Investigating resins for solid phase organic synthesis: the relationship between swelling and microenvironment as probed by EPR and fluorescence spectroscopy

The relationship between observed swelling of two cross-linked polystyrene resins and the microenvironment within polymer matrixes has been examined. Polystyrene cross-linked with either divinyl benzene (Merrifield resin) or 1,4-bis(4-vinylphenoxy)butane (JandaJel) was investigated with fluorescence and electron-paramagnetic resonance spectroscopy. Fluorescence spectroscopy revealed a superior correlation between observed swelling and solvation effects using a dansyl probe with JandaJel than with Merrifield resin. However, the internal viscosity of pre-swollen JandaJel is higher than Merrifield resin, as determined by EPR measurements. The combination of these two analytical methods provides insights into the physical differences observed between these two chemically similar resins and suggests caution should be used if using singular physical techniques to probe the microenvironment of polymeric matrixes.     

EPR investigation of the influence of side chain protecting groups on peptide-resin solvation of the Asx and Glx model containing peptides

In spite of all progressive efforts aiming to optimize SPPS, serious problems mainly affecting the assembly of aggregating sequences have persisted. Following the study intended to unravel the complex solvation phenomenon of peptide-resin beads, the XING and XAAAA model aggregating segments were labeled with a paramagnetic probe and studied via EPR spectroscopy. Low and high substituted resins were also comparatively used, with the X residue being Asx or Glx containing the main protecting groups used in the SPPS. Notably, the cyclo-hexyl group used for Asp and Glu residues in Boc-chemistry induced greater chain immobilization than its tert-butyl partner-protecting group of the Fmoc strategy. Otherwise, the most impressive peptide chain immobilization occurred when the large trytil group was used for Asn and Gln protection in Fmoc-chemistry. These surprising results thus seem to stress the possibility of the relevant influence of the amino-acid side chain protecting groups in the overall peptide synthesis yield.     

Influence of the Molecular Character and Degree of Crosslinking on the Interaction of Crosslinked Polyacrylamide-Supported Amines with Cu(Ii) Ions

Abstract

Amino groups were incorporated into polyacrylamides with 2?20 mol% of crosslinking agents by transamidation with ethylenediamine. Divinylbenzene, N,Nv′-methylene-bis-acrylamide, and tetraethyleneglycol diacrylate were used as the crosslinking agents. The complexation of these resins, which contain ligand functions in different macromolecular structural environments, was investigated with Cu(II) ions. The Cu(II) uptake of these different resins was correlated with the molecular character and degree of crosslinking in the polymer matrix. The time course and kinetics of complexation depend on the nature of the crosslinking agent in the polymer matrix. The swelling behavior of the uncomplexed and complexed resins, structural characteristics, and thermal decomposition behavior were followed by IR, EPR, and thermal analysis. The swelling characteristics of the complexed resins are lower than those of the uncomplexed resins. Complexation resulted in shifting of the IR absorptions. The EPR parameters depend on the nature of crosslinking and are in agreement with the distorted tetragonal geometry of the Cu(II) complexes. The thermal decomposition behavior also depends on the nature and the degree of crosslinking in the polymer matrix.     

High-resolution diffusion-ordered spectroscopy to probe the microenvironment of JandaJel and Merrifield resins

The success of organic reactions performed on a gel-phase resin is highly dependent on the accessibility of solvents, catalysts, and reagents to the interior of the resin. A variety of techniques including EPR, fluorescence, and Hildebrand solubility parameters (delta) have been used to probe reaction capabilities and in particular the microenvironment of a gel-phase resin. To provide a more detailed picture of the matrix in question, researchers have turned to NMR for the determination of the diffusion coefficients of solvents and small molecules in swollen beads to provide a means to compare the microenvironment of swollen beads. Since Merrifield and JandaJel resins display different swelling properties and have significantly different kinetic behavior, we undertook a comparative study of the diffusion coefficients of solvents and small molecules in both resins by high-resolution (1)H DOSY NMR. Our results show the following: (1) diffusion values for all studied solvents and small molecules are 20-30% higher in JandaJel compared to Merrifield resins, (2) in the absence of interactions between the resin and a given molecule, the diffusion values mirror the swelling properties of the resin, and (3) in the presence of strong intermolecular interactions between the gel and the considered molecule, the diffusion behavior in the gel is primarily influenced by the strength of the interactions and secondarily by the swelling properties of the resin. These results clearly show that the microenvironment of JandaJels is more "solution-like" than that of Merrifield resins, presumably due to the higher swelling capacity.     

