2-(6,7-Dimethoxy-2,3-Naphthalimido)ethyl Trifluoromethanesulfonate as a Labeling Reagent for Carboxylic Acids in Liquid Chromatography

A new, highly reactive and sensitive labeling reagent, 2-(6,7-dimethoxy-2,3-naphthalimido)ethyl trifluoromethanesulfonate, (6,7-DMNE-OTf), has been synthesized and used to determine several carboxylic acids by high-performance liquid chromatography. Reactions of the reagent with long-chain, saturated monocarboxylic acids proceed to completion within 5 min in acetone at room temperature in the presence of 18-crown-6 as a catalyst. The acid derivatives could be readily eluted with acetonitrile-water (90:10). The detection limits (signal-to-noise RATIO = 3) with ultraviolet and fluorescence detection are 10 fmol (λ_max = 282 nm) and 3 fmol (λ_ex = 282 nm, λ_em = 456 nm), respectively.     

The fluorescence of biliverdin dimethyl ester

Freshly prepared solutions of biliverdin dimethyl ester ( 2 ) in ethanol showed fluorescence maxima at 710 and 770 nm [Φ_F = 1.1. 10^?4 (room temperature) and 5.0 10^?4 (77 K)]. The maxima of monoprotonated 2 at 77 K were shifted to 725 and 806 nm and the quantum yield was increased to 2.6. 10^?2. This acid effect was reversible by neutralization with base. When a neutral solution was kept standing in the dark at room temperature, or when an acidic solution was neutralized by base, an additional fluorescence maximum at 500 nm with a mirror image excitation spectrum with λ_max = 470 nm developed, which disappeared on addition of acid and which is attributed to a chemical change of 2 .     

Thermodynamique de composes azotes III. Etude Thermochimique de la glycine et de la l-α-alanine

The enthalpies of formation of glycine and L-α-alanine are determined by two complementary techniques: combustion calorimetry and change of state (sublimation) calorimetry.

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The values of the experimental and theoretical energy of conjugation of these two molecules (partially conjugated) and the energetical value of the C---N bond in Laidler's scheme, derived from the experimental enthalpy of atomization, are also given.

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Résumé

Les enthalpies de combustion à létat condensé de la glycine et de la L-α-alanine ont été déterminées à d'un calorìmetre isopéribolique à bombe rotative. Leurs valeurs à l'état standard à 298,15 K sont: δH°_c = −(972,98±0,19) kJ mol⁻¹ pour la glycine et ΔHδ_c = −(1621,45 − 0,48) kJ mol^-1 pour la L-α-alanine. Les enthalpies de sublimation à l'état standard à 298,15 K ont été obtenues par calori-     

Time-resolved emission studies of poly(n-v1nyl carbazole) using pulsed laser excitation

Time-resolved fluorescence spectroscope of poly(N-vinyl carbazole) in tetrahydrofuran solution has revealed the presence of three emitting species, namely a normal excimer emitting at λ_max = 420 nm, a higher-energy excimer site at λ_max = 370 nm which interconverts with the “normal” site, and a third species which emits at 370 nm but not at 420 nm. and which does not convert to the normal excimer. The identity of this species, and the kinetics of fluorescence of the polymer, are discussed.     

Colorimetric determination of palladium (II) by means of promazine hydrochloride

In acetate buffer medium palladium(II) ions form with promazine hydrochloride (PM) two complexes: an orange one of a formula [Pd(C₁₇H₂₀N₂S)]²⁺ (λ_max = 460 nm, ε = 4.5 × 10³, at 20 °C and pH = 2) and a violet one of a formula [Pd(C₁₇H₂₀N₂S)₂]²⁺ (λ_max = 540 nm, ε = 8.8 × 10³ at 20 °C and pH = 2).The values for instability constants determined by Bjerrum's method amount to pK₁ = 3.95; pK₂ = 3.07; pβ₁ = 3.95; pβ₂ = 7.02, respectively.A colorimetric method of the determination of palladium(II) has been elaborated. The method consists in a measurement of the absorbance of the violet complex of palladium(II) with promazine hydrochloride at λ = 540 nm. The method permits the determination of 2–17 μg Pd/ml with an error of ±2%. The time of the determination is 20 min. Iron(III), Ce(IV), Pt(IV), V(V), Cr(VI), and HNO₃ interfere with the determination.     

