A molecular dynamics study of the thermodynamic properties of calcium apatites. 1. Hexagonal phases

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(PO4)6(X)2 (X = OH, F, Cl, Br), were investigated using an all-atom Born-Huggins-Mayer potential by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of ca. 4% for the haloapatites and 8% for hydroxyapatite. The standard molar lattice enthalpy, delta(lat)H298(o), of the apatites was calculated and compared with previously published experimental results, the agreement being better than 2%. The molar heat capacity at constant pressure, C(p,m), in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, H(m) = (H(m,298) - 298C(p,m)) + C(p,m)T, yielding C(p,m) = 694 +/- 68 J x mol(-1) x K(-1), C(p,m) = 646 +/- 26 J x mol(-1) x K(-1), C(p,m) = 530 +/- 34 J x mol(-1) x K(-1), and C(p,m) = 811 +/- 42 J x mol(-1) x K(-1) for hydroxy-, fluor-, chlor-, and bromapatite, respectively. High-pressure simulation runs, in the range 0.5-75 kbar, were performed in order to estimate the isothermal compressibility coefficient, kappaT, of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state with an accuracy better than 1%.     

Binding of sodium dodecyl sulfate to linear and star homopolymers of the nonionic poly(methoxyhexa(ethylene glycol) methacrylate) and the polycation poly(2-(dimethylamino)ethyl methacrylate): electromotive force, isothermal titration calorimetry, surface tension, and small-angle neutron scattering measurements

We investigated the binding of sodium dodecyl sulfate (SDS) to various linear and star polymers of the nonionic methoxyhexa(ethylene glycol) methacrylate (PMHEGMA) and the ionic 2-(dimethylamino)ethyl methacrylate (PDMAEMA), the latter being a polycation at low pH. The dodecyl sulfate ion selective electrode (EMF), isothermal titration calorimetry (ITC), and surface tension (ST) were applied to gain detailed information about interactions. In all cases there is evidence of significant binding of SDS over an extensive SDS concentration range spanning from ca. 10(-6) to 0.1 mol dm(-3). At pH 3, the polymer PDMAEMA is a strong polycation and here the binding is dominated by electrostatic 1:1 charge neutralization with the anionic surfactant. At their natural pH of 8.6, PMHEGMA and PDMAEMA polymers are essentially nonionic and bind SDS in the form of polymer-bound aggregates in the concentration range of ca. 1 x 10(-3) to 3 x 10(-2) mol dm(-3). All the polymers also bind SDS to a lesser extent at concentrations below 1 x 10(-3) mol dm(-3) reaching as low as 10(-7) mol dm(-3). This low concentration binding process involves the polymer and nonassociated SDS monomers. As far as we are aware, this is the first example that such a low concentration noncooperative binding process could be observed in SDS/neutral polymer systems by EMF and ST. We also showed that the nonionic surfactant hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) and the cationic cetyltrimethylammonium bromide (C16TAB) interact with star PDMAEMA. We believe that the interaction of C12EO6 and CTAB is of similar noncooperative type as the first SDS binding process in the range from ca. 10(-5) to 0.3 x 10(-3) mol dm(-3). At the high concentration binding limit Csat of SDS, the above polymers become fully saturated with bound SDS micelles. We applied small angle neutron scattering (SANS) to determine the structure and aggregation numbers of the star polymer/bound SDS micelles and calculated the stoichiometry of such supramolecular complexes. The SANS data on PDMAEMA star polymers in the presence of C12EO6 showed only a limited monomer binding in contrast to linear PDMAEMA, which showed monomer C12EO6 binding at low concentrations but micellar aggregates at 6 x 10(-3) mol dm(-3).     

On the Binding Strength Sequence for Nucleic Acid Bases and C(60) with Density Functional and Dispersion-corrected Density Functional Theories: Whether C(60) could protect nucleic acid bases from radiation-induced damage?

