Measurement of concentration gradients in swelling crosslinked polymer beads

The concentration of solvent in the gel phase of a swelling polystyrene–divinylbenzene copolymer bead has been measured as a function of the radius by optical interference techniques. It is found that an appreciable gradient exists even shortly after the core disappears. Results are presented for two cases: a very lightly crosslinked bead that exhibits negligible birefringence while swelling, and a more tightly crosslinked network in which the birefringence must be considered. It is also found that the refractive index of the unswollen bead is considerably less than that found by extrapolation from the swollen state; this effect is reasonable when the “free volume” in the bulk polymer is considered.     

Electron paramagnetic resonance spin probe study of carbon dioxide‐induced polymer plasticization

Steady‐state electron paramagnetic resonance (EPR) spectroscopy using nitroxide spin probes has been used to investigate the plasticization of poly(vinyl acetate) and poly(ethyl methacrylate) by carbon dioxide. By varying the CO₂ pressure at constant ambient temperature, the glass transition for each polymer could be depressed to 25 °C. This effect has been quantified by a parameter P_50G, obtained by plotting the EPR spectral width as a function of CO₂ pressure. Certain spin probes showed free volume distribution effects, manifested in the EPR spectra as “double peaks.” Possible reasons for this phenomenon are presented and discussed, and the efficacy of CO₂ as a plasticizer is clearly demonstrated by direct comparison with di‐n‐butyl phthalate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2097–2108, 2005     

The effective and specific isolation of antibodies from human serum by using thiophilic paramagnetic polymer beads

A rapid and effective method to specifically isolate immunoglobulin G (IgG) from human serum by thiophilic paramagnetic polymer beads was developed. The thiophilic paramagnetic beads were synthesized with vinyl acetate and divinylbenzene by microsuspension polymerization in the presence of magnetite nanoparticles. Divinylsulfone and 2-mercapto-benzothiazole were subsequently used to modify the surface of these beads, resulting in thiophilic particles that exhibited a high specificity to the antibodies in serum at low salt concentration. The adsorbed IgG was eluted by 0.8 M KCl and 72% of the IgG in the serum was recovered. The purity of the isolated IgG reached 98.4% and the bioactivity was fully maintained (>99%). The high efficiency, mild conditions and simplicity make this technology suitable for the economic purification of antibodies in a large scale.     

Electrochemistry of tetracyanoquinodimethane polyer-modified electrodes: Effect of polymer film composition on site-site interactions

Charge transport through polymer films containing the tetracyanoquinodimethane (tcnq) structure in the polymer chain and swollen with acetonitrile solvent has been studied as a function of film composition. Variation of the film composition was achieved by synthesis of a tcnq terpolymer and by casting blended films of the tcnq polyester and an isomorphic polyester containing a benzene ring in place of the tcnq structure. Formation of uniform blends at room temperature and below was indicated for the neutral polymers, but phase separation takes place on repeated cycling of the films between the 0 and −1 oxidation states. Redox center site-site interactions were probed by following dimer formation of the tcnq^.− radical anion and by the charge transport process as measured by chronocoulometric Cottrell slopes. ESR and cyclic voltammetric data indicate that interchain, and not interchain, interactions occur in the reduced films.     

Isolation and detection of single molecules on paramagnetic beads using sequential fluid flows in microfabricated polymer array assemblies

We report a method for isolating individual paramagnetic beads in arrays of femtolitre-sized wells and detecting single enzyme-labeled proteins on these beads using sequential fluid flows in microfabricated polymer array assemblies. Arrays of femtolitre-sized wells were fabricated in cyclic olefin polymer (COP) using injection moulding based on DVD manufacturing. These arrays were bonded to a complementary fluidic structure that was also moulded in COP to create an enclosed device to allow delivery of liquids to the arrays. Enzyme-associated, paramagnetic beads suspended in aqueous solutions of enzyme substrate were delivered fluidically to the array such that one bead per well was loaded by gravity. A fluorocarbon oil was then flowed into the device to remove excess beads from the surface of the array, and to seal and isolate the femtolitre-sized wells containing beads and enzyme substrate. The device was then imaged using standard fluorescence imaging to determine which wells contained single enzyme molecules. The analytical performance of this device as the detector for digital ELISA compared favourably to the standard method, i.e., glass arrays mechanically sealed against a silicone gasket; prostate specific antigen (PSA) could be detected from 0.011 pg mL(-1) up to 100 pg mL(-1). The use of an enclosed fluidic device to isolate beads in single-molecule arrays offers a multitude of advantages for low-cost manufacturing, ease of automation, and instrument development to enable applications in biomarker validation and medical diagnosis.     

