Thermodynamic investigations of methyl tert-butyl ether and water mixtures

Interactions between methyl tert-butyl ether (MTBE) and water have been investigated by scanning calorimetry, isothermal titration calorimetry, densitometry, IR-spectroscopy, and gas chromatography. The solubilization of MTBE in water at 25 °C at infinite dilution has ΔH° = -17.0 ± 0.6 kJ mol(-1); ΔS° = -80 ± 2 J mol(-1) K(-1); ΔC(p) = +332 ± 15 J mol(-1) K(-1); ΔV° = -18 ± 2 cm(3) mol(-1). The signs of these thermodynamic functions are consistent with hydrophobic interactions. The occurrence of hydrophobic interaction is further substantiated as IR absorption spectra of MTBE-water mixtures show that MTBE strengthens the hydrogen bond network of water. Solubilization of MTBE in water is exothermic whereas solubilization of water in MTBE is endothermic with ΔH° = +5.3 ± 0.6 kJ mol(-1). The negative mixing volume is explained by a large negative contribution due to size differences between water and MTBE and by a positive contribution due to changes in the water structure around MTBE. Henry's law constants, K(H), were determined from vapor pressure measurements of mixtures equilibrated at different temperatures. A van't Hoff analysis of K(H) gave ΔH(H)° = 50 ± 1 kJ mol(-1) and ΔS(H)° = 166 ± 5 J mol(-1) K(-1) for the solution to gas transfer. MTBE is excluded from the ice phase water upon freezing MTBE-water mixtures.     

Spectral behaviour of eosin Y in different solvents and aqueous surfactant media

Photophysical behaviour of the anionic xanthene dye, eosin Y (EY) was investigated in solvents of different polarities as well as in the presence of aqueous cationic surfactants. From the correlation between E(T)(30) and Kosower Z values of EY in different solvents, subsequent parameters for EY were determined in the presence of surfactants. A red shift, both in the absorption and emission spectra of EY, was observed with decreasing solvent polarity. Dimerisation of EY was found to be dependent on solvent polarity. Cationic surfactants retarded the process of dimerisation, which were evident from the lower dimerisation constant (K(D)) values, compared to that of in pure water. Dye-surfactant interaction constants were determined at different temperatures (298-318 K) and subsequently the thermodynamic parameters, viz., ΔG°, ΔH° and ΔS° were evaluated using the interaction constant values. The fluorescence spectra of EY followed the same trend as in the absorption spectra, although with lesser extents. Stokes shifts were calculated and correlated with the polarity of the medium. Fluorescence of EY was initially quenched by the cationic surfactants in their pre-micellar region, which then followed a red shift with intensity enhancement. Fluorescence quenching was found to be of Stern-Volmer type where the excited state lifetime of EY remained unchanged in different surfactant media. However, the anisotropy value of EY was changed in the post micellar region of surfactants.     

The experimental and theoretical QM/MM study of interaction of chloridazon herbicide with ds-DNA

We report a multispectroscopic, voltammetric and theoretical hybrid of QM/MM study of the interaction between double-stranded DNA containing both adenine-thymine and guanine-cytosine alternating sequences and chloridazon (CHL) herbicide. The electrochemical behavior of CHL was studied by cyclic voltammetry on HMDE, and the interaction of ds-DNA with CHL was investigated by both cathodic differential pulse voltammetry (CDPV) at a hanging mercury drop electrode (HMDE) and anodic differential pulse voltammetry (ADPV) at a glassy carbon electrode (GCE). The constant bonding of CHL-DNA complex that was obtained by UV/vis, CDPV and ADPV was 2.1×10(4), 5.1×10(4) and 2.6×10(4), respectively. The competition fluorescence studies revealed that the CHL quenches the fluorescence of DNA-ethidium bromide complex significantly and the apparent Stern-Volmer quenching constant has been estimated to be 1.71×10(4). Thermal denaturation study of DNA with CHL revealed the ΔTm of 8.0±0.2°C. Thermodynamic parameters, i.e., enthalpy (ΔH), entropy (ΔS°), and Gibbs free energy (ΔG) were 98.45 kJ mol(-1), 406.3 J mol(-1) and -22.627 kJ mol(-1), respectively. The ONIOM, based on the hybridization of QM/MM (DFT, 6.31++G(d,p)/UFF) methodology, was also performed using Gaussian 2003 package. The results revealed that the interaction is base sequence dependent, and the CHL has more interaction with ds-DNA via the GC base sequence. The results revealed that CHL may have an interaction with ds-DNA via the intercalation mode.     

