Scattering Behavior of Restructured Aggregates: A Simulation Study

Simulations of diffusion-limited cluster-cluster aggregation (DLCA) with no restructuring, full restructuring, and partial restructuring have been performed. The scattering patterns produced from these aggregates have been simulated using the Rayleigh-Gans-Debye approximation. Pure DLCA aggregates produced a scattering pattern with the slope of the fractal region being about -1.8. In contrast, the slope of the fractal region of the scattering pattern for fully restructured aggregates was about -2.1, indicating an increase in fractal dimension. Partial restructuring at large length scales produced an upward turn in the scattering pattern at low qr(o), while at high qr(o) the fractal section of the pure DLCA aggregate was retained. This last result was expected and is consistent with the results and postulations of several other workers. This simulation shows that the type of scattering pattern often obtained from orthokinetic or sheared aggregation can be produced by restructuring of aggregates at large length scales. Copyright 2001 Academic Press.     

Several convenient methods for the synthesis of 2-amido substituted furans

Several new methods for the synthesis of differently substituted 2-amidofurans are described. The thermolysis of furan-2-carbonyl azide results in a Curtius rearrangement and the resulting furanyl isocyanate was trapped with various organometallic reagents. A second method consists of a C-N cross-coupling reaction of a bromo-substituted furan with various amides, carbamates, and lactams. The CuI-catalyzed cross-coupling reaction between furanyl bromides and amides furnished 2- and 3-substituted amidofurans in 45-95% yield. The third protocol used involves the reaction of cyclic carbinol amides with triflic anhydride. The reaction proceeds under very mild conditions to provide alpha-(trifluoromethyl)sulfonamido-substituted furans in high yield. The resulting iminium ion derived from the reaction of the hydroxy pyrrolidinone with Tf(2)O undergoes a facile ring opening as a consequence of the adjacent hydroxyl group to produce an imino triflate intermediate. Subsequent cyclization of this highly electrophilic imine with the oxygen atom of the adjacent carbonyl group leads to an imino dihydrofuran that reacts further with another equivalent of Tf(2)O to give the observed product.     

Simultaneous determination of DTPA,EDTA, and NTA by capillary electrophoresis after complexation with copper

We present a method for simultaneous determination of the aminopolycarboxylic acids DTPA, EDTA and NTA in dishwashing detergents, paper mill waters, and natural waters by capillary electrophoresis (CE). The complexing agents were examined as their copper(II) complexes and separated by conventional CE with reversed polarity of the applied voltage. The optimum separation conditions were established by varying the pH and phosphate and tetradecyltrimethylammonium bromide (TTAB) concentrations in the run buffer. The separations were carried out in a fused-silica capillary (61 cm×75 m i.d.) filled with phosphate buffer (80 mmol L^–1, TTAB concentration 0.5 mmol L^–1, pH 7.1, voltage –20 kV) using direct UV detection at 191 and 254 nm. With this CE method all the peaks in the electropherograms were properly separated, the calibration plots gave good correlation coefficients and all three complexing agents could be detected in less than 4 min. Linear calibration plots were obtained for CuDTPA, CuEDTA and CuNTA; limits of detection were 0.03 mmol L^–1 for all complexing agents and recoveries for all tested samples were within the range 104±7%. Results obtained from dishwashing detergent samples were found to be reliable and comparable with those from HPLC (R²=0.989) and UV–Vis (R²=0.985) methods.     

A new ionic liquid dimethyldinonylammonium bromide as a flow modifier for the simultaneous determination of eight carboxylates by capillary electrophoresis

Two new methods of capillary zone electrophoresis based on aqueous phosphate running buffers with UV spectrophotometric detection were developed and optimized for the determination of eight carboxylates as copper complexes. Metalcomplexes are negatively charged, so measurements were made as anion analyses with flow reversal in the capillary. Two flow modifiers were used: a common tetradecyltrimethylammonium bromide (TTAB) and a new ionic liquid dimethyldinonylammonium bromide (DMDNAB). The methods were compared to each other. Better separation was achieved with DMDNAB as the flow modifier. Method development was done using a fused silica capillary (61 cm x 50 microm i.d.).Optimization was done using 95 mmol L(-1) phosphate buffer with TTAB or DMDNAB in the concentration 0.5 mmol L(-1) at pH 7.1. A -20 kV voltage and direct UV detection at 254 nm was used in measurements. In both CE methods all the peaks in the electropherograms were properly separated, the calibration plots gave good correlation coefficients and all eight carboxylates were detected in less than 7.5 min. The two methods were tested with natural water samples and a paper mill sample, and proved to be feasible.     