Kinetic studies of NHx radical reactions

Combined experimental and theoretical studies of the reactions

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and

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were carried out in the present work. The rate constant of reaction (1) was investigated in the temperature range 293 K T612 K using the laser photolysis continuous-wave laser-induced fluorescence technique for the production and detection of NH₂. The results are well described by k₁(T)=5.43×T^−4.02 exp(−1034K/T) cm³ molecule⁻¹ s⁻¹. Stationary points on the potential energy surface were characterized using the gaussian-2 (G2) ab initio method. The surface is complex, with hydrogen-atom transfers and cis—trans isomerization connecting five stable adduct species.The product distribution of reaction (2) was studied at room temperature using the discharge flow technique with mass spectrometric detection of the reaction products. Measured branching fractions for the production of N₂O+H(D) were k_2a/k₂ = 0.84±0.4 for NH+NO and k_2a/k₂ = 0.87±0.17 for ND+NO. Stationary points on the ground ²A′ surface were calculated using the G2 method. The transition state energy for the dissociation of the cis isomer into H+N₂O was found to be lower than the transition state energy for dissociation into OH+N₂. Additionally, trans-HNNO was found to isomerize to cis-HNNO before dissociation.For reaction (3), the molecular properties of all relevant intermediates and transition states on the ground state potential energy surface were determined using the G2 method. The results predict the formation of three intermediates, H₂NO, trans-HNOH and cis-HNOH, all exothermic relative to the reactants. The transition states separating these intermediates from one another and their products (H₂+NO, H+HNO(¹A′) or NH+OH) were also characterized, several for the first time.     

Photoluminescence of biphenyl-containing polymers

Poly(4-vinylbiphenyl) and copolymers of methyl methacrylate and 4-vinylbiphenyl show both monomeric (λ_max = 325 nm) and excimer (λ_max = 380 nm) fluorescence. The quantum yield of excimer emission increases and the monomeric emission decreases with increase in the fraction of vinylbiphenyl units in the copolymer. The decrease of the monomeric emission is closely related to a decrease in singlet lifetime. These results are interpreted in terms of a kinetic controlled excimer formation. Comparison of the emission in the homo and copolymers with that of the dimeric model compound shows that excimer formation in the polymer strongly depends upon the possibility of energy migration along sequences of vinylbiphenyl units. This conclusion is considered as of particular relevance due to the change in geometry of the biphenyl unit upon excitation.     

Characterizations of iron-rich clay modified electrodes and their applications in optical recognition

Optical recognition was studied with modified electrodes based on iron-rich clays. According to energy-dispersive X-ray spectroscopy (EDX), several clay minerals including montmorillonite K10 (mont. K10) contain a lot of iron. Cyclic voltammetry suggested that electrochemically active iron species exist in most iron-rich clays and are likely to reside at different sites. The associated electrochemical activity is strongly pH-dependent and photosensitive. Under UV irradiation (λ ≤ 420 nm), these iron species were activated, and a pronounced photocurrent resulted. When these electrodes were flow-injected with 2-pyridylcarboxylic acids (λ_ab,max = 260 nm), the originally monotonic photocurrent could be modulated into a more recognizable a.c. pattern and the 260 nm optical signal became distinguishable. The photoresponse was highly reproducible, and the response time (t₉₀) was less than 10s.     

Normal and second excimers in macromolecules—I: Poly(1-methoxy-4-vinylnaphthalene) in fluid solution

The emission properties of poly(1-methoxy-4-vinylnaphthalene) (PMVN) in 2-methyltetrahydrofuran solution have been investigated over the range 77–350 K. It exhibits, in addition to the monomer fluorescence (348 and 360 nm), two structureless emissions derived from two different types of excimer, viz. the normal excimer (420 nm) and the second excimer (380 nm), the latter having a partially overlapping structure of aromatic rings. The intensity of second excimer emission of PMVN increases with decreasing temperature, while the normal excimer emission shows a maximum at 215 K. Kinetic analysis of transient decay curves for the fluorescence of PMVN gave results consistent with the previously reported kinetic scheme for the dimer model, 1,3-bis(4-methoxy-1-naphthyl)propane, showing that the second excimer is not formed from or converted to the normal excimer and that two types of excimer are formed independently from the excited monomer.     