The major objective of this paper is to address a controversial binding sequence between nucleic acid bases (NABs) and C(60) by investigating adsorptions of NABs and their cations on C(60) fullerene with a variety of density functional theories including two novel hybrid meta-GGA functionals, M05-2x and M06-2x, as well as a dispersion-corrected density functional, PBE-D. The M05-2x/6-311++G** provides the same binding sequence as previously reported, guanine(G) > cytosine(C) > adenine (A) > thymine (T); however, M06-2x switches the binding strengths of A and C, and PBE-D eventually results in the following sequence, G>A>T>C, which is the same as the widely accepted hierarchy for the stacking of NABs on other carbon nanomaterials such as single-walled carbon nanotube and graphite. The results indicate that the questionable relative binding strength is due to insufficient electron correlation treatment with the M05-2x or even the M06-2x method. The binding energy of G@C(60) obtained with the M06-2x/6-311++G(d,p) and the PBE-D/cc-pVDZ is -7.10 and -8.07 kcal/mol, respectively, and the latter is only slightly weaker than that predicted by the MP2/6-31G(d,p) (-8.10kca/mol). Thus, the PDE-D performs better than the M06-2x for the observed NAB@C(60) π-stacked complexes. To discuss whether C(60) could prevent NABs from radiation-induced damage, ionization potentials of NABs and C(60), and frontier molecular orbitals of the complexes NABs@C(60) and (NABs@C(60))(+) are also extensively investigated. These results revealed that when an electron escapes from the complexes, a hole was preferentially created in C(60) for T and C complexes, while for G and A the hole delocalizes over the entire complex, rather than a localization on the C(60) moiety. The interesting finding might open a new strategy for protecting DNA from radiation-induced damage and offer a new idea for designing C(60)-based antiradiation drugs.     

Metal dilution effects on the spin-crossover properties of the three-dimensional coordination polymer Fe(pyrazine)[Pt(CN)4

The 1A1 left arrow over right arrow 5T2 spin transition has been investigated in the solid solutions of Fe(x)M(1-x)(pyrazine)[Pt(CN)4] (M = Ni or Co, 0 < or = x < or = 1) having a three-dimensional polynuclear structure. Both Ni and Co dilutions tend to decrease the hysteresis width and smooth the transition curves. The enthalpy (entropy) change associated with the spin transition was found to decrease from 26 kJ mol(-1) (84 J K(-1) mol(-1)) for x = 1 to 12 kJ mol(-1) (47 J K(-1) mol(-1)) for 47% Co dilution and to 15 kJ mol(-1) (54 J K(-1) mol(-1)) for 59% Ni dilution. Raman spectroscopy revealed a mixed one- and two-mode behavior in the solid solutions. For the first time, a correlation between vibrational frequencies exhibiting one-mode behavior and the entropy change, which drives the spin crossover, is established.     

Study on the electrochemiluminescence behavior of ABEI and its application in DNA hybridization analysis

The electrogenerated chemiluminescence (ECL) behavior of N-(4-aminobutyl)-N-ethylisoluminol (ABEI) was studied and it was found that ABEI could produce emission light when oxidized at a +1.0 V (vs. Ag/AgCl) potential in alkaline solution. The addition of H2O2 markedly improved the ECL sensitivity. The pH value of the solution as well as the H2O2 concentration and working potential all have influences on the ECL response. Under optimal conditions, ABEI can be detected in the range 1.3 x 10(-6)-6.5 x 10(-12) mol L(-1). A detection limit of 2.2 x 10(-12) mol L(-1) for ABEI was obtained at a signal-to-noise ratio of 3. ABEI was then used as a marker to label a known sequence oligonucleotide, which was used as a DNA probe for identifying a target ssDNA immobilized on a PPy modified electrode based on a specific hybridization reaction. The hybridization events were evaluated by the ECL measurements. The results showed that only a complementary sequence could form a double-stranded DNA with the DNA probe and give a strong ECL response. A three-base mismatch sequence and non-complementary sequence have no response. The intensity of the ECL was linearly related to the concentration of the complementary sequence in the range 9.6 x 10(-11)-9.6 x 10(-8) mol L(-1), the detection limit was 3.0 x 10(-11) mol L(-1).     