Changes in defect concentration within polymer crystals: annealing of isotropic and drawn polyethylene

An estimation of the changes in the level of chain defects (branches) accommodated within crystals of undrawn and drawn branched polyethylene, subsequently annealed, is carried out in the light of unit cell expansion data. The results reveal that annealing affects the estimated crystalline defect concentration differently depending on whether the material was drawn or undrawn. Thus, annealing of drawn polyethylene with a defect concentration of 3 % results in a healing of crystal defects, due to a preferential rejection of branches from the crystals. In contrast, annealing of melt crystallized branched polyethylene does not influence the average concentration of chain defects within the crystal despite the increase in lamellar thickness. The branches are trapped into the much wider lamellar crystals with fewer grain boundaries available and defect sequences cannot diffuse out of the crystals.Cordially dedicated to Prof. Dr. H.-G. Kilian on the occasion of his 60th birthday.     

Transition metal ions in black tea: an electron paramagnetic resonance study

Tea (Camellia Sinensis) is the most widely consumed beverage in the world and is known to have therapeutic, antioxidant and nutritional effects. It contains dimeric flavanols and polyphenols which are known as the most important organic compounds in tea infusions, and can make strong and stable complexes with metal ions. In this study, we carried out a series of electron paramagnetic resonance experiments on well-known paramagnetic transition metal ions, namely Mn²⁺, Fe³⁺, Cu²⁺, VO²⁺, and Cr³⁺ doped in black tea cultivated along the shore of Black Sea, Turkey, to see the effects and structures formed.     

Electron paramagnetic resonance study of thermal-oxidative degradation of nylon 6

Electron paramagnetic resonance studies were conducted on thermally degraded nylon 6 polymers. A nitroxide radical was observed from nonterminated and amineterminated nylon 6 samples heat-treated in the persence of air. The nitroxide concentration in the latter was about twice that of the former under the same conditions. The nitroxide radical was not observed in acid-terminated nylon 6 samples. A mechanism of nitroxide radical formation was proposed based on the oxidation of secondary amines. A one-line EPR signal was also observed from nonterminated, amine-terminated, and acid-terminated nylon 6 samples heated in the presence of air. This singlet signal was attributed to conjugated systems with unpaired electrons.     

G-tensors of the flavin adenine dinucleotide radicals in glucose oxidase: a comparative multifrequency electron paramagnetic resonance and electron-nuclear double resonance study

The flavin adenine dinucleotide (FAD) cofactor of Aspergillus niger glucose oxidase (GO) in its anionic (FAD*-) and neutral (FADH*) radical form was investigated by electron paramagnetic resonance (EPR) at high microwave frequencies (93.9 and 360 GHz) and correspondingly high magnetic fields and by pulsed electron-nuclear double resonance (ENDOR) spectroscopy at 9.7 GHz. Because of the high spectral resolution of the frozen-solution continuous-wave EPR spectrum recorded at 360 GHz, the anisotropy of the g-tensor of FAD*- could be fully resolved. By least-squares fittings of spectral simulations to experimental data, the principal values of g have been established with high precision: gX=2.00429(3), gY=2.00389(3), gZ=2.00216(3) (X, Y, and Z are the principal axes of g) yielding giso=2.00345(3). The gY-component of FAD*- from GO is moderately shifted upon deprotonation of FADH*, rendering the g-tensor of FAD*- slightly more axially symmetric as compared to that of FADH*. In contrast, significantly altered proton hyperfine couplings were observed by ENDOR upon transforming the neutral FADH* radical into the anionic FAD*- radical by pH titration of GO. That the g-principal values of both protonation forms remain largely identical demonstrates the robustness of g against local changes in the electron-spin density distribution of flavins. Thus, in flavins, the g-tensor reflects more global changes in the electronic structure and, therefore, appears to be ideally suited to identify chemically different flavin radicals.     

Multi-frequency electron paramagnetic resonance study of irradiated human finger phalanxes

Electron paramagnetic resonance (EPR) is often used in dosimetry using biological samples such as teeth and bones. It is generally assumed that the radicals, formed after irradiation, are similar in both tissues as the mineral part of bone and tooth is carbonated hydroxyapatite. However, there is a lack of experimental evidence to support this assumption. The aim of the present study was to contribute to that field by studying powder and block samples of human finger phalanxes that were irradiated and analyzed by multi-frequency EPR. The results obtained from bones are different from the ones obtained in enamel by several respects: the ordering of the apatite crystallites is much smaller in bone, complicating the assignment of the observed CO2- radicals to a specific location, and one type of CO3(3-) radical was only found in enamel. Moreover, a major difference was found in the non-CO2- and non-CO3(3-) signals. The elucidation of the nature of these native signals (in bone and tooth enamel) still represents a big challenge.     