Spectrophotometric determination of molybdenum by extraction of its thiosulphate complex

A simple and rapid spectrophotometric determination of molybdenum is described. The molybdenum thiosulphate complex is extracted into isoamyl alcohol from 1·0–1·5M hydrochloric acid containing 36–40 mg of Na₂S₂O₃·5H₂O per ml. The absorbance at λ_max = 475 nm obeys Beer's law over the range 0–32 μg of Mo per ml of solvent phase. Up to 5 mg/ml of Ti(IV), V(V), Cr(VI), Fe(III), Co(II), Ni(II), U(VI), W(VI), Sb(III), 1 mg/ml of Cu(II), Sn(II), Bi(V) and 10 μg/ml of Pt(IV) and Pd(II) do not interfere. Large amounts of complexing agents interfere. The method has been applied to analysis of synthetic and industrial samples.     

Spectroscopic evidence for triplet excitation energy transfer among carotenoids in the LH2 complex from photosynthetic bacterium <Emphasis Type="Italic">Rhodopseudomonas palustris</Emphasis>

The LH2 complex from Rhodopsudomonas (Rps.) palustris is unique in the heterogeneous carotenoid compositions. The dynamics of triplet excited state Carotenoids (³Car* has been investigated by means of sub-microsecond time-resolved absorption spectroscopy both at physiological temperature (295 K) and at cryogenic temperature (77K). Broad and asymmetric T _n ←T ₁ transient absorption was observed at room temperature following the photo-excitation of Car at 532 nm, which suggests the contribution from various carotenoid compositions having different numbers of conjugated C=C double bonds (N_c=c). The triplet absorption bands of different carotenoids, which superimposed at room temperature, could be clearly distinguished upon decreasing the temperature down to 77 K. At room temperature the shorter-wavelength side of the main T_n04T₁ absorption band decayed rapidly to reach a spectral equilibration with a characteristic time constant of ∽1 μs, the same spectral dynamics, however, was not observed at 77 K. The aforementioned spectral dynamics can be explained in terms of the triplet-excitation transfer among heterogeneous carotenoid compositions. Global spectral analysis was applied to the time-resolved spectra at room temperature, which revealed two spectral components peaked at 545 and 565 nm, and assignable to the T_n04 T₁ absorption of Cars with Nc=c=11 and Nc=c=13, respectively. Surprisingly, the decay time constant of a shorter-conjugated Car, i.e. 0.72 ώs (aerobic) and 1.36 ώs (anaerobic), is smaller than that of a longer-conjugated Car, i.e. 2.12 us (aerobic) and 3.75 ώs (anaerobic), which is contradictory to the general rule of carotenoids and relative polyenes. The results are explained in terms of triplet-excitation transfer among different types of Cars. It is postulated that two Cars with different conjugation lengths coexist in an α, β-subunit in the LH2 complex.     

Preparation and characterization of stable monodisperse silver nanoparticles via photoreduction

Silver nanoparticles were synthesized by UV irradiation of [Ag(NH₃)₂]⁺ aqueous solution using poly(N-vinyl-2-pyrrolidone) (PVP) as both reducing and stabilizing agents. The formation of silver nanoparticles was confirmed from the appearance of surface plasmon absorption maxima around 420 nm. It was found that the formation rate of silver nanoparticles from Ag₂O was much quicker than that from AgNO₃, and the absorption intensity increased with PVP concentration as well as irradiation time. The maximum absorption wavelength (λ_max) was blue shift with increasing PVP content until 8 times concentration of [Ag(NH₃)₂]⁺ (wt%). The transmission electron microscopy (TEM) showed the resultant particles were 4–6 nm in size, monodisperse and uniform particle size distribution. X-ray diffraction (XRD) demonstrated that the colloidal nanoparticles were the pure silver. In addition, the silver nanoparticles prepared by the method were stable in aqueous solution over a period of 6 months at room temperature (25 °C).     