Determination of human plasma protein binding of baicalin by ultrafiltration and high-performance liquid chromatography

A simple and selective HPLC assay was developed and utilized for determination of human plasma protein binding of baicalin. The method involved solid-phase extraction and reversed-phase chromatographic separation with a mobile phase of acetonitrile-0.02 mol/L phosphate buffer (pH 2.5; 25:75, v/v) and UV detection at 276 nm. The standard curve for baicalin was linear over the concentration range 0.1-20 microg/mL and the limit of detection was 0.02 microg/mL. The absolute recovery was greater than 76%. The intra-day and inter-day variations were less than 10%. Ultrafiltration technique was applied to determining the plasma protein binding of baicalin in human plasma. Results show the plasma protein binding of baicalin was in the range 86-92% over all the concentrations studied and the protein binding association constant was determined to be 1.21 x 10(5) L/mol at 4 degrees C.     

流动注射化学发光抑制法测定抗坏血酸

基于抗坏血酸对Ｌｕｍｉｎｏｌ－ＫＩＯ４－Ｈ２Ｏ２体系化学发光反应的抑制作用,建立了化学发光抑制快速测定抗坏血酸的新方法。该方法线性范围为１．０＊１０＾－７－１．０＊１０＾－５ｍｏｌ／Ｌ,检出限为６．０＊１０＾－８ｍｏｌ／Ｌ,对８．０＊１０＾－７ｍｏｌ／Ｌ抗坏血酸１１次平行测定的相对标准偏差为１．０％。用于维生素Ｃ片剂及注射液中抗坏血酸含量的测定,结果令人满意。     

Reentrant behavior of poly(N-isopropylacrylamide) brushes in water-methanol mixtures investigated with a quartz crystal microbalance

The solvent composition induced reentrant behavior of poly(N-isopropylacrylamide) (PNIPAM) chains grafted on a SiO2 surface in water-methanol mixtures was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) at 20 degrees C. The frequency and energy dissipation responses showed that the grafted PNIPAM chains sharply collapse when the methanol content (x(m)) reaches approximately 17 mol %. In the range 17-50 mol %, the grafted chains remain in a collapsed state. Further increase of the methanol content leads to an abrupt reswelling of the collapsed chains at x(m) > approximately 50 mol %. The sharp reentrant swelling-to-collapse-to-swelling transition was attributed to the water-methanol complexation instead of the preferential adsorption effect. Our results also suggest that the water-methanol complexation is not induced by hydrophobic interaction but by hydrogen bonding.     

多菌灵在玻碳电极上的伏安行为及其应用

应用循环伏安法、微分脉冲伏安法对多菌灵在破碳电极上的电化学行为及其测定进行了研究。在ｐＨ＝９．０的２ｍｏｌ／Ｌ ＮＨ３－ＮＨ４Ｃｌ底液中，对其进行循环伏安扫描，发现于０．６１Ｖ（ｖｓ．Ａｇ／ＡｇＣｌ）产生一灵敏的氧化峰。微分脉冲伏安法殉菌灵的检测限为４×１０＾－８ｍｏｌ／Ｌ。多菌灵的浓度在５．０×１０＾－７￣１．０×１０＾－５ｍｏｌ／Ｌ间与微分脉冲伏安峰电流呈线性关系（ｒ＝０．９９４２）。对于１×     

Adsorptive behavior and electrochemical determination of the anti-fungal agent ketoconazole

The adsorptive properties and electrochemical behavior of ketoconazole, an oral anti-fungal agent, are demonstrated at a glassy carbon electrode. The adsorption of the compound obeys the Frumkin isotherm with an interaction factor (alpha) of 0.985 and adsorptive coefficient (beta) of 1.98 x 10(6) L mol(-1). The Gibbs energy of adsorption (deltaG) is -3.59 x 10(4) J mol(-1) at 25 degrees C. A very sensitive electroanalytical method has been developed for determination of the drug with a detection limit of 4.0 x 10(-11) mol L(-1). Relationships between stripping current and concentration of ketoconazole were linear in the range 10(-6)-10(-10) mol L(-1) with different preconcentration periods. The method has been used to measure the ketoconazole content of tablets.     