Comparative density-functional study of the electron paramagnetic resonance parameters of amavadin

Electronic g tensors and hyperfine coupling tensors have been calculated for amavadin, an unusual eight-coordinate vanadium(IV) complex isolated from Amanita muscaria mushrooms. Different density-functional methods have been compared, ranging from local via gradient-corrected to hybrid functionals with a variable Hartree-Fock exchange admixture. For both electron paramagnetic resonance (EPR) properties, hybrid functionals with an appreciable exact-exchange admixture provide the closest agreement with experimental data. Second-order spin-orbit corrections provide non-negligible contributions to the 51V hyperfine tensor. The orientation of g and A tensors relative to each other also depends on spin-orbit corrections to the A tensor. A rationalization for the close resemblance of the EPR parameters of amavadin to those of the structurally rather different vanadyl complexes is provided, based on the nature of the relevant frontier orbitals.     

A study of Roman glass by reflectance and electron paramagnetic resonance spectroscopies

Reflectance and electron paramagnetic resonance (EPR) spectroscopies were used to study 25 fragments of Roman glass. Colour coordinates were used for an unbiased classification of the glasses in colour groups, which accounted for the presence of blue, blue-green, green, yellow-green, yellow and purple samples. Reflectance spectra were recorded in the 250–2500 nm wavelength range and showed absorption bands characteristic of Fe^II, Fe^III and Mn^III ions; furthermore, Co^II and Cu^II bands were observed in the spectra of the blue glasses. A decrease of the absorbance ratio of Fe^II to Fe^III ions was observed moving from blue-green to green and yellow-green glasses; however, yellow fragments still proved to be reduced glasses. EPR spectra displayed the characteristic patterns of Fe^III and Mn^II ions, with g-values in the 2–5 interval and spectral features depending on the relative content of the two elements. The characteristic pattern of the V^IV ion (g ≈ 2) and signals due to the formation of iron-sulphur complexes (g ≈ 6) appeared in the spectrum of a dark yellow glass, recorded at 77 K.     

Nucleophile addition of reduced glutathione on 2-methyl-2-nitroso compound: A combined electron paramagnetic resonance spectroscopy and electrospray tandem mass spectrometry study

Mass spectrometry (MS) was used in conjunction with electron paramagnetic resonance (EPR) to characterize products arising from reactions between reduced glutathione (GSH) and 2-methyl 2-nitroso propane (MNP) in an oxidative medium, to evaluate the reactivity of this tripeptide as a nucleophile toward a nitroso compound. Depending on the experimental conditions, different radical species could be detected by EPR, which allowed some structural assumptions. These samples were then submitted to electrospray ionization, in both positive and negative ion modes, for structural elucidation in tandem mass spectrometry. Although the primary nitroxide products could not be detected in MS, structurally related compounds such as hydroxylamine and O-methyl hydroxylamine could be fully characterized. In the absence of light, a S-adduct was formed via a Forrester-Hepburn reaction, that is, a nucleophile addition of MNP onto the thiol function in reduced glutathione to yield a hydroxylamine intermediate, further oxidized into nitroxide. In contrast, irradiating the reaction medium with visible light could allow an inverted spin trapping reaction to take place, involving the oxidation of both MNP and GSH before the nucleophilic addition of the sulfenic acid function onto the nitrogen of MNP, yielding a so-called O-adduct. It was also found that dilution of the reaction medium with methanol for the purpose of electrospray ionization could allow nitroxides to be indirectly observed either as hydroxylamine or O-methyl hydroxylamine species.     

Observation of a photoexcited state of a paramagnetic transition metal complex by time-resolved electron paramagnetic resonance spectroscopy

The first observation of a spin polarized excited state of a paramagnetic metal-complex using time-resolved electron paramagnetic resonance (TREPR) spectroscopy is reported for octaethylporphinatooxovanadium(iv). The TREPR spectra show well resolved orientation dependent hyperfine splitting to the I = 7/2 vanadium nucleus. The reduction of the hyperfine splitting by a factor of 3 compared to the ground state and the observation of a multiplet pattern of spin polarization allow the TREPR spectra to be assigned to the excited quartet state of the complex. The spin polarization patterns evolve with time and it is postulated that this is a result of the equilibration between the lowest excited quartet and doublet states.     

Photoexcited triplet-state dynamics of novel porphyrinoids: octaethylcorrphycene and octaethylhemiporphycene. Time-resolved electron paramagnetic resonance study

The photoexcited triplet state of the two new porphyrin isomers, octaethylhemiporphycene (OEHPC) and octaethylcorrphycene (OECPC), embedded in isotropic (toluene) and an anisotropic liquid crystal matrices was studied by time-resolved electron paramagnetic resonance (EPR) spectroscopy in the temperature range of 150–300 K. The magnetic, kinetic and orientation parameters were determined and interpreted in terms of structure, symmetry and dynamic. Analysis of the results suggests that at T250 K, the orientation and packing of the chromophores result from a discrete solid-like jumps mechanism, which is more efficient for OECPC. This difference is rationalized in terms of differences in the symmetry of the two chromophores.