Fluorimetric investigation of supramolecular system by modified β-cyclodextrin and its analytical application

The supramolecular interaction of MAH-β-cyclodextrin (MAH-β-CD, a modified β-cyclodextrin carrying seven vinyl carboxylic acid groups) and meferamic acid (MF) has been studied by spectrofluorimetry. The results showed that MAH-β-CD reacted with MF to form a host-guest complex (MAH-β-CD-MF) with stoichiometry (1:1) and the inclusion constant (K=7.15×10(2) L/mol) was ascertained by the typical double reciprocal plots. From the phase-solubility diagram, an increase in the water solubility of the drug was observed and the apparent stability constant (K(1:1)) was calculated to be 8.62×10(2) L/mol. Furthermore, the thermodynamic parameters (ΔG°, ΔH° and ΔS°) of MAH-β-CD-MF were obtained and the inclusion mechanism was also preliminarily discussed. In order to further confirm the experimental results, investigation on the molecular modeling was performed. On the basis of the significant enhancement of the fluorescence intensity of MF, a spectrofluorimetric method for MF determination in bulk aqueous solution in the presence of MAH-β-CD was developed. The linear range was 2.00×10(-8)-9.00×10(-5) mol/L and the detection limit was 3.36×10(-9) mol/L. The proposed method was successfully applied to determine MF in tablets, serum and urine with the satisfactory result.     

Kinetic study of the thermal inactivation of glucose oxidase in the presence of denaturant and stabilizer by means of bioelectrocatalysis method

The thermal inactivation of glucose oxidase (GOD) in aqueous solution has been studied by the electrochemical method to follow the bioelectrocatalytic current due to the oxidation of glucose by GOD. Exponential time-dependent decrease in bioelectrocatalytic current, that is, the decrease in the enzymatic activity of GOD, was observed at given temperatures to determine the rate constant (k) of a simple inactivation process: GOD (active) → GOD (inactive). The ln[k] vs. T(-1) plots gave straight lines with all solution conditions tested, so that the resulting Arrhenius activation parameters including ΔH(?) and ΔS(?) can be compared with each other. In the 50 mmol/L phosphate buffer at 70°C, k was determined to be (6.6 ± 1.6)× 10(-4) s(-1), and ΔH(?) and ΔS(?) were calculated to be 202 ± 13 kJ mol(-1) and 282 ± 39 J K(-1) mol(-1), respectively. By addition of 3 mol/L guanidine hydrochloride, the k was increased to (4.7 ± 0.6)× 10(-3) s(-1), indicating that the denaturant accelerates the thermal inactivation. In this case, ΔH(?) was significantly reduced. By addition of 1 g/L ε-poly-L-lysine, which may adsorb onto the GOD surface to reduce the local disorder, k was decreased to (1.8 ± 0.6)× 10(-4) s(-1). In this case, ΔS(?) was reduced but ΔH(?) was not decreased much. This can be used as an important indication for selection of the enzyme stabilizer in solution.     

An X-ray and neutron scattering study of the equilibrium between trimethylamine N-oxide and urea in aqueous solution

The interaction of the osmolytes trimethylamine N-oxide (TMAO) and urea in aqueous solutions at 40 °C was investigated by isotopic substitution neutron scattering at a TMAO mole fraction of 0.05 and TMAO/urea concentration ratios of 1?:?2 and 1?:?4. The partial pair distribution functions obtained by the empirical potential structure refinement method are consistent with those obtained previously for similar pure TMAO and 1?:?1 TMAO-urea solutions and indicate that urea progressively replaces the water molecules in the first coordination shell of the TMAO oxygen atom. The apparent association constant for the TMAO?:?urea complex (K(1)) was calculated to be 0.14 M(-1), which is of the same order as the experimental urea-protein binding constants per site reported in the literature. This confirms that the two osmolytes act independently at least in the physiological range.     