A novel water-soluble fluorescent probe for the determination of methylamine

A novel water-soluble fluorescent probe, monosodium 7-(4,6-dichloro-1,3,5-triazinylamino)-1,3-naphthalenedisulfonic acid (DTND), was synthesized by reacting cyanuric chloride with 7-amino-1,3-naphthalenedisulfonic acid monopotassium salt at 0-5 degrees C. This new reagent was used for the determination of methylamine. The linear range is 3x10(-6)-2x10(-4) mol L(-1) with a detection limit (S/N=3) of 7.2x10(-8) mol L(-1), and the relative S.D. is 1.3% for ten replicate determinations of 1x10(-5) mol L(-1) CH3NH2. Common species in the aqueous environment have no or only slight influence on the determination. The method can be used to determine methylamine in real water samples.     

Binding of the bioactive component daphnetin to human serum albumin demonstrated using tryptophan fluorescence quenching

Daphnetin (7,8-dihydroxycoumarin), one of the major bioactive components isolated from Daphne koreane Nakai, has been used in traditional Chinese medicine for the treatment of coagulation disorders. It is also a chelator, an antioxidant and a protein kinase inhibitor. In this paper, a combination of intrinsic fluorescence, Fourier transform infrared (FT-IR) spectroscopy and circular dichroic (CD) spectroscopy has been used to characterize the binding between daphnetin and human serum albumin (HSA) under physiological conditions with drug concentrations of 6.7 x 10(-6) - 2.3 x 10(-5) mol x L(-1), and a HSA concentration of 1.5 x 10(-6) mol x L(-1). Changes in the CD spectra and FT-IR spectra were observed upon ligand binding, and the degree of tryptophan fluorescence quenching did change significantly in the complexes. These data have proved the change in protein secondary structure accompanying ligand binding. The change in tryptophan fluorescence intensity was used to determine the binding constants. The thermodynamic parameters, the enthalpy change (DeltaH) and the entropy change (DeltaS) were calculated to be -12.45 kJ x mol(-1)and 52.48 J x mol(-1) x K(-1) according to the van't Hoff equation, which indicated that hydrophobic and electrostatic interactions played the main role in the binding of daphnetin to HSA, in accordance with the results of calculations performed on a Silicon Graphics Ocatane2 workstation. In addition, the binding distance between daphnetin and HSA was obtained (4.02 nm) based on the Forster energy transfer theory.     

Sensitive flow-injection amperometric detection of iodide using Mn3+ and As3+.

A rapid, selective, and sensitive kinetic flow-injection method for iodide content determination with amperometric detection on a platinum electrode was developed. The method is based on the catalytic effect of iodide on the Mn3+ reaction with As3+ in the presence of sulfuric acid. The calibration curve was linear in the concentration range from 5.0 x 10(-7) to 1.0 x 10(-4) mol/L iodide. The limit of detection (LOD) was found to be 5.0 x 10(-9) mol/L iodide. The relative standard deviations (RSD) were 1.68% and 3.03% for 1.0 x 10(-3) mol/L standard and 1.0 x 10(-6) mol/L iodide solution (n = 6), respectively. The method has been successfully applied for determination of iodide in waters, table salts, fodder, organic substances and human blood sera. The results were compared with those obtained by a standard AOAC (Association of Official Analytical Chemists) method, as well as with those obtained by a kinetic spectrophotometric procedure for determination of iodide.     

