Nonlinear Least-squares Curve-fitting Program Research Articles

A Selective Fluorescent Optode for Lead(II) Based on the Dansylamidopropyl Pendant Arm Derivative of 1,4-Dioxa-7,13-dithia-10-azacyclopentadecane ([15]aneNS2O2)

In this study, a novel highly sensitive and selective fluorescent optode membrane aimed at the determination of Pb(II) ion is proposed by incorporating N-(3-(1,4-dioxa-7,13-dithia-10-azacyclopentadecan-10-yl)propyl)-5-(dimethylamino)naphthalene-1-sulfonamide (L) as fluoroionophore in polyvinyl chloride (PVC) containing 2-nitrophenyl octylether (NPOE) as a plasticizer. In addition to high stability and reproducibility, the proposed optosensor showed a unique selectivity toward Pb(II) ion, with a wide linear range of molar concentrations (1.0 × 10−9–1.0 × 10−3 M) and a low detection limit of 7.5 × 10−10 M in solution at pH 5.0. The formation constants of the Pb(II) complexes with the fluoroionophore were evaluated by fitting the fluorescence data with a nonlinear least-squares curve-fitting program, and further information about the structures of the complexes were evaluated based on hybrid-DFT calculations. The optosensor exhibited a fast response time of less than three min, being easily regenerated by exposure to a solution of dithiothreitol. The sensor was applied to the determination of Pb(II) in real samples (canned tuna fish), and it provided satisfactory results comparable to those obtained via atomic absorption spectrometry (AAS).

19F-nuclear magnetic resonance spectroscopy as a tool to investigate host-guest complexation of some antidepressant drugs with natural and modified cyclodextrins

Purpose: 19 F-Nuclear magnetic resonance spectroscopy ( 19 F-NMR) was used to study host-guest complexation of three fluorine containing antidepressant drugs, viz, fluoxetine hydrochloride, citalopram hydrobromide and fluvoxamine maleate, with various cyclodextrins (CD), including α-, β-cyclodextrin, methylated α-cyclodextrin (M-α-CD), diamino derivative of methylated α-cyclodextrin, (DAM-α-CD) and tetramino derivative of methylated α-cyclodextrin (TAM-α-CD). Methods: Using the mole ratio method, a 1:1 stoichiometry was determined for the resulting inclusion complexes. 19 F chemical shifts were used to determine the formation constant of the complexes. Experiments were performed with solutions containing 0.001 M drug and various concentrations of CDs. NMR data were plotted as 19 F chemical shift versus CD/drug mole ratio, and fitted using the nonlinear least-squares curve fitting program, KINFIT, to obtain the formation constant of CD-drug complex. Molecular modeling (MM) calculations were used to predict the geometry of the complex of fluvoxamine and β-CD. Molecular modeling studies were performed in vacuum phase, employing empirical force fields and semi-empirical quantum theory using AM1 Hamiltonian. Results: Complex formation caused separation of the fluorine peaks that can be assigned to the two enantiomers of fluoxetine hydrochloride. Molecular modeling data suggest that fluvoxamine/β-CD inclusion complexes have a 1:1 stoichiometry and that the CF3-substituted ring of fluvoxamine is embedded in the cavity of β-CD, indicating a good agreement between molecular modeling calculation and experimental data (NMR data). Conclusion: One-dimensional 19 F-NMR is a fast and convenient method for the determination of complex stoichiometry and complexation constants of natural and modified CDs and fluorinated drugs. Keywords: Antidepressant drugs, Cyclodextrins, Complexation, Inclusion complex, Formation constant, 19 F-NMR

Effects of glutathione, l-cysteine, and l-methionine on the kinetics and mechanism of the peroxyoxalate–rubrene–H2O2 chemiluminescence system