A FLUORESCENCE STUDY OF THE BINDING OF HOECHST 33258 and DAPI TO HALOGENATED DNAs

We have studied the time-resolved and the steady-state fluorescence of the DNA groove binders 4',6-diamidino-2-phenylindole (DAPI) and Hoechst 33258 with the double stranded DNAs poly(dA-dU) and poly(dI-dC) and their halogenated analogs, poly(dA-I5dU) and poly(dI-Br5dC). These studies were prompted by earlier observations that steady-state fluorescence of Hoechst 33258 is quenched on binding to halogenated DNAs (presumably due to an intermolecular heavy atom effect involving the halogen atom in the major groove), and recent studies which clearly point to a binding-site in the minor groove of DNA. Measurements of the time resolved fluorescence decay demonstrate that the fluorescence of Hoechst 33258 is quenched on binding to the halogenated DNAs, in agreement with previous observations. However, quenching studies carried out using the free halogenated bases IdUrd and BrdCyd in solution yielded bimolecular rate constants more than one order of magnitude larger than those expected for an intermolecular heavy atom effect. Moreover, the quenching of the Hoechst 33258 fluorescence was accompanied by an accelerated photochemical destruction of Hoechst 33258. We therefore conclude that the fluorescence quenching observed with halogenated DNAs is probably due to a photochemical reaction involving Hoechst 33258, rather than direct contact of Hoechst 33258 with the halogen substituents in the major groove of the DNA. The fluorescence decay measurements however, do provide clear evidence for at least two different modes of binding. Taking into account the alternating sequences used in this study and the possibility of two different conformations for bound dye, at least four different modes of binding are plausible. Our present data do not allow us to distinguish between these alternatives. The time-resolved fluorescence decays and fluorescence quantum yields of DAPI are not affected by the presence of the heavy atom substituents in the DNA major groove. Based on this observation and earlier reports that DAPI binds in one of the DNA grooves, we conclude that the high affinity sites for DAPI on DNA are located in the minor groove.     

Solution behavior of iron(III) and iron(II) porphyrins in DMSO and reaction with superoxide. Effect of neighboring positive charge on thermodynamics, kinetics and nature of iron-(su)peroxo product

The solution behavior of iron(III) and iron(II) complexes of 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5,10,15,20-tetraphenylporphyrin (H(2)tBuTPP) and the reaction with superoxide (KO(2)) in DMSO have been studied in detail. Applying temperature and pressure dependent NMR studies, the thermodynamics of the low-spin/high-spin equilibrium between bis- and mono-DMSO Fe(II) forms have been quantified (K(DMSO) = 0.082 ± 0.002 at 298.2 K, ΔH° = +36 ± 1 kJ mol(-1), ΔS° = +101 ± 4 J K(-1) mol(-1), ΔV° = +16 ± 2 cm(3) mol(-1)). This is a key activation step for substitution and inner-sphere electron transfer. The superoxide binding constant to the iron(II) form of the studied porphyrin complex was found to be (9 ± 0.5) × 10(3) M(-1), and does not change significantly in the presence of the externally added crown ether in DMSO (11 ± 4) × 10(3) M(-1). The rate constants for the superoxide binding (k(on) = (1.30 ± 0.01) × 10(5) M(-1) s(-1)) and release (k(off) = 11.6 ± 0.7 s(-1)) are not affected by the presence of the external crown ether in solution. The resulting iron(II)-superoxide adduct has been characterized (mass spectrometry, EPR, high-pressure UV/Vis spectroscopy) and upon controlled addition of a proton source it regenerates the starting iron(II) complex. Based on DFT calculations, the reaction product without neighboring positive charge has iron(II)-superoxo character in both high-spin side-on and low-spin end-on forms. The results are compared to those obtained for the analogous complex with covalently attached crown ether, and more general conclusions regarding the spin-state equilibrium of iron(II) porphyrins, their reaction with superoxide and the electronic structure of the product species are drawn.     