Determination of myoglobin based on its enzymatic activity by stopped-flow spectrophotometry

A new method has been developed for the determination of myoglobin (Mb) based on its enzymatic activity for the oxidation of o-phenylenediamine (OPDA) with hydrogen peroxide. Stopped-flow spectrophotometry was used to study the kinetic behavior of the oxidation reaction. The catalytic activity of Mb was compared to other three kinds of catalyst. The time dependent absorbance of the reaction product, 2,3-diamimophenazine (DAPN), at a wavelength of 426 nm was recorded. The initial reaction rate obtained at 40 degrees C was found to be proportional to the concentration of Mb in the range of 1.0 x 10(-6) to 4.0 x 10(-9)mol L(-1). The detection limit of Mb was found to be 9.93 x 10(-10)mol L(-1). The relative standard deviations were within 5% for the determination of different concentrations of Mb. Excess of bovine serum albumin (BSA), Ca(II), Mg(II), Cu(II), glucose, caffeine, lactose and uric acid did not interfere.     

Direct determination of brucine by square wave voltammetry on 4-amino-2-mercaptopyrimidine self-assembled monolayer gold electrode

4-Amino-2-mercaptopyrimidine self-assembled monolayer (AMP SAMs/Au) was prepared on a gold electrode. The AMP SAMs/Au was characterized by using attenuated total reflection-fourier transform infrared (ATR-FTIR) and A.C. Impedance. The electrochemical behavior of brucine on AMP SAMs/Au was studied by cyclic voltammetry (CV) and square wave adsorptive stripping voltammetry (SWASV). The modified electrode showed an excellent electrocatalytic activity for the redox of brucine. The catalytic current increased linearly with the concentration of brucine in the range of 4.0 x 10(-7) to 2.0 x 10(-4) mol l(-1) by square wave voltammetry response. The detection limit was 6.0 x 10(-8) mol l(-1).     

Synthesis, structure, texture, and CO sensing behavior of nanocrystalline tin oxide doped with scandia

Weakly agglomerated nanocrystalline scandia doped tin oxide powders with high surface area (170-220 m(2)/g) and uniform size (3-4 nm) were synthesized for the first time by a two-step hydrothermal process in the presence of urea, followed by the calcination between 500 and 1200 degrees C. The structure and texture of the binary oxide system were characterized by thermogravimetry and differential thermal analysis, Brunauer-Emmett-Teller-specific surface area analysis, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. A metastable scandium tin oxide solid solution in tetragonal structure was formed for the scandia content lower than 6 mol % as the samples were calcined at 800 degrees C, and the excess Sc atoms were dispersed at the surface of the crystallites above this limit. The solid solution was metastable, so scandium migrated toward the surface region of the crystallites and produced a second phase of Sc(4)Sn(3)O(12) during calcining at high temperatures over 1000-1200 degrees C. In the case of the samples with higher dopant concentration (>15 mol %), the calcination at the temperature between 500 and 800 degrees C caused the precipitation of Sc(2)O(3), and the calcination over 1000-1200 degrees C led to the formation of more Sc(4)Sn(3)O(12). Textural analysis showed that doping an appropriate amount of Sc(2)O(3) into nanosized SnO(2) could effectively inhibit the grain growth and stabilize the surface area against high-temperature calcinations below 1000 degrees C. CO gas-sensing property measurements revealed that the dispersion of Sc at the surfaces of the SnO(2) nanocrystallites could improve the CO sensitivity significantly, and the pellet sample with scandia content of 10 mol % sintered at 800 degrees C showed the best CO gas-sensing property in the operation temperature range of 300-400 degrees C. On the basis of the structural and textural analysis, the correlation between the structure/texture and the sensitivity to CO for the as-calcined (SnO(2))(1-x)(Sc(2)O(3))(x) nanocrystallites has been established and explained.     