Vibronic effects in Cu(II)-doped Ba2Zn(HCO2)6 x 4 H2O

The temperature-dependent electron paramagnetic resonance (EPR) spectrum of approximately 1% Cu(II) ions doped into Ba 2Zn(HCO2)6 x 4 H2O was analyzed at the Q-band frequencies over the temperature range 100-350 K to obtain structural information about the local environment. It can be concluded that the host crystal imparts a large orthorhombic strain which mainly corresponds to a tetragonal compression imposed onto the Cu(II)O6 species. This results in a copper center which adopts an orthorhombically distorted elongated geometry with the elongated axis perpendicular to the direction of the tetragonal compression due to the host crystal. There are two possible axes of elongation, and these represent two conformers separated by approximately 320 cm(-1). The thermal population of the higher energy level averages the g values, giving the observed temperature-dependent EPR spectra. The averaging process is between vibronic levels that are localized at two different minima of a single ground-state potential energy surface. These vibronic levels correspond to vibrational levels having different electronic properties. The determination of the host lattice strain parameters from the Cu(II) EPR spectra means that the guest ion is used as a probe of the environment of the Zn(II) site. The structural data derived from the lattice strain parameters are correlated with those from the Ba 2Zn(HCO2)6 x 4 H2O crystal structure.     

Recent progress in EPR study of spin labeled polymers and spin probed polymer systems

The EPR technique is commonly used for the detection and characterization of paramagnetic centers in chemical science. This method can provides a lot of information, such as identity, structure, dynamics, interaction, orientation, glass transition temperature, adsorption behavior, functionality, phase behavior, nano-inhomogeneities, and conformation of the free-radical portion of the polymer chain. Most polymers intrinsically possess diamagnetic properties, so in order to study polymers with EPR, paramagnetic centers need to be incorporated into the polymer systems. Spin labeling and spin probing are main methods of covalently attaching paramagnetic centers to polymer chains or embedding them in polymer matrices through non-covalent interactions, respectively. Spin labeling and spin probing techniques for polymers and polymer systems (especially with nitroxide radicals) have also been studied, which have a profound impact on polymer science. This review focuses on the continuous wave EPR technique and introduces the recent advances in spin labeled polymers and spin probed polymer systems in EPR research.     

Gel-type polyacrylic resins cross-linked with trimethylolpropanetrimethacrylate: the issue of their nanostructure and molecular accessibility unveiled with a combination of inverse steric exclusion chromatography (ISEC), and ESR and CP-MAS 13C NMR spectroscopy

Six gel-type functional resins, that is, three poly-DMAA-co-TMPTP (DMAA = N,N-dimethylacrylamide, TMPTP = trimethylolpropyltrimethacrylate) samples with different degrees of cross-linking (0.6, 1.2, 1.7 % mol) and three poly-DMAA-co-MA-co-TMPTP (MA = methacrylic acid, ca. 5.5 % mol) samples with 1.7, 3.5, and 7 % mol cross-linking were investigated with ISEC (inverse steric exclusion chromatography), and ESR and CP-MAS (cross polarization magic angle spinning) 13C NMR spectroscopy after swelling in water and other solvents. This unprecedented combination of conceptually independent physicochemical techniques provides a thorough overall consistent picture of the morphology of the resins on the nanometer scale and of the molecular accessibility of the swollen polymer framework to the paramagnetic probe TEMPONE (2,2,6,6-tetramethyl-4-oxo-1-oxypiperidine) and to selected solvents.     

Characterisation of solute mobility in hypercross-linked resins in solvents of different polarity: two promising supports for catalysis

Two hypercross-linked resins stemming from a gel-type poly-chloromethylated styrene-divinylbenzene resin (GT) in beaded form are investigated with a combination of spectroscopic techniques (EPR and time-domain (TD)-NMR spectroscopy) to evaluate their use as supports for the development of operationally flexible heterogeneous metal catalysts, suitable to be employed in liquid and gas phase. The first resin (HGT) is the direct product of the hypercross-linking reaction, whereas the second one (HGS) is the sulphonated analogue of HGT obtained by exchanging approximately 3?wt?% of the chloromethyl groups with sulphonic groups. HGT and HGS absorb both polar and apolar solvents in the permanent nanoporosity created by the hypercross-linking, and NMR data highlight that the pore size is not affected by the different properties of the investigated liquid media. The EPR analysis of the dry resins reveals that during the hypercross-linking process paramagnetic species are formed in the HGT beads, which persist in the sulphonated resin. The mobility of solutes inside the polymers framework was investigated with EPR spectroscopy upon soaking the resins with solutions of two spin probes (2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL)) in THF, toluene, n-heptane and water. The EPR spectra show that, depending on the solvent, the two resins can act as sorbents, able to trap the solutes in the polymer framework, or as simple supports that allow free diffusion of the solutes. Our results suggest that HGT and HGS are promising supporting materials for metal catalysts, provided one chooses carefully the solvent to be employed for the catalysed reaction as this choice strongly affects the mobility of the substrates and, thus their effective reactivity.     

