Nucleophilic dephosphorylation of p-nitrophenyl diphenyl phosphate in cationic micellar media

The kinetics of nucleophilic dephosphorylation of p-nitrophenyl diphenyl phosphate by hydroxamate ions (R'(C=O)N(RO-)) have been investigated in aqueous cationic micellar media at pH 9.12 and 27 degrees C. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. N-Substituted hydroxamate ion shows higher reactivity over the unsubstituted hydroxamate ions in cationic micellar media. The kinetic data are discussed in terms of the pseudophase ion exchange model.     

Catalytic hydrolysis of phosphodiesters by nucleophilic ions in gemini micellar media

The catalytic hydrolysis of bis(4‐nitrophenyl)phosphate (BNPP) and bis(2,4‐dinitrophenyl)phosphate (BDNPP) catalyzed by α‐nucleophiles in gemini micellar media was investigated at 27 °C. The cationic gemini surfactants, i.e., alkanediyl‐α‐ω‐bis(hydroxyethylmethylhexadecylammonium bromide) (16‐s‐16 MEA 2Br^?, where s = 4 and 6) were used. Nucleophilic reactivity of α‐nucleophiles such as hydroperoxide (HOO^?), acetohydroxamate (AHA^?), and butane 2,3‐dione monoximate ions (BDMO^?) were compared. The kinetic rate data were treated by applying the pseudophase model. The cationic gemini surfactants show unusual rate acceleration toward the cleavage of phosphodiesters with nucleophiles. These studies reveal that the hydroperoxide ion shows the highest catalytic activity reported so far with an unprecedented acceleration rate, 10⁷ times faster than that of the uncatalyzed reaction. The possible mechanism for the BNPP and BDNPP cleavage promoted by α‐nucleophiles is proposed on the basis of kinetic analysis. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrolytic Cleavage of Paraoxon by Octanohydroxamate Ion in Cationic Microemulsions

The hydrolysis reaction of O,O‐diethyl O‐p‐nitrophenylphosphate (Paraoxon) with the octanohydroxamate ion (OHA^?) was studied in a cationic oil‐in‐water (O/W) microemulsion system over a pH range 7.5–12.0 at 300 K. The O/W systems are stabilized by using cationic surfactant, cetyltrimethylammonium bromide (CTAB), and n‐butanol as cosurfactants. In a microemulsion, the rate enhancement by OHA^? is greater toward the cleavage of paraoxon than its spontaneous (2.1 × 10⁷ s^?1) hydrolysis. The k_obs values for the reaction of paraoxon with OHA^? were determined in different microemulsion compositions with varying chain length of alcohols (n‐butanol, n‐pentanol, n‐octanol, and n‐dodecanol) and alkanes (n‐hexane, n‐heptane, and n‐decane). The effects of water content, pH, and size of the oil pool have been discussed.     

Studies of Nucleophilic Substitution Reactions of p‐Nitrophenyl Acetate with Some Dihydroxamate Ions in Cationic Micellar Media

Pseudo‐first‐order rate constants have been determined for the nucleophilic substitution reactions of p‐nitrophenyl acetate with oxalo, malono, and succinodihydroxamate ions (^?ONHC(O)(CH₂)_nC(O)NHO^?) in phosphate buffer (pH=7.9) at 27°C. The rate data of the reaction revealed that the nucleophilic reactivity sequence of these hydroxamate ions is generally ODHA>MDHA>SDHA. The k_obs value increases upon addition of cationic surfactants to the reaction medium which is typical behavior of micelle‐assisted bimolecular reactions. The pseudo‐phase ion exchange model has been successfully applied to determine binding constant.     

Reactivity studies of carbon,phosphorus and sulfur‐based acyl sites with tertiary oximes in gemini surfactants

Kinetic studies of the reactions of tertiary oximes (monoisonitroso acetone; MINA and butane 2,3 dione monooxime; BDMO) with some carboxylate (p‐nitrophenyl acetate and p‐nitrophenyl benzoate), phosphate (p‐nitrophenyl diphenyl phosphate and bis (2,4‐dinitrophenyl) phosphate) and sulfonate (p‐nitrophenyl p‐toluene sulphonate) esters in gemini surfactants have been conducted. The observed first‐order rate constant versus surfactant profiles show micelle‐assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. Experimental results showed that MINA exhibited better nucleophilic activity towards ester cleavage than BDMO. Pseudophase model has been applied in order to determine micellar second‐order rate constants and binding constants. Copyright © 2013 John Wiley & Sons, Ltd.     

Adsorption Kinetics and Binding Studies of Protein Quantum Dots Interaction: A Spectroscopic Approach

Protein Quantum dots interaction is crucial to investigate for better understanding of the biological interactions of QDs. Here in, the model protein Bovine serum albumin (BSA) was used to evaluate the process of protein QDs interaction and adsorption on QDs surface. The modified Stern-Volmer quenching constant (K_a), number of binding sites (n) at different temperatures (298 308 and 318 K?±?1) and corresponding thermodynamic parameters (ΔG?<?0, ΔH?<?0, and ΔS?>?0) were calculated. The quenching constant (K_s) and number of binding sites (n) is found to be inversely proportional to temperature. It signified that static quenching mechanism is dominant over dynamic quenching. The standard free energy change (ΔG?<?0) implies that the binding process is spontaneous, while the enthalpy change (ΔH?<?0) suggest that the binding of QDs to BSA is an enthalpy-driven process. The standard entropy change (ΔS?>?0) suggest that hydrophobic force played a pivotal role in the interaction process. The adsorption process were assessed and evaluated by pseudofirst-order, pseudosecond-order kinetic model, and intraparticle diffusion model.     