Facile synthesis of non-ionic dimeric molecular resonance imaging contrast agent: its relaxation and luminescence studies

A bis-polyazamacrocycle, 10'-bis(acetamido)ethane-bis[1,4,7-tri(carboxymethane)-1,4,7,10-tetraazacyclododecane] (DO3A-AME-DO3A) was synthesized for application in magnetic resonance imaging. The efficacy of DO3A-AME-DO3A as non ionic magnetic contrast agent was tested by performing relaxometric studies on its gadolinium complex. The longitudinal relaxivity, r(1) and transverse relaxivity, r(2) values were found to be 5.84 mM(-1)s(-1) and 6.82 mM(-1)s(-1), per Gd(III) at pH 7.0, 37 °C. The luminescence properties of europium complex of DO3A-AME-DO3A were investigated in aqueous medium. The lifetime of Eu(2)-DO3A-AME-DO3A in water was found to be 0.786 ms. Emission and luminescence lifetime measurements on the europium complex of DO3A-AME-DO3A gives a hydration number of q = 1.9. The reaction enthalpy and entropy were found to be, ΔH(0) = -(6.2 ± 2) kJ mol(-1), ΔS(0) = - (1.8 ± 0.4) kJ mol(-1)K(-1), and K(Eu)(298) = (1.8 ± 0.1).     

Photoreversible transformation between seconds and hours time-scales: threading of pillar[5]arene onto the azobenzene-end of a viologen derivative

Photoswitching of the transformation between seconds and hours time-scales is demonstrated using the threading of per-hydroxylated pillar[5]arene onto the azobenzene-end of a viologen derivative. When the azobenzene moiety was in the trans form, the threading of per-hydroxylated pillar[5]arene quickly took place at 25 °C and could not be monitored directly. The exchange rate (k) and half-life time (t(1/2)) examined by 2D EXSY NMR spectroscopy were found to be 0.209 ± 0.013 s(-1) and 3.33 ± 0.21 s, respectively. In contrast, the cis form of the azobenzene moiety required very long time (k = 2.14 ± 0.27 × 10(-5) s(-1), t(1/2) = 9.13 ± 1.2 h) to thread per-hydroxylated pillar[5]arene at 25 °C. Photoisomerization from the trans to the cis form generated the following increment of free energy of activation at 25 °C: ΔG(in)(?) (cis form) - ΔG(in)(?) (trans form) = 22.8 ± 0.24 kJ mol(-1), which led to the time-scale transformation. The tuning of the threading was also accomplished by heating/cooling: the rate constants increased on heating and decreased on cooling.     

二氯苯的蒸发焓和升华焓测定

在进行分子内非键合相互作用的热化学研究中,我们已用燃烧量热法测定了各种取代的氯代苯在凝聚态的标准生成焓。为了排除分子间的相互作用,以得到准确的气态生成焓,需要这些化合物的标准蒸发焓或升华焓。文献中这类数据不多,且相当一部分是用测量蒸气压随温度变化按Clapeyron-Clausius方程计算的,不如直接量热法准确。对高取代的氯代苯,由于蒸气压很低,实验困难,数据更缺。为此,我们用量热法测定了三个二氯苯异构体的蒸发焓和升华焓,并结合已有的文献数据,建立了氯代苯标准蒸发焓和升华焓与其沸点的线性关系,估算了其余未知的数据。     

7Li NMR studies on complexation reactions of lithium ion with cryptand C211 in ionic liquids: comparison with corresponding reactions in nonaqueous solvents