Direct determination of amino acids by pressurized capillary electrochromatography with chemiluminescence detection

A pressurized CEC (pCEC) coupled with on-column chemiluminescence (CL) detection was developed for direct determination of amino acids, which was based on the principle of an enhanced effect of Cu(II)-amino acid complexes on the CL reaction between luminol and hydrogen peroxide in alkaline solution. The effects of some important factors on pCEC separation and CL intensity were systemically investigated. Baseline separation of amino acids including L-histidine (L-His), L-threonine (L-Thr), and L-tyrosine (L-Tyr) was achieved by using a monolithic column with a mobile phase of 5.0x10(-3) mol/L phosphate buffer at pH 8.0 that contained 25% v/v methanol and 5.0x10(-4) mol/L luminol and 1.0x10(-5) mol/L Cu(II) at an applied voltage of -5 kV. The calibration curves of the analytes by plotting the peak height against corresponding concentration were linear over the range of 3.2x10(-6)-3.2x10(-4) mol/L for L-His, 4.1x10(-6)-4.1x10(-4) mol/L for L-Thr, and 6.0x10(-7)-3.0x10(-4) mol/L for L-Tyr. The LODs for L-His, L-Thr, and L-Tyr were 6.4x10(-7), 8.4x10(-7), and 3.0x10(-7) mol/L (S/N = 2), respectively. The proposed method was applied to the analysis of amino acid injection sample with satisfactory results. Mean recoveries for three amino acids were from 84.3 to 89.6%.     

Long-range cooperativity in molecular recognition of RNA by oligodeoxynucleotides with multiple C5-(1-propynyl) pyrimidines.

A heptamer composed of C5-(1-propynyl) pyrimidines (Y(p)'s) is a potent and specific antisense agent against the mRNA of SV40 large T antigen (Wagner, R. W.; Matteucci, M. D.; Grant, D.; Huang, T.; Froehler, B. C. Nat. Biotechnol. 1996, 14, 840-844). To characterize the role of the propynyl groups in molecular recognition, thermodynamic increments associated with substitutions in DNA:RNA duplexes, such as 5'-dCCUCCUU-3':3'-rGAGGAGGAAAU-5', have been measured by UV melting experiments. For nucleotides tested, an unpaired dangling end stabilizes unmodified and propynylated duplexes similarly, except that addition of a 5' unpaired rA is 1.4 kcal/mol more stabilizing on the propynylated, PODN:RNA, duplex than on the DNA:RNA duplex. Free energy increments for addition of single propynyl groups range from 0 to -4.0 kcal/mol, depending on the final number and locations of substitutions. A preliminary model for predicting the stabilities of Y(p)-containing hybrid duplexes is presented. Eliminating one amino group, and therefore a hydrogen bond, by substituting inosine (I) for guanosine (G), to give 5'-dC(p)C(p)U(p)C(p)C(p)U(p)U(p)-3':3'-rGAGIAGGAAAU-5', destabilizes the duplex by 3.9 kcal/mol, compared to 1.7 kcal/mol for the same change within the unpropynylated duplex. This 2.2 kcal/mol difference is eliminated by removing a single propynyl group three base pairs away. CD spectra suggest that single propynyl deletions within the PODN:RNA duplex have position-dependent effects on helix geometry. The results suggest long-range cooperativity between propynyl groups and provide insights for rationally programming oligonucleotides with enhanced binding and specificity. This can be exploited in developing technologies that are dependent upon nucleic acid-based molecular recognition.     

Determination of a cationic surfactant with naphthalene black 12B by the resonance light scattering technique

A new determination method for a cationic surfactant, zephiramine (Zeph), was developed with resonance light scattering (RLS) technique, based on the interaction of naphthalene black 12B with Zeph. The resonance light scattering (RLS) and UV characteristics of interaction between naphthalene black 12B and Zeph were studied. The RLS intensity of naphthalene black 12B at 363 nm was greatly enhanced in the presence of Zeph at pH 6.0. The enhanced RLS is proportional to the concentration of Zeph in the range of 3.20 x 10(-7) - 1.44 x 10(-5) mol L(-1). The limit of detection was 8.8 x 10(-8) mol L(-1). The proposed method was applied to the determination of Zeph in synthetic and spiked water samples with the recovery of 96.2-104% and RSD of 1.1-2.5%.