Effect of cationic gemini surfactants on the hydrolysis of carboxylate and phosphate esters using hydroxamate ions

The kinetics of the hydrolysis of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenyl phosphate (PNPDPP) by hydroxamate ions mediated by gemini surfactants with quaternary ammonium bromide (16-n-16,2Br⁻, n = 3, 4, 6, 12) and pyridinium chloride (12py-n-py12,2Cl⁻, n = 3, 4) head group have been investigated at 27 °C. The gemini surfactant with the pyridinium head group, 12-py-4-py12,2Cl⁻ (tetramethylene-1,4 bis dodecylpyridinium chloride) shows a large rate acceleration effect than that with an ammonium head group, 16-12-16,2Br⁻, relative to those in water. The apparent pK _a of the hydroxamic acids have been determined in the presence of gemini surfactants. Catalytic system N-phenylbenzohydroxamate/12py-4-py12,2Cl⁻ demonstrated over ~1,590-fold and ~255-fold rate enhancement in the hydrolysis of PNPA and PNPDPP, respectively, for the identical reaction performed in buffer aqueous media at 27 °C. The second order rate constant and binding constants for reactions were determined employing pseudophase model for micellar catalysis.     

The α‐effect in micelles: Nucleophilic substitution reaction of p‐nitrophenyl acetate with N‐phenylbenzohydroxamate ion

Pseudo‐first‐order rate constants have been determined for the nucleophilic substitution reactions of p‐nitrophenyl acetate with p‐chlorophenoxide (4‐ClC₆H₄O^?) and N‐phenylbenzohydroxamate (C₆H₅CON(C₆H₅)O^?) ions in phosphate buffer (pH 7.7) at 27°C. The effect of cationic, (CTAB, TTAB, DTAB), anionic (SDS), and nonionic (Brij‐35) surfactants has been studied. The k_obs value increases upon addition of CTAB and TTAB. The effect of DTAB and other surfactants on the reaction is not very significant. The micellar catalysis and α‐effect shown by hydroxamate ion have been explained. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 26–31, 2006     

Interaction of Folic Acid with Mn2+ Doped CdTe/ZnS Quantum Dots: In Situ Detection of Folic Acid

To utilize the nanomaterials as an effective carrier for the drug delivery applications, it is important to study the interaction between nanomaterials and drug or biomolecules. In this study GSH functionalized Mn²⁺-doped CdTe/ZnS QDs has been utilized as a model nanomaterial due to its high luminescence property. Folic acid (FA) gradually quenches the FL of GSH functionalized Mn²⁺???doped CdTe/ZnS QDs. The Stern-Volmer quenching constant (K_sv), binding constant (K_s) and effective quenching constant (Ka) for the FA-QDs system is calculated to be 1.32?×?10⁵ M^?1, 1.92?×?10⁵ and 0.27?×?10⁵ M^?1, respectively under optimized condition (Temp. 300 K, pH 8.0, incubation time 40 min.). The effects of temperature, pH, and incubation time on FA-QDs system have also been studied. Statistical analysis of the quenched FL intensity versus FA concentration revealed a linear range from 1?×?10^?7 to 5.0?×?10^?5 for FA detection. The LOD of the current nano-sensor for FA was calculated to be 0.2 μM. The effect of common interfering metal ions and other relevant biomolecules on the detection of FA (12.0 μM) have also been investigated. L-cysteine and glutathione displayed moderate effect on FA detection. Similarly, the common metal ions (Na⁺, K⁺, Ca²⁺ and Mg²⁺) produced minute interference while Zn²⁺ Cu²⁺ and Fe³⁺ exert moderate interference. Toxic metal ions (Hg²⁺ and Pb²⁺) produced severe interferences in FA detection.

Graphical abstract

GSH-Mn²⁺ CdTe/ZnS QDs based Fluorescence Nanosensor for Folic acid

     

Green Luminescent CdTe Quantum Dot Based Fluorescence Nano-Sensor for Sensitive Detection of Arsenic (III)

Arsenic (As³⁺) is a hazardous and ubiquitous element; hence the quantitative detection of arsenic in various kinds of environmental sample is an important issue. Herein, we reported L-cysteine capped CdTe Quantum dot based optical sensor for the fluorometric detection of arsenic (III) in real water sample. The method is based on the fluorescence quenching of QDs with the addition of arsenic solution that caused the reduction in fluorescence intensity due to strong interaction between As³⁺ and L-cysteine to form As(Cys)₃. The calibration curve was linear over 2.0 nM-0.5 μM arsenic with limit of detection (LOD) of 2.0 nM, correlation coefficient (r²) of 0.9698, and relative standard deviation (RSD %) of 5.2%. The Stern-Volmer constant for the quenching of CdTe QDs with As³⁺ at optimized condition was evaluated to be 1.17 × 10⁸ L mol^?1 s^?1. The feasibility of the sensor has been analyzed by checking the inference of common metal ions available in the water such as K⁺, Na⁺, Mg²⁺, Ca²⁺, Ba²⁺, Cu²⁺, Ni²⁺, Zn²⁺, Al³⁺, Co²⁺, Cr²⁺, Fe³⁺ and its higher oxidation state As⁵⁺.

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Graphical Abstract Schematic representation of As³⁺ detection by L-Cysteine capped CdTe QDs