(7)Li NMR spectra of DEME-TFSA [DEME=N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium; TFSA=bis(trifluoromethanesulfonyl)amide], EMI-TFSA (EMI=1-ethyl-3-methylimidazolium), MPP-TFSA (MPP = N-methyl-N-propylpyridinium), DEME-PFSA [PFSA=bis(pentafluoroethanesulfonyl)amide], and DEME-HFSA [HFSA=bis(heptafluoropropanesulfonyl)amide] ionic liquid (IL) solutions containing LiX (X=TFSA, PFSA, or HFSA) and C211 (4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane) were measured at various temperatures. As a result, it was found that the uncomplexed Li(I) species existing as [Li(X)(2)](-) in the present ILs exchange with the complexed Li(I) ([Li·C211](+)) and that the exchange reactions proceed through the bimolecular mechanism, [Li·C211](+) + [*Li(X)(2)](-)=[*Li·C211](+) + [Li(X)(2)](-). Kinetic parameters [k(s)/(kg m(-1) s(-1)) at 25 °C, ΔH(++)/(kJ mol(-1)), ΔS(++)/(J K(-1) mol(-1))] are as follows: 5.57×10(-2), 69.8 ± 0.4, and -34.9 ± 1.0 for the DEME-TFSA system; 5.77×10(-2), 70.6 ± 0.2, and -31.9 ± 0.6 for the EMI-TFSA system, 6.13×10(-2), 69.0 ± 0.3, and -36.7 ± 0.7 for the MPP-TFSA system; 1.35 × 10(-1), 65.2 ± 0.5, and -43.1 ± 1.4 for the DEME-PFSA system; 1.14×10(-1), 64.4 ± 0.3, and -47.1 ± 0.6 for the DEME-HFSA system. To compare these kinetic data with those in conventional nonaqueous solvents, the exchange reactions of Li(I) between [Li·C211](+) and solvated Li(I) in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were also examined. These Li(I) exchange reactions were found to be independent of the concentrations of the solvated Li(I) and hence proposed to proceed through the dissociative mechanism. Kinetic parameters [k(s)/s(-1) at 25 °C, ΔH(++)/(kJ mol(-1)), ΔS(++)/(J K(-1) mol(-1))] are as follows: 1.10 × 10(-2), 68.9 ± 0.2, and -51.3 ± 0.4 for the DMF system; 1.13×10(-2), 76.3 ± 0.3, and -26.3 ± 0.8 for the DMSO system. The differences in reactivities between ILs and nonaqueous solvents were proposed to be attributed to those in the chemical forms of the uncomplexed Li(I) species, i.e., the negatively charged species ([Li(X)(2)](-)) in ILs, and the positively charged ones ([Li(solvent)(n)](+)) in nonaqueous solvents.     

Study binding of Al-curcumin complex to ds-DNA, monitoring by multispectroscopic and voltammetric techniques

In this work a complex of Al3+ with curcumin ([Al(curcumin) (EtOH)2](NO3)2) was synthesized and characterized by UV-vis, FT-IR, elemental analysis and spectrophotometric titration techniques. The mole ratio plot revealed a 1:1 complex between Al3+ and curcumin in solution. For binding studies of this complex to calf thymus-DNA various methods such as: UV-vis, fluorescence, circular dichroism (CD), FT-IR spectroscopy and cyclic voltammetry were used. The intrinsic binding constant of ACC with DNA at 25°C was calculated by UV-vis and cyclic voltammetry as 2.1×10(4) and 2.6×10(4), respectively. The thermodynamic studies showed that the reaction is enthalpy and entropy favored. The CD results showed that only the Δ-ACC interacts with DNA and the Δ-ACC form has not any tendency to interact with DNA, also the pure curcumin has not any stereoselective interaction with CT-DNA. Fluorimetric studies showed that fluorescence enhancement was initiated by a static process in the ground state. The cyclic voltammetry showed that ACC interact with DNA with a binding site size of 2. From the FT-IR we concluded that the Δ-ACC interacts with DNA via partial electrostatic and minor groove binding. In comparison with previous works it was concluded that curcumin significantly reduced the affinity of Al3+ to the DNA.     