The quenching effect of glutathione, l-cysteine, and l-methionine on the strong chemiluminescence (CL) intensity of bis(2,4,6-trichlorophenyl)oxalate (TCPO)–H2O2 in the presence of rubrene (5,6,11,12-tetraphenylnaphthacene) as a fluorescer was investigated. Kinetic parameters for the peroxyoxalate chemiluminescence (PO–CL) including the intensity at maximum CL, time at maximum intensity, total light yield, theoretical maximum level of intensity and pseudo-first order rate constants for the rise and fall of the CL burst (k r and k f ) were also calculated using the pooled intermediate model by a computerized non-linear least-squares curve fitting program (KINFIT). These systems resulted in Stern–Volmer plots in the quencher concentration range of 2.5 × 10−5 to 2.25 × 10−4 M, with k Q values of 2.5 × 104, 2.1 × 104 and 1.1 × 104 M−1, for glutathione, l-cysteine, and l-metionine, in order.

Effect of glutathione on peroxyoxalate chemiluminescence of hypericin as the fluorophore

Herein, the effect of amino acid Glutathione (GSH) on Peroxyoxalate Chemiluminescence was studied for the first time. Hypericin (HYP) was employed as the efficient fluorophore. The investigated parameters included rise and fall rate constant for the chemiluminescence burst, theoretical and experimental maximum intensity, the time-needed to reach maximum intensity and the total light yield emission which theoretically was evaluated using the pooled intermediate model by a computerized non-linear least-squares curve fitting program (KINFIT). Furthermore, based on observed quenching effect of GSH, the Stern–Volmer plot in quencher concentration range of 2.8×10−6 to 3.4×10−5M with KQ value of 1.59×104 was calculated. The bimolecular quenching rate constant (Kq) was also estimated about 2.8×1012 and M−1S−1. Moreover the system was applied successfully to determine glutathione in biological samples.

Quenching effect of l-tyrosine on peroxyoxalate chemiluminescence of berberine as the fluorophore

The present work deals with the first attempt to study the effect of L-tyrosine on the characteristics of the peroxyoxalate chemiluminescence. Berberine was applied as an efficient fluorophore. The investigated parameters include rise and fall rate constant for the chemilumines- cence burst, theoretical and experimental maximum intensity, the time needed to reach maximum intensity and the total light yield emission, which is theoretically eval- uated using the pooled intermediate model by a comput- erized non-linear least-squares curve fitting program (KINFIT). Furthermore, based on the observed quenching effect of tyrosine, the Stern-Volmer plot with KQ value of 7.7 9 10 4 M -1 in the quencher concentration range 4 9 10 -6 -5 9 10 -5 M. Moreover, this method is applied

A study of chemiluminescence characteristics of a novel peroxyoxalate system using berberine as the fluorophore

In the present work the chemiluminescence characteristics of a peroxyoxalate system using berberine as a novel fluorophore has been investigated. Berberine can accept energy via the formation of a dioxetane and emits intense yellow light. The characteristics of the chemiluminescence for this system under various concentrations of reagents including bis (trichlorophenyl)oxalate, sodium salicylate (as catalyst), hydrogen peroxide and the fluorophore were also studied. These parameters including the rise and fall rate constants for the chemiluminescence emission, theoretical and experimental maximum intensity, theoretical and experimental time to reach maximum intensity and total light yield emission were evaluated from intensity–time profiles using the pooled intermediate model by a non-linear least-squares curve fitting program, KINFIT.

Hypericin from St. John’s Wort (hypericum perforatum) as a novel natural fluorophore for chemiluminescence reaction of bis (2,4,6-trichlorophenyl) oxalate–H2O2–imidazole and quenching effect of some natural lipophilic hydrogen peroxide scavengers