In‐Situ Spin Labeling of His‐Tagged Proteins: Distance Measurements under In‐Cell Conditions

New spin labeling strategies have immense potential in studying protein structure and dynamics under physiological conditions with electron paramagnetic resonance (EPR) spectroscopy. Here, a new spin‐labeled chemical recognition unit for switchable and concomitantly high affinity binding to His‐tagged proteins was synthesized. In combination with an orthogonal site‐directed spin label, this novel spin probe, Proxyl‐trisNTA (P‐trisNTA) allows the extraction of structural constraints within proteins and macromolecular complexes by EPR. By using the multisubunit maltose import system of E. coli: 1) the topology of the substrate‐binding protein, 2) its substrate‐dependent conformational change, and 3) the formation of the membrane multiprotein complex can be extracted. Notably, the same distance information was retrieved both in vitro and in situ allowing for site‐specific spin labeling in cell lysates under in‐cell conditions. This approach will open new avenues towards in‐cell EPR.     

ESR imaging of polymers and solid materials

EPR imaging was applied to the study of several important processes in polymers and materials. The solvent diffusion and induced swelling was visualized in the water/DMF or toluene/DMF interactions with polycarbonate or polystyrene molded rods. The cross-sectional images revealed non-homogeneous solute/solvent diffusion, rod expansion and crack development. The ESR lineshapes in all cases above confirmed that the nitroxide imaging agent did not interact significantly with the polymer matrix; thus the paramagnetic distribution reflected bulk solvent distribution quite well. The diffusion of an electrolyte buffer (Krebs) solution containing a nitroxide spin probe was followed in a cross-sectional image of a polyacrylamide gel rod. A one-dimensional radial diffusion constant was calculated for the solvent piperidinol nitroxide spin probe, TEMPOL. Lastly, solid coal rods were monitored during mild pyrolysis at 150°C in air. Here the naturally occurring organic radical composition was imaged. Even with the increase in total ESR signal and subsequent decline to ca. the initial levels, the coal rod cross-section was essentially homogeneous in paramagnetic centers throughout the heating process. All of the imaging above utilized a homemade L-band, flat loop surface coil spectrometer.     

Characterisation of Solute Mobility in Hypercross‐Linked Resins in Solvents of Different Polarity: Two Promising Supports for Catalysis

Two hypercross‐linked resins stemming from a gel‐type poly‐chloromethylated styrene‐divinylbenzene resin (GT) in beaded form are investigated with a combination of spectroscopic techniques (EPR and time‐domain (TD)‐NMR spectroscopy) to evaluate their use as supports for the development of operationally flexible heterogeneous metal catalysts, suitable to be employed in liquid and gas phase. The first resin (HGT) is the direct product of the hypercross‐linking reaction, whereas the second one (HGS) is the sulphonated analogue of HGT obtained by exchanging approximately 3 wt % of the chloromethyl groups with sulphonic groups. HGT and HGS absorb both polar and apolar solvents in the permanent nanoporosity created by the hypercross‐linking, and NMR data highlight that the pore size is not affected by the different properties of the investigated liquid media. The EPR analysis of the dry resins reveals that during the hypercross‐linking process paramagnetic species are formed in the HGT beads, which persist in the sulphonated resin. The mobility of solutes inside the polymers framework was investigated with EPR spectroscopy upon soaking the resins with solutions of two spin probes (2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) and 4‐hydroxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPOL)) in THF, toluene, n‐heptane and water. The EPR spectra show that, depending on the solvent, the two resins can act as sorbents, able to trap the solutes in the polymer framework, or as simple supports that allow free diffusion of the solutes. Our results suggest that HGT and HGS are promising supporting materials for metal catalysts, provided one chooses carefully the solvent to be employed for the catalysed reaction as this choice strongly affects the mobility of the substrates and, thus their effective reactivity.     

A 13C‐Labeled Triarylmethyl Radical as an EPR Spin Probe Highly Sensitive to Molecular Tumbling

A stable triarylmethyl spin probe whose electron paramagnetic resonance (EPR) spectrum is highly sensitive to molecular tumbling is reported. The strong anisotropy of the hyperfine coupling tensor with the central carbon of a ¹³C₁‐labeled triarylmethyl radical enables the measurement of the probe rotational correlation time with applications to measure microviscosity and molecular dynamics.     