Molecular dynamics study of the hydration of the hydroxyl radical at body temperature

Classical molecular dynamics (MD) simulation of ˙OH in liquid water at 37 °C has been performed using flexible models of the solute and solvent molecules. We derived the Morse function describing the bond stretching of the radical and the potential for ˙OH-H(2)O interactions, including short-range interactions of hydrogen atoms. Scans of the potential energy surface of the ˙OH-H(2)O complex have been performed using the DFT method with the B3LYP functional and the 6-311G(d,p) basis set. The DFT-derived partial charges, ±0.375e, and the equilibrium bond-length, 0.975 ?, of ˙OH resulted in the dipole moment of 1.76 D. The radical-water radial distribution functions revealed that ˙OH is not built into the solvent structure but it rather occupies distortions or cavities in the hydrogen-bonded network. The solvent structure at 37 °C has been found to be the same as that of pure water. The hydration cage of the radical comprises 13-14 water molecules. The estimated hydration enthalpy -42 ± 5 kJ mol(-1) is comparable with the experimental value -39 ± 6 kJ mol(-1) for 25 °C. Inspection of hydrogen bonds showed the importance of short-range interaction of hydrogen atoms and indicated that neglect of the angular condition greatly overestimates the number of the H-acceptor radical-water bonds. The mean number ?n = 0.85 of radical-water H-bonds has been calculated using geometric definition of H-bond and ?n = 0.62 has been obtained when the energetic condition, E(da)≤-8 kJ mol(-1), was additionally considered. The continuous lifetimes of 0.033 ps and 0.024 ps have been estimated for the radical H-donor and the H-acceptor bonds, respectively. Within statistical uncertainty the radical self-diffusion coefficient, (2.9 ± 0.6) × 10(-9) m(2) s(-1), is the same as (3.1 ± 0.5) × 10(-9) m(2) s(-1) calculated for water in solution and in pure solvent. To the best of our knowledge, this is the first study of the ˙OH(aq) properties at a biologically relevant body temperature.     

Supramolecular assembly of Hoechst-33258 with cucurbit[7]uril macrocycle

Molecular assemblies of potential guest molecules through non-covalent host-guest interactions have found immense use in many applied areas. In this study supramolecular interaction of a biologically important dye Hoechst-33258 (H33258) has been investigated in aqueous solutions at different pHs, in the presence of a macrocyclic host, namely, cucurbit[7]uril (CB7). The pH dependent emission behaviour of H33258 is inherently connected with its protolytic equilibria which allow the dye in different geometrical conformations. This pH dependent structural orientation is greatly affected by the complexation with CB7. The significant structural changes in the monocationic H33258 brought out by CB7 at pH 7 have been documented in the fluorescence emission and lifetime data, which are comparatively less affected in case of the dicationic form, which is prominent in dye solutions at pH 4.5. The strong ion-dipole interactions provided by the carbonyl portals of the CB7 host adequately stabilize the CB7-H33258 complex, both in 1:1 and 2:1 stoichiometries at both the pH conditions. The Job's plot method, fluorescence anisotropy, NMR measurements and geometry optimization calculations confirm the stoichiometric arrangement and are found to be tunable with the addition of metal ions. The non-covalently stabilized assembly brings out large enhancement in the fluorescence emission due to the unique structural orientation attained by H33258, which reduces the non-radiative relaxation pathways. Comparison of the spectral data of the dye at different pH conditions in the absence and presence of CB7 proposes a large upward pK(a) shift due to CB7 encapsulation, thus providing a handy tool to modulate the photophysical characteristics of the guest molecules.     

NMR study of ligand exchange and electron self-exchange between oxo-centered trinuclear clusters [Fe3(μ3-O)(μ-O2CR)6(4-R'py)3](+/0)

