charged surroundings about protons in the glucuronide and nucleotide are markedly evident from larger than unity k₊/k- and k_o/k- ratios. In the case of small charged target molecules such as 6–8 , the electrostatic field seems to envelop the entire molecule. In general, a lack of statistically significant differences in the k_attraction/k_repulsion or k_neutral/k_repulsion ratios as a function of proton distance from the charged functionality was observed. This finding (with the possible exception of the methyl protons in 7 ) suggests that the present version of this method is not sensitive enough to map subtle differences in charged environments around small and conformationally flexible or dynamic target molecules.     

Fluorescence study on drying of ι-carrageenan gels at different temperatures prepared with various CaCl<Subscript>2</Subscript> content

The influence of temperature and salt content on drying was investigated by using steady-state fluorescence (SSF) technique. Supporting gravimetric and volumetric measurements were also carried out during drying of gels at various temperatures. ι-Carrageenan gels were prepared with various CaCl₂ content. Pyranine was introduced as a fluorescence probe during gel preparation. Apparent fluorescence intensity, I, was measured during in situ drying process at each temperature and it was observed that fluorescence intensity values decreased for all gel samples. A simple model consisting of Case II diffusion was used to produce the packing constants, k ₀, for helixes. It was observed that k ₀ increased as the drying temperature was increased. On the other hand at each temperature, it was seen that k ₀ decreased as CaCl₂ content was increased. Packing energies for drying processes were obtained from fluorescence, volumetric, and gravimetric measurements separately.     

Synthesis and Characterization of Pyridine Compounds for Amperometric Measurements of Leukocyte Esterase

We introduce a new class of substrates (compounds I – III ) for leukocyte esterase (LE) that react with LE yielding anodic current in direct proportion to LE activity. The kinetic constants K_m and k_cat for the enzymatic reactions were determined by amperometry at a glassy carbon electrode. The binding affinity of I – III for LE was two orders of magnitude better than that of existing optical LE substrates. The specificity constant k_cat/K_m was equal to 2.7, 3.8, and 5.8×10⁵ m ^?1 s^?1 for compounds containing the pyridine ( I ), methoxypyridine ( II ), and (methoxycarbonyl)pyridine ( III ), respectively, thus showing an increase in catalytic efficiency in this order. Compound III had the lowest octanol/water partition coefficient (log p=0.33) along with the highest topological surface area (tPSA=222 Ų) and the best aqueous solubility (4.0 mg mL^?1). The average enzymatic activity of LE released from a single leukocyte was equal to 4.5 nU when measured with compound III .     

Multiparameter Kinetic Analysis for Covalent Fragment Optimization by Using Quantitative Irreversible Tethering (qIT)

Chemical probes that covalently modify cysteine residues in a protein-specific manner are valuable tools for biological investigations. Covalent fragments are increasingly implemented as probe starting points, but the complex relationship between fragment structure and binding kinetics makes covalent fragment optimization uniquely challenging. We describe a new technique in covalent probe discovery that enables data-driven optimization of covalent fragment potency and selectivity. This platform extends beyond the existing repertoire of methods for identifying covalent fragment hits by facilitating rapid multiparameter kinetic analysis of covalent structure–activity relationships through the simultaneous determination of K_i, k_inact and intrinsic reactivity. By applying this approach to develop novel probes against electrophile-sensitive kinases, we showcase the utility of the platform in hit identification and highlight how multiparameter kinetic analysis enabled a successful fragment-merging strategy.     

Kinetics for unidirectional endwise chain depolymerizations. Experiments with amylose in aqueous alkaline solutions

A kinetic model for the unidirectional endwise chain-propagated depolymerization of a linear polymer is described in terms of three pseudo-first-order rate constants: k₁ for the unzipping itself; k₂ for termination through the formation of stable endgroups; and k₃ for termination through complete degradation of polymer chains. It is shown that the calculated zip length, ν = k₁/(k₂ + k₃), will decrease as the initial substrate D.P. is reduced. For the dimer, a maximum value of ν = 1 is expected. During the anaerobic degradation of potato amylose in aqueous alkaline solutions, k₁ decreases and k₂ increases in value as the initial amylose concentration is raised. As a result, quantitative depolymerization occurs at low substrate concentrations, while at raised starch levels an alkali-stable residue is formed. It is proposed that intermolecular association between polymer chains causes these kinetic differences. For amylose, the constants k₁, k₂, and k₃ are approximately related by the ratio 1000:1:1 or 1000:0:1; and for the homologous disaccharide, the ratio is 10:1:10. The relevance of these findings to the kinetics of cellulose decomposition in aqueous alkali is discussed.     