Hypericin (HYP) molecule is a natural photoactive pigment, which plays a role as an effective photoreceptor in some plants of the Hypericum species (the most common of which is Saint John’s Wort) and some insect species. The present work deals with the first attempt to the study of peroxyoxalate chemiluminescence (POCL) system in the presense of HYP as a natural fluorophore. Reaction of bis (2,4,6-trichlorophenyl) oxalate(TCPO)–H2O2–imidazole can transfer energy to a HYP via formation of dioxetane through the chemically initiated electron exchange luminescence (CIEEL) mechanism and can emits a very intense red light. The effects of HYP, hydrogen peroxide, TCPO and imidazole concentrations on kinetic chemiluminescence parameters were also studied. These parameters including rise and fall rate constant for the chemiluminescence burst, theoretical and experimental maximum intensity, theoretical and experimental time to reach maximum intensity and total light yield emission were evaluated by using a pooled intermediate model for a non-linear least-squares curve fitting program, KINFIT. Moreover, quenching effect of two lipophilic natural antioxidant, Quercetin and β-carotene on it system was also investigated. The measurable concentration range of 7×10−6M to 7.5×10−5M of antioxidants were evaluated from the proper Stern–Volmer plots with satisfactory RSD% and corresponding detection limits of 2.2×10−6 and 3.7×10−6 for β-carotene and quercetin respectively.

Juxtamembranous Region of the Amino Terminus of the Family B G Protein-coupled Calcitonin Receptor Plays a Critical Role in Small-molecule Agonist Action

The Family B G protein-coupled calcitonin receptor is an important drug target. The aim of this work was to elucidate the molecular mechanism of action of small-molecule agonist ligands acting at this receptor, comparing it with the action mechanism of the receptor's natural peptide ligand. cAMP responses to four non-peptidyl ligands and calcitonin were studied in COS-1 cells expressing wild-type and chimeric calcitonin-secretin receptors. All compounds were full agonists at the calcitonin receptor with no activity at the secretin receptor. Only chimeric constructs including the calcitonin receptor amino terminus exhibited responses to any of these ligands. We progressively truncated this domain and tested constructs for cAMP responses. Although calcitonin was able to activate the calcitonin receptor fully with the first 58 residues absent, its potency was 3 orders of magnitude lower than that at the wild-type receptor. After truncation of 114 residues, there was no response to calcitonin. In contrast, small-molecule ligands were fully active at receptors having up to 149 amino-terminal residues absent. Those compounds finally became inactive after truncation of 153 residues. Deletion and/or alanine replacement of the region of the calcitonin receptor between residues 150 and 153 resulted in marked reduction in cAMP responses to these compounds, with some compound-specific differences observed, supporting a critical role for this region. Binding studies further supported distinct sites of action of small molecules relative to that of calcitonin. These findings focus attention on the potential importance of the juxtamembranous region of the amino terminus of the Family B calcitonin receptor for agonist drug action.

Liquid−Liquid Equilibria in Binary Mixtures of Water with Malonitrile, Succinonitrile, and Glutaronitrile

Liquid−liquid equilibria of binary mixtures of water and dinitrile solvents malononitrile, succinonitrile, and glutraronitrile have been studied in temperature range of 15 to 70 °C. The phase diagrams show that the critical mole fraction and consolate temperature (°C) (xc, Tc) are (0.25, 58) for malononitrile and (0.19, 56) and (0.12, 68) for succinonitrile and glutraronitrile, respectively. The nonrandom two-liquid (NRTL) equation has been used to correlate of the activity coefficients of the components in binary mixtures. The parameters needed for the calculation are the energy parameters (g12 − g22) and (g21 − g11) and the nonrandom parameter α, which are determined by solving the NRTL equation with the aid of a nonlinear least-squares curve-fitting program. From the parameters of this equation, the excess Gibbs energies, GE, were calculated. Good agreement between the experimental data of phase diagrams and model results was achieved.

Regulatory effect of cholecystokinin on subsequent insulin binding to pancreatic acini