The syntheses, single crystal X-ray structures, and magnetic properties of the homometallic μ?-oxo trinuclear clusters [Fe?(μ?-O)(μ-O?CCH?)?(4-Phpy)?](ClO?) (1) and [Fe?(μ?-O)(μ-O?CAd)?(4-Mepy)?](NO?) (2) are reported (Ad = adamantane). The persistence of the trinuclear structure within 1 and 2 in CD?Cl? and C?D?Cl? solutions in the temperature range 190-390 K is demonstrated by 1H NMR. An equilibrium between the mixed pyridine clusters [Fe?(μ?-O)(μ-O?CAd)?(4-Mepy)(3-x)(4-Phpy)(x)](NO?) (x = 0, 1, 2, 3) with a close to statistical distribution of these species is observed in CD?Cl? solutions. Variable-temperature NMR line-broadening made it possible to quantify the coordinated/free 4-Rpy exchanges at the iron centers of 1 and 2: k(ex)2?? = 6.5 ± 1.3 × 10?1 s?1, ΔH(?) = 89.47 ± 2 kJ mol?1, and ΔS(?) = +51.8 ± 6 J K?1 mol?1 for 1 and k(ex)2?? = 3.4 ± 0.5 × 10?1 s?1, ΔH(?) = 91.13 ± 2 kJ mol?1, and ΔS(?) = +51.9 ± 5 J K?1 mol?1 for 2. A limiting D mechanism is assigned for these ligand exchange reactions on the basis of first-order rate laws and positive and large entropies of activation. The exchange rates are 4 orders of magnitude slower than those observed for the ligand exchange on the reduced heterovalent cluster [Fe(III)?Fe(II)(μ?-O)(μ-O?CCH?)?(4-Phpy)?] (3). In 3, the intramolecular Fe(III)/Fe(II) electron exchange is too fast to be observed. At low temperatures, the 1/3 intermolecular second-order electron self-exchange reaction is faster than the 4-Phpy ligand exchange reactions on these two clusters, suggesting an outer-sphere mechanism: k?2?? = 72.4 ± 1.0 × 103 M?1 s?1, ΔH(?) = 18.18 ± 0.3 kJ mol?1, and ΔS(?) = -90.88 ± 1.0 J K?1 mol?1. The [Fe?(μ?-O)(μ-O?CCH?)?(4-Phpy)?](+/0) electron self-exchange reaction is compared with the more than 3 orders of magnitude faster [Ru?(μ?-O)(μ-O?CCH?)?(py)?](+/0) self-exchange reaction (ΔΔG(exptl)(?298) = 18.2 kJ mol?1). The theoretical estimated self-exchange rate constants for both processes compare reasonably well with the experimental values. The equilibrium constant for the formation of the precursor to the electron-transfer and the free energy of activation contribution for the solvent reorganization to reach the electron transfer step are taken to be the same for both redox couples. The larger ΔG(exptl)(?298) for the 1/3 iron self-exchange is attributed to the larger (11.1 kJ mol?1) inner-sphere reorganization energy of the 1 and 3 iron clusters in addition to a supplementary energy (6.1 kJ mol?1) which arises as a result of the fact that each encounter is not electron-transfer spin-allowed for the iron redox couple.     

Fluorescence spectroscopic study of dip coated sol-gel thin film internal environment using fluorescent probes Hoechst33258 and Pyranine

The characterization of physicochemical properties of the internal environment of sol-gel thin films is required for understanding and designing applications in optical biosensors. We have investigated the dip coated tetraethyl-orthosilicate (TEOS) derived sol-gel thin films deposited on microscopic glass cover slips using molar ratio (water or ethanol / TEOS) R=32 using fluorescence spectroscopic measurements (emission, lifetime and anisotropy) on entrapped fluorescent probes. The effect of water and/or ethanol was studied as a function of storage (60 days) using fluorescent probes Hoechst 33258 (H258) and Pyranine (PY). Distribution of fluorescent probes in thin film was studied using confocal microscope. Emission maxima of H258 entrapped thin films from sol prepared using water as solvent showed emission maximum at 503 nm indicating the presence of water like environment which did not change during storage. On the contrary, PY entrapped thin films depicted emission bands at 434 nm and 513 nm, characteristics of ethanol and water respectively, up to the first few weeks and then the band at 434 nm prevailed (60 days), suggesting heterogeneous internal environment. Thin films from sol prepared using ethanol as solvent showed presence of ethanol through out storage. Fluorescence lifetime data of these probes in both sol-gel and thin films also suggested presence of heterogeneous internal environment. Thin films prepared from sol-gel using water as solvent suggested release of ethanol in the pores during hydrolysis and condensation reaction, which were clearly indicated by PY. The effect of sodium phosphate buffer was also studied in sol-gel and thin films. The results of these measurements showed that both the probes H258 and PY could be used effectively in monitoring the physicochemical properties of internal environment of thin films and sol-gel as a function of storage.     