Kinetics of 4,4′-diaminodiphenylmethane curing of bisphenol-S epoxy resin

The kinetics of the cure reaction for a system of bisphenol-S epoxy resin (BPSER), with 4,4′-diaminodiphenylmethane (DDM) as a curing agent, were studied by means of differential scanning calorimetry (DSC). Analysis of DSC data indicated that an autocatalytic behavior showed in the first stages of the cure, with the model proposed by Kamal, which includes two rate constants, k₁ and k₂, and two reaction orders, m and n. Rate constants k₁ and k₂ were observed to be greater when curing temperature increased. The over-all reaction order, m + n, is in the range of 2.5 ∼ 3. The activation energies for k₁ and k₂ were 55 kJ/mol and 57 kJ/mol, respectively. Diffusion control is incorporated to describe the cure in the latter stages. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1799–1803, 1999     

Kinetics of copolycondensation of bis(4-hydroxybutyl) terephthalate and bis(2-hydroxyethyl) terephthalate by ester-interchange reaction

The kinetics of polycondensation and copolycondensation reactions were investigated using bis(4-hydroxybutyl) terephthalate (BHBT) and bis (2-hydroxyethyl) terephthalate (BHET) as monomers. BHBT was prepared by ester interchange reaction of dimethyl terephthalate and 1,4-butanediol. BHBT and BHET were polymerized at 270°C in the presence of titanium tetrabutoxide (TBT) as a catalyst. Applying second-order kinetics for polycondensation, the rate constants of polycondensation of BHBT and BHET, k₁₁ and k₂₂, were calculated as 3.872 min⁻¹ and 2.238 min⁻¹, respectively. BHBT and BHET were also copolymerized at 270°C using TBT. The rate constants of crossreactions in the copolycondensation of BHBT and BHET, k₁₂ and k₂₁, were obtained by using the results obtained from a proton nuclear magnetic resonance (¹H-NMR) spectroscopy and a high-performance liquid chromatography (HPLC). It was found that the rate constants during the copolycondensation of BHBT and BHET at 270°C decreased in the order k₂₁ > k₁₁> k₂₂ > k₁₂ and the monomer reactivity ratio of BHBT was four or five times larger than that of BHET. In calculating the crossreactions, the method by the ¹H-NMR spectroscopy gave more accurate results than that by the HPLC. © 1996 John Wiley & Sons, Inc.     

Absolute rate constants for radical-monomer reactions

Summary A method is described for determining the absolute rate constants for the first few propagation steps in radical polymerization. The procedure involves a product analysis of the oligomeric alkoxyamines formed when an initiator is decomposed in monomer containing a very low concentration of a nitroxide radical scavenger. The method is illustrated with analysis of data for methyl acrylate. The rate constants for the first two propagation steps for polymerization of this monomer,k _p(1) andk _p(2), are at least an order of magnitude greater thank _p(average). Values of the absolute rate constants for reactions of phenyl and primary alkyl radicals with methyl acrylate are also estimated.     

Binding properties of peptidic affinity ligands for plasmid DNA capture and detection

Peptides constructed from α‐helical subunits of the Lac repressor protein (LacI) were designed then tailored to achieve particular binding kinetics and dissociation constants for plasmid DNA purification and detection. Surface plasmon resonance was employed for quantification and characterization of the binding of double stranded Escherichia coli plasmid DNA (pUC19) via the lac operon (lacO) to “biomimics” of the DNA binding domain of LacI. Equilibrium dissociation constants (K_D), association (k_a), and dissociation rates (k_d) for the interaction between a suite of peptide sequences and pUC19 were determined. K_D values measured for the binding of pUC19 to the 47mer, 27mer, 16mer, and 14mer peptides were 8.8 ± 1.3 × 10^?10 M, 7.2 ± 0.6 × 10^?10 M, 4.5 ± 0.5 × 10^?8 M, and 6.2 ± 0.9 × 10^?6 M, respectively. These findings show that affinity peptides, composed of subunits from a naturally occurring operon–repressor interaction, can be designed to achieve binding characteristics suitable for affinity chromatography and biosensor devices. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Competitive primary amine/epoxy and secondary amine/epoxy reactions: Effect on the isothermal time-to-vitrify

The effect of temperature on the ratio of the kinetic rate constants, k₂/k₁, has been investigated using FTIR spectroscopy for the competing reactions of epoxy with secondary amine (k₂) and primary amine groups (k₁) in a trimethylene glycol di-p-aminobenzoate/diglycidyl ether of the bisphenol A system. The ratio of the rate constants increases from 0.16 to 0.33 in the temperature range 100?160°C. The corresponding difference in the activation energies for the competing reactions is about 3.7 kcal/mol. The effect of the ratio on the time-to-vitrify contour of the isothermal time–temperature–transformation (TTT) cure diagram is discussed: The effect is more significant at higher curing temperatures.     