Quinoprotein glucose dehydrogenase modified thick-film electrodes for the amperometric detection of phenolic compounds in flow injection analysis

The use of thick-film electrodes as basic transducers for highly sensitive amperometric biosensors using PQQ (pyrroloquinoline quinone) dependent glucose dehydrogenase (GDH) with short response times is described. The enzyme is embedded in a polyurethane matrix on top of a platinum based thick film electrode and its ability to reduce oxidized phenolic compounds is exploited. The electrochemical amplification is based on the oxidation of the analyte on the surface of the electrode followed by its enzymatic reduction. Different parameters of the glucose dehydrogenase electrode system using dopamine as a model analyte were optimized, e.g., membrane thickness, pH value, buffer system, flow rate and storage conditions. Using optimized parameters the sensitivity and detection limits for various phenolic compounds were evaluated. The comparison of electrodes from the identical as well as from different batches shows the ability to produce a number of well reproducible sensors showing remarkably small differences with respect to parameters as sensitivity, response times and measuring range.     

PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING II: REACTIONS INVOLVING PYRIMIDINE RADICAL ANION INTERMEDIATES

A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor Nα-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1′-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product.     

PHOTO-CIDNP DETECTION OF PYRIMIDINE DIMER RADICAL CATIONS IN ANTHRAQUINONESULFONATE- SENSITIZED SPLITTING

Anthraquinone-2-sulfonate (AQS) photosensitizes pyrimidine dimer splitting. Electron abstraction from the dimer is thought to induce dimer splitting, but direct evidence for the existence and intermediacy of dimer radical cations has been lacking. By employing photochemically induced dynamic nuclear polarization, we have found emission signals in the NMR spectra of dimers upon photolysis of dimers in the presence of anthraquinone-2-sulfonate. The two dimers employed were cis, syn-thymine dimer in which the N(1)-positions were linked by a three-carbon bridge and the N(3), N(3')-dimethyl derivative of that compound. The anthraquinone-2-sulfonate sensitized photochemically induced dynamic nuclear polarization spectrum of the methylated derivative exhibited an emission signal from the dimer-C(6) hydrogens. This result implied the existence of a dimer radical cation (mD+.) formed by electron abstraction by excited anthraquinone-2-sulfonate and nuclear spin sorting within a solvent caged radical ion pair [mD+. AQS-.]. Product pyrimidine photochemically induced dynamic nuclear polarization signals were also seen [enhanced absorption by C(6)-hydrogens and emission by C(5)-methyl groups]. Nuclear spin polarization in the product resulted from spin sorting in one or more of its precursors, including mD+. The results support the conclusion that dimer radical cations not only exist but are intermediates in the photosensitized splitting of pyrimidine dimers by anthraquinonesulfonate.     

Strong coupling expansion for classical statistical dynamics

We discuss the simple, randomly driven systemdx/dt = –x –x³ +f(t), wheref(t) is a Gaussian random function or stirring force with f(t)f(t) = (t – t). We show how to obtain approximately the coefficients of the expansion of the equal-time Green's functions as power series in (1/R)ⁿ, whereR is the internal Reynolds number ()^1/2/, by using a new expansion for the path integral representation of the generating functional for the correlation functions. Exploiting the fact that the action for the randomly driven system is related to that of a quantum mechanical anharmonic oscillator with Hamiltonianp ²/2 +m ² x ²/2 +vx ⁴ +x ⁶/2, we evaluate the path integral on a lattice by assuming that thex ⁶ term dominates the action. This gives an expansion of the lattice theory Green's functions as power series in 1/(a)^1/3, wherea is the lattice spacing. Using Padé approximants to extrapolate toa = 0, we obtain the desired large-Reynolds-number expansion of the two-point function.Supported financially by the National Science Foundation and the U.S. Department of Energy.     

Matrix identities of the fast fourier transform

Let A_n(ω) be the nxn matrix A_n(ω)=(a_ij with a_ij=ω^ij, 0?i,j?n?1, ωⁿ=1. For n=rs we show

$\text{A}{}_{\mathrm{n}}\text{(w)P}{}_{\mathrm{s}}{}^{\mathrm{r}}\text{P}{}_{\mathrm{r}}{}^{\mathrm{s}}\text{∵}\text{0}\text{s?1}\text{A}{}_{\mathrm{r}}\text{(w}{}^{\mathrm{s}}\text{)}\text{P}{}_{\mathrm{s}}{}^{\mathrm{r}}\text{{T}{}_{\mathrm{r}}{}^{\mathrm{s}}\text{(w)}}\text{∵}\text{0}\text{r?1}\text{A}{}_{\mathrm{s}}\text{(w}{}^{\mathrm{r}}\text{)}$

=(A_r?I_s)T^s_r(I_r?A_s). When r and s are relatively prime this identity implies a wide class of identities of the form PA_n(ω)Q^T=A_r(ω^αs)?A_s(ω^βr). The matrices P^s_r, P^r_s, P, and Q are permutation matrices corresponding to the “data shuffling” required in a computer implementation of the FFT, and T^s_r is a diagonal matrix whose nonzeros are called “twiddle factors.” We establish these identities and discuss their algorithmic significance.