Study on the interaction between salvianic acid A sodium and bovine serum albumin by spectroscopic methods

The interaction between salvianic acid A sodium (SAS) and bovine serum albumin (BSA) was investigated using fluorescence and ultraviolet spectroscopy at different temperatures under imitated physiological conditions. The experimental results showed that the fluorescence of BSA was quenched by SAS through a static quenching procedure. The binding constants of SAS with BSA were 2.03, 1.17 and 0.71×10(5) L mol(-1) at 291, 298 and 305 K, respectively. Negative values of ΔG, ΔH, and ΔS indicate that the interaction between SAS and BSA is driven by hydrogen bonds and van der Waals forces. According to F?rster non-radiation energy transfer theory, the binding distance between BSA and SAS was calculated to be about 2.92 nm. The effect of SAS on the conformation of BSA was analyzed using synchronous fluorescence spectroscopy. In addition, the effect of some metal ions Cu(2+), Ca(2+), Mg(2+), and Zn(2+) on the binding constant between SAS and BSA was examined.     

Micelle formation of Tyr-Phe dipeptide and Val-Tyr-Val tripeptide in aqueous solution and their influence on the aggregation of SDS and PEO-PPO-PEO copolymer micelles

The aggregation properties of Tyr-Phe dipeptide and Val-Tyr-Val tripeptide were studied in aqueous solution and in the presence of SDS and SDS-polymer environments using UV-visible, surface tension, fluorescence and circular dichroism (CD) techniques. Both the peptides formed micelles. The cmc values obtained for dipeptide and tripeptide are 2×10(-5) and 4×10(-5) M, respectively in aqueous solution at 25°C. The presence of additives (SDS and polymer) hindered the micelle formation of peptides. The cmc values obtained by various methods are in good agreement with each other. Effect of peptides on the aggregation properties of SDS also was investigated. The cmc of SDS was decreased in presence of peptides and were reduced with increase in temperature. Using monophasic micellization concept, the association constant (K(A)) for the SDS-peptide mixed micellar systems was determined. Using biphasic model, the thermodynamic parameters viz; ΔG°(m), ΔH°(m) and ΔS°(m) for SDS-water and SDS-peptide-water mixed micellar systems, the standard free energy for transfer of SDS from aqueous to peptide additive environments were estimated at various temperatures. These results suggest that the SDS is more stable in micellized form in the SDS-water-peptide ternary systems compared to the situation in the corresponding SDS-water binary systems.     

Rate coefficients for the gas-phase reaction of hydroxyl radicals with 2-methoxyphenol (guaiacol) and related compounds

2-Methoxyphenol (guaiacol) and its derivatives are potential marker compounds for wood smoke emissions in the atmosphere. To investigate the atmospheric reactivity of this type of compounds, rate coefficients for their reactions with hydroxyl (OH) radicals have been determined at 294 ± 2 K and 1 atm using the relative rate method with gas chromatography for chemical analysis. The rate coefficients (in units of cm3 molecule?1 s?1) are: 2-methoxyphenol, (7.53 ± 0.41) × 10?11; 3-methoxyphenol, (9.80 ± 0.46) × 10?11; 4-methoxyphenol, (9.50 ± 0.55) × 10?11; 2-methoxy-4-methylphenol, (9.45 ± 0.59) × 10?11; and methoxybenzene, (2.20 ± 0.15) × 10?11. The estimated atmospheric lifetime for 2-methoxyphenol is ~2 h, indicating that it is too reactive to be used as a tracer for wood smoke emissions. The reactivity of the methoxyphenols is compared with other substituted aromatics and interpreted in relation to the type, number, and positions of the different substituents on the aromatic ring. The atmospheric implications of the reactions are also discussed.