QUANTIFYING THE POSITION OF THE STATIONARY STATE TRAJECTORY BETWEEN THE MICHAELIS-MENTEN AND BRIGGS-HALDANE EQUATIONS AS A FUNCTION OF THE INITIAL ENZYME CONCENTRATION

For single intermediate enzyme kinetic systems, a relationship between a defined dimensionless parameter alpha (α) and the rate constants and initial enzyme concentration of the system was derived to describe the stationary-state trajectory of d(P)/dt versus (S) between the upper and lower bounds of the Michaelis-Menten and Briggs-Haldane systems. It was found that α is a function of E _o /k _m and the parameter Ω, which is a function of k ₂ and k ₃. The development of the α parameter provides a new method for directly estimating all the rate constants in the enzyme kinetic model, given (S) and (P) data as a function of time. This method for estimating the three rate constants and initial enzyme concentration is tested on four sets of simulated discrete (isothermal) data covering a range of different trajectories and on experimental data for the horseradish-peroxidase enzyme system.     

Multi‐Step Controlled Kinetics of the Transesterification of Crude Soybean Oil with Methanol by Mg(OCH3)2

This work focuses on a kinetic model that can be expressed as three significant controlled regions, i.e., a mass transfer controlled region in the internal surface of a heterogeneous catalyst, an irreversible chemical reaction controlled region in the pseudo‐homogenous fluid body and a reversible equilibrium chemical reaction controlled region near to the transesterification equilibrium stage. With the help of MATLAB7.0 software, the apparent reaction rate constants, k₂, k₂⁺ and k₂^–, were calculated and the corresponding apparent activation energies were calculated to be 67.450, 60.680 and 58.279 kJ·mol^–1, respectively. According to the confirmation experiments, it is indicated that the results can be applied for predicting the FAME yield at other reaction temperatures.     

Role of hydrophobic bonding in the contraction of nylon 66 in phenolic solutions

Rate constants for intermolecular bond breaking (k₁) and bond re-formation (k₂) were calculated from contraction measurements. Variation of ΔH? and ΔS? for k₁ with phenol substituents and concentration suggests the existence of hydrophobic bonding between the solution and the polymer activated complex. This behavior has been substantiated by correlating ΔH? and ΔS? for k₁ with a parameter related to hydrophobicity.     

Cryo-kinetics Reveal Dynamic Effects on the Chemistry of Human Dihydrofolate Reductase

Effects of isotopic substitution on the rate constants of human dihydrofolate reductase (HsDHFR), an important target for anti-cancer drugs, have not previously been characterized due to its complex fast kinetics. Here, we report the results of cryo-measurements of the kinetics of the HsDHFR catalyzed reaction and the effects of protein motion on catalysis. Isotopic enzyme labeling revealed an enzyme KIE (k_H^LE/k_H^HE) close to unity above 0 °C; however, the enzyme KIE was increased to 1.72±0.15 at −20 °C, indicating that the coupling of protein motions to the chemical step is minimized under optimal conditions but enhanced at non-physiological temperatures. The presented cryogenic approach provides an opportunity to probe the kinetics of mammalian DHFRs, thereby laying the foundation for characterizing their transition state structure.     

Rheo‐kinetic evaluation on the formation of urethane networks based on hydroxyl‐terminated polybutadiene

Rheo‐kinetic studies on bulk polymerization reaction between hydroxyl‐terminated polybutadiene (HTPB) and di‐isocyanates such as toluene‐di‐isocyanate (TDI), hexamethylene‐di‐isocyanate (HMDI), and isophorone‐di‐isocyanate (IPDI) were undertaken by following the buildup of viscosity of the reaction mixture during the cure reaction. Rheo‐kinetic plots were obtained by plotting ln (viscosity) vs. time. The cure reaction was found to proceed in two stages with TDI and IPDI, and in a single stage with HMDI. The rate constants for the two stages k₁ and k₂ were determined from the rheo‐kinetic plots. The rate constants in both the stages were found to increase with catalyst concentration and decrease with NCO/OH equivalent ratio (r‐value). The ratio between the rate constants, k₁/k₂ also increased with catalyst concentration and r‐value. The extent of cure reaction at the point of stage separation (x_i) increased with catalyst concentration and r‐value. Increase in temperature caused merger of stages. Arrhenus parameters for the uncatalyzed HTPB‐isocyanate reactions were evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1869–1876, 2001     