We investigated the regulatory effect of cholecystokinin (CCK) on subsequent insulin binding to pancreatic acinar cells. Rat isolated acini were preincubated with various concentrations of CCK octapeptide (CCK-8) at 37 degrees C. Acini were then washed, resuspended in the binding buffer, and incubated with 8.3 pM 125I-labeled insulin for 60 min at 37 degrees C. Pretreatment with CCK-8 caused inhibition of subsequent 125I-insulin binding that was time and concentration dependent. Significant inhibition was observed with 3 pM CCK-8. Computer analysis of the competition-inhibition study with a nonlinear least-squares curve-fitting program revealed that CCK-8 pretreatment of acini reduced the receptor affinity of the high-affinity binding site. This inhibitory action of CCK-8 was not due to the alteration in degradation or internalization of the tracer. When acini were pretreated with 100 pM CCK-8 for 120 min at 4 degrees C, a reduction in the receptor affinity of the high-affinity binding site was also observed. In pancreatic membrane prepared from acini preincubated with 100 pM CCK-8 for 120 min at 37 degrees C, displacement of 125I-insulin (83 pM) by unlabeled insulin (24 degrees C, 1 h) revealed that CCK-8 inhibited 125I-insulin binding by altering the receptor affinity of the high-affinity binding site. In acinar preparations the inhibitory effect of CCK-8 on 125I-insulin binding was abolished when acini were preincubated with CCK-8 and CCK receptor antagonist L 374718 at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of degradation of 2′-5′ oligoadenylates

We have studied the mechanisms of breakdown of 2′-5′ oligoadenylates. We monitored the time-courses of degradation of ppp(A2′p5′) nA (dimer to tetramer) and of 5′OH-(A2′p5′) nA (dimer to pentamer) in unfractionated L1210 cell extract. The 5′ triphosphorylated 2′-5′ oligoadenylates are converted by a phosphatase activity. However, 2′-5′ oligoadenylates are degraded mainly by phosphodiesterase activity which splits the 2′-5′ phosphodiester bond sequentially at the 2′ end to yield 5′ AMP and one-unit-shorter oligomers. The nonlinear least-squares curve-fitting program CONSAM was used to fit these kinetics and to determine the degradation rate constant of each oligomer. Trimers and tetramers, whether 5′ triphosphorylated or not, are degraded at the same rate, whereas 5′ triphosphorylated dimer is rapidly hydrolyzed and 5′-OH dimer is the most stable oligomer. The interaction between degradation enzymes and the substrate strongly depends on the presence of a 5′ phosphate group in the vicinity of the phosphodiester bond to be hydrolyzed; indeed, when this 5′ phosphate group is present, as in pp/pA2′p5′ A/or A2′/p5′ A2′p5′ A/, affinity is high and maximal velocity is low. Such a degradation pattern can control the concentration of 2′-5′ oligoadenylates active on RNAse L either by limiting their synthesis (5′ triphosphorylated dimer is the primer necessary for the formation of longer oligomers) and/or by converting them into inhibitory ( e.g., monophosphorylated trimer) or inactive ( e.g., nonphosphorylated oligomers) molecules.

Degradation kinetics of vinblastine sulphate in aqueous solutions

The degradation kinetics of vinblastine sulphate (VBL) have been studied over the H 0/pH range −4 up to 14 at 80°C. A stability indicating HPLC-system with UV-detection was used to separate the parent compound and the degradation products. The influences on the degradation rate of the buffer concentration, ionic strength and temperature have been investigated. The degradation kinetics were modeled with a non-linear least-squares curve-fitting program.

A 1H nuclear magnetic resonance study of the deprotonation of L-dopa and adrenaline

The acid–base chemistries of L-3,4-dihydroxyphenylalanine (dopa), L-adrenaline (ad), 3,4-dimethoxyphenylethylamine (dmpe), and 3,4-dihydroxyphenylpropionic acid (dhpa) have been studied by 1H n.m.r. spectroscopy in D2O solution. Macroscopic dissociation constants for dmpe and dhpa and macroscopic and microscopic dissociation constants for the phenolic OD and the side-chain ammonium groups of dopa and ad have been calculated from the 1H chemical shift titration curves for the aryl and allyl protons by a non-linear least-squares curve-fitting program. The isotope effect on the acidity has been determined for several amines and phenols in order to estimate the microscopic dissociation constants of dopa and ad in H2O from the values obtained in D2O. The results indicate that the phenolic hydroxy-groups of dopa and of ad are more acidic than their respective side-chain ammonium groups.