Study of the diffusion of pyrene in silicone polymer coatings by steady state fluorescence technique: effects of pyrene concentration

The concentration dependence of the diffusion coefficient of pyrene in single component and two-component room temperature curing silicone polymer coatings is investigated by the steady state fluorescence technique by measuring the pyrene excimer fluorescence intensity. At pyrene concentrations lower than 10 mM, the intensity of excimer fluorescence is proportional to the concentration and at higher concentrations it deviates from this trend due to concentration quenching. Thermal aging studies show that this concentration quenching can be removed by thermal annealing and the excimer emission intensity approaches the value expected from the trend at lower concentrations. The diffusion coefficient of pyrene at low concentrations in silicone polymer coatings is obtained using the approximate solution of one-dimensional diffusion equation. A modified approach is employed to estimate the diffusion coefficient at higher pyrene concentrations. In this method, the excimer intensity and time scale are shifted, respectively to I_max the maximum value of excimer intensity attained during annealing and t_max, the time taken to reach this. The estimated diffusion coefficients at different pyrene concentrations show a negligible dependence on pyrene concentration in both types of polymers. These results are attributed to the high structural mobility of silicone polymer chains due to their molecular structure.     

Studies on Simultaneous Fluorescence-Spectrophotometric Determination of Ultratrace Niobium(V), Tantalum(V), and Zirconium(IV) Using Partial Least-Squares Algorithm

In the presence of cetyltrimethylammonium bromide, a cationic surfactant, highly sensitive molecular fluorescence reactions occur between Nb(V), Ta(V), and Zr(IV) ions and morin (3, 5, 7, 2′, 4′-pentahydroxyflavone) in acidic medium to form stable ternary micellar complexes. Their λ_ex(max)/λ_em(max)values are 421.0/492.2, 416.2/489.6, and 424.2/507.8 nm, respectively, and their λ_em(max)values are 490.5, 488.6, and 507.2 nm, respectively, at the same fixed λ_exof 420.5 nm, indicating their seriously overlapping fluorescence excitation spectra and fluorescence emission spectra. The linear ranges of their regression calibration curves are 0 to 0.20, 0 to 0.50, and 0 to 0.20 mg/liter, respectively, with 0.5 ng/ml for all of sensitivities. The simultaneous molecular fluorescence-spectrophotometric determination of ultratrace or trace Nb(V), Ta(V), and Zr(IV) without separation was made using a partial least-squares (PLS) algorithm and other algorithms. The optimum PLS computation conditions are wavelength point number of 25 and corresponding wavelength range from 450 to 550 nm oriented from λ_em500 nm to two sides at combined intervals of 2.5 and 5.0 nm at a fixed λ_exof 420.5 nm with an optimum calibration sample number of 14 and respective optimum abstracted factor numbers of 6, 4, and 3. With respect to both accuracy and precision of the obtained results, the PLS algorithm is superior to the ordinary least-squares algorithm.     

Reversible photomagnetic properties of the molecular compound [{Cu(bipy)2}2{Mo(CN)8}]·9H2O·CH3OH

The optical and photomagnetic properties of [{Cu^II(bipy)₂}₂{Mo^IV(CN)₈}]·9H₂O·CH₃OH (1) have been reinvestigated. A comparison between spectra in solution and in the solid state revealed the presence of an intervalence band (or Metal–Metal Charge Transfer, hereafter noted MMCT) at 570 nm. The photomagnetic properties have been performed in a Superconducting QUantum Interference Device at 10 K with irradiation in the range of the MMCT: 488 nm, 520 nm and 647 nm at 10 K. An important increase of the magnetic signal has been measured after 1 h of irradiation at 488 nm, whereas a weaker increase has been obtained for the irradiation at 520 nm in the same conditions. Moreover, after an excitation at 488 nm, an irradiation at 647 nm has induced a decrease of the magnetic moment, which corresponds to a partial deexcitation. The complete characterization of the photoproduct has been realised after an irradiation of 4 h at 488 nm. The photomagnetic properties have shown an increase of the paramagnetism of 1 at low temperature. After a thermal heating at 300 K, the material goes back to its initial state before irradiation. It is the first time that a fully reversible photomagnetic behaviour for the compound [{Cu^II(bipy)₂}₂{Mo^IV(CN)₈}]·9H₂O·CH₃OH has been described. The observed properties have been discussed in terms of an electron transfer mechanism Mo → Cu.     