Friction dynamics of human hair treated with water or cationic surfactant aqueous solution

Cationic surfactants are adsorbed on the surface of human hair and exhibit a lubricating effect. Here, we evaluated the friction when the hair treated with water or 1 wt% cationic surfactant (cetyltrimethylammonium bromide) aqueous solution was rubbed by a contact probe equipped in a sinusoidal motion friction evaluation system. Because of the rough structure of the cuticle on the human hair surface, an oscillatory phenomenon with a frequency of 50–70 Hz was observed when untreated hair was rubbed with a contact probe. On the other hand, the oscillatory phenomenon was not observed when the hair contained a large amount of water because stick–slip phenomena were inhibited on the soft, swollen hair surface. Furthermore, the change in kinetic friction coefficient and delay time, which is a normalized value of the time difference between the reaction of the friction force to accelerated motion, from untreated hair, Δμ_k and Δδ, was almost zero. However, we found a large difference in Δμ_k and Δδ for the hair treated with cationic surfactant aqueous solution. The treatment with cationic surfactant reduced both friction parameters, Δμ_k and Δδ, indicating that the treatment induced not only the frictional force but also the profile at the beginning of frictional sliding. The significant lubrication is due to cationic surfactant adsorption on the hair surface. These data imply that the smoothness of hair treated with a cationic surfactant is related to a reduction in friction coefficient and delay time δ.     

Verteilungseigenschaften von Benzolderivaten. III. Zur Kinetik der diffusionskontrollierten Verteilung von mono- und disubstituierten Benzolderivaten im System n-Octanol/Wasser

Distribution Properties of Benzene Derivatives. III. On the Kinetics of the Diffusion-controlled Distribution of Mono- and Disubstituted Benzene Derivatives in the System n-Octanol/Water The kinetics of the diffusion-controlled distribution of 22 mono- and disubstituted benzenes in the solvent system n-octanol/water were investigated spectrophotometrically in the UV-VIS range at various temperatures between 288 and 333 K. The first-order rate constants k₁ and k₂ and the activation parameters were determined graphically. In this way, an interesting deviation from theory could be observed in the temperature range from 300 up to 308 K, which is discussed on the basis of known anomalies of water at the interface. Studies on relationships between rate constants (lg k₁, lgk₂) and hydrophobicity (lgP) show a nearly constancy of k₁, while the rate of transport from the organic to the aqueous phase (k₂) strongly depends on hydrophobicity described by lgP. The calculated regression equations correspond to analogous relationships found for mechanically influenced distributions and can therefore be explained by the same kinetical model.     

A kinetic study on the autocatalytic cure reaction of a cyanate ester resin

The cure of a novolac‐type cyanate ester monomer, which reacts to form a polycyanurate network, was investigated by using differential scanning calorimeter. The conversions and the rates of cure were determined from the exothermic curves at several isothermal temperatures (513–553 K). The experimental data, showing an autocatalytic behavior, conforms to the kinetic model proposed by Kamal, which includes two reaction orders, m and n, and two rate constants, k₁ and k₂. These kinetic parameters for each curing temperature were obtained by using Kenny's graphic‐analytical technique. The overall reaction order was about 1.99 (m = 0.99, n = 1.0) and the activation energies for the rate constants, k₁ and k₂, were 80.9 and 82.3 kJ/mol, respectively. The results show that the autocatalytic model predicted the curing kinetics very well at high curing temperatures. However, at low curing temperatures, deviation from experimental data was observed after gelation occurred. The kinetic model was, therefore, modified to predict the cure kinetics over the whole range of conversion. After modification, the overall reaction order slightly decreased to be 1.94 (m = 0.95, n = 0.99), and the activation energies for the rate constants, k₁ and k₂, were found to be 86.4 and 80.2 kJ/mol. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3067–3079, 2004     

Chemical Properties of 2‐Bromodecanoic Acid

Abstract

Some chemical equilibrium constants for 2‐bromodecanoic acid were investigated. The dimerization constant of 2‐bromodecanoic acid, k₂ =278 M^?1, in tert‐butylbenzene was first derived from IR spectroscopy measurements. Secondly, the distribution coefficient, k _d =799, was found by combining the value of k₂ with distribution data obtained from solvent extraction experiments evaluated with the aid of neutron activation analysis. Finally the dissociation constant, k _a =3.18 ?10^?3 M, was estimated from two‐phase titrations. A theoretical calculation was made based on the obtained constants and this calculation was validated by a second solvent extraction experiment that gave a good correspondence between calculated and experimental values.