Detection of point mutations using pyrene-labeled DNA probes

Pyrene-labeled oligodeoxyribonucleotide probes were shown to be suitable for the detection of point mutations. Reagents based on homochiral 2,4-dihydroxybutyramides were used to introduce pyrene residues at the 3"- and 5"- ends of oligonucleotide pairs. The oligonucleotide pair forms a tandem complex with a complementary target, giving rise to an excimer signal (_max 470—490 nm) in the fluorescence spectra when the pyrene residues come into close proximity. The maximum ratio of the intensity of the excimer signal to the monomer signal (_max 380—400 nm) is attained when (S)-N-(1-pyrenylmethyl)-3,3-dimethyl-2,4-dihydroxybutyramide is used to introduce the pyrene residue. The excimer fluorescence completely disappears with an increase in the distance between the pyrene residues (upon the introduction of an additional nucleotide in the target) or in the presence of a mismatch near the contact site of the probes.     

Binding of 6-propyl-2-thiouracil to human serum albumin destabilized by chemical denaturants

We compared the binding affinity of 6-propyl-2-thiouracil (PTU) with native and destabilized human serum albumin (HSA) as a model to assess the binding ability of albumin in patients suffering from chronic liver or renal diseases. Urea (U) and guanidine hydrochloride (Gu·HCl) at a concentration of 3.0 M were used as denaturation agents.Increasing the concentration of PTU from 0.8 × 10⁻⁵ to 1.20 × 10⁻⁴ M in the systems with HSA causes a decrease in fluorescence intensity of the protein excited with both 280 and 295 nm wavelengths. The results indicate that urea and Gu·HCl bind to the carbonyl group and then to the NH-group. To determine binding constants we used the Scatchard plots. The presence of two classes of HSA–PTU binding sites was observed. The binding constants (K_b) are equal to 1.99 × 10⁴ M⁻¹ and 1.50 × 10⁴ M⁻¹ at λ_ex = 280 nm, 5.20 × 10⁴ M⁻¹ and 1.65 × 10⁴ M⁻¹ at λ_ex = 295 nm. At λ_ex = 280 nm the number of drug molecules per protein molecule is a_I = 1.45 and a_II = 1.32 for I and II binding sites, respectively. At λ_ex = 295 nm they are a_I = 0.63 and a_II = 1.54 for the I and II binding sites.The estimation of the binding ability of changed albumin in the uremic and diabetic patients suffering from chronic liver or renal diseases is very important for safety and effective therapy.     

Luminescence studies in polymers—IV. Effect of orientation,tacticity and crystallinity on polystyrene and polyvinylcarbazole fluorescence

The fluorescence spectra of amorphous atactic, amorphous isotactic and crystallized isotactic polystyrene films have been compared. The effect of chain orientation has also been analysed on amorphous atactic samples. The results show that the fluorescence yield increases with crystallinity at room temperature and 77°K. The contribution of excimer fluorescence at 77°K increases according to the sequence: atactic < atactic oriented < isotactic amorphous < isotactic crystallized. An increase of the fluorescence yield with crystallinity was also observed for polyvinylcarbazole samples although the contribution of excimer fluorescence at 77°K is independent of crystallinity for this polymer. The results are interpreted in terms of energy migration.     

The spectrophotometric determination of molybdenum and uranium with quinalizarin

Quinalizarin reacts very sensitively with molybdenum and uranium to form colored chelates having λ_max at 520 and 630 nm, respectively. The molar ratio for both the chelates is 1:1 (metal:reagent). Optimum conditions including the range for adherence to Beer's law, effect of pH on color intensity, effect of excess of the reagent, sensitvity, and interference of the foreign ions has been reported for the photometric determination of these metal ions using quinalizarin, in 50% ethanolic medium and at 30 °C.