Meso-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Understanding electronic communication among interacting chromophores provides the foundation for a variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin dyads has been investigated by a novel molecular design strategy that entails introduction of a 13C-atom (*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier (highest occupied) molecular orbital. The site of 13C substitution is at a meso-position, either the site of attachment of the linker (proximal, "P") or the site trans to the linker (distal, "D"). The substituents (R) at the non-linking meso-positions are mesityl, tridec-7-yl ("swallowtail"), or p-tolyl groups. Altogether five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy. The introduction of the meso-13C label provides a "clock" (via the hyperfine interactions) that allows investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural abundance 14N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer, which has been previously shown to be fast on the time scale of the 14N hyperfine clock ( approximately 220 ns), remains fast on the time scale of the 13C hyperfine clock ( approximately 50 ns).     

EPR studies of VO2+ ions in kainite single crystals

Electron paramagnetic resonance (EPR) spectra of VO2+ ions doped in Kainite (a mineral salt) single crystals and powder were recorded at room temperature at X-band frequencies.The angular variation studies of the spectra indicate that the VO2+ ion enters Mg2+ ion site substitutionally. The principal values of g and A-tensors were determined from the EPR spectral studies. Using these EPR parameters, the molecular orbital bonding parameters of VO2+ ion in the lattice have been evaluated and discussed.     

Fourier Transform EPR Spectroscopy of Trityl Radicals for Multifunctional Assessment of Chemical Microenvironment

Pulse techniques in electron paramagnetic resonance (EPR) allow for a reduction in measurement times and increase in sensitivity but require the synthesis of paramagnetic probes with long relaxation times. Here it is shown that the recently synthesized phosphonated trityl radical possesses long relaxation times that are sensitive to probe the microenvironment, such as oxygenation and acidity of an aqueous solution. In principle, application of Fourier transform EPR (FT‐EPR) spectroscopy makes it possible to acquire the entire EPR spectrum of the trityl probe and assess these microenvironmental parameters within a few microseconds. The performed analysis of the FT‐EPR spectra takes into consideration oxygen‐, proton‐, buffer‐, and concentration‐induced contributions to the spectral shape, therefore enabling quantitative and discriminative assessment of pH, pO₂, and concentrations of the probe and inorganic phosphate.     

EFFECT OF THE DEGREE OF DVB CROSSLINKING ON THE METAL ION SPECIFICITY OF POLYACRYLAMIDE-SUPPORTED GLYCINES

Metal ion specificity studies of divinylbenzene (DVB)-crosslinked polyacrylamide-supported glycines in different structural environments were investigated. The effect of the degree of crosslinking on the specific rebinding of the desorbed metal ion was investigated towards Co(II), Ni(II), Cu(II), and Zn(II) ions. The metal ion-desorbed resins showed specificity for the desorbed metal ion and the specificity characteristics increases with an increasing degree of the crosslinking agent. The polymeric ligands and metal complexes were characterized by IR, UV-visible and EPR spectra, and by SEM analysis. The swelling and solvation characteristics of the crosslinked polymers, polymeric ligands and metal complexes, the effect of the pH dependence on metal ion binding and rebinding and the kinetics of metal ion binding and rebinding were also followed. The complexation resulted in the downfield shift of the carboxylate peak in the IR spectra. The EPR parameters are in agreement with a distorted tetragonal geometry. The Cu(II) ion-desorbed resins selectively rebinds Cu(II) ions from a mixture of Cu(II) and Co(II) and Cu(II) and Ni(II) ions. The resin could be regenerated several times without loss of capacity and effective for the specific and selective rebinding of Cu(II) ions.     

Determination of long-range distances and dynamic order parameters by dipolar recoupling in high-resolution magic-angle spinning NMR spectroscopy

By introducing dipolar recoupling methods to high-resolution magic-angle spinning (HRMAS) NMR spectroscopy, a class of experiments has been delevoped that allows the measurement of residual dipole-dipole couplings of approximately 1 Hz in weakly immobilized molecules. Using homonuclear 1H-1H recoupling, distances of up to approximately 8 A can be selectively determined, while heteronuclear 1H-13C recoupling provides access to dynamic order parameters of individual molecular segments on the order of approximately 10-3. The experiments are demonstrated on functionalized oligopeptides that are attached to polymer resins.     

Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles

Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles (polyacrylamide, PAAm) was reported. Ultrafast fluorescence dynamics of polymer/water solution was monitored using a fluorescent probe molecule (C153). In the femtosecond time-resolved fluorescence measurement at 480 nm, slowly decay components having lifetimes of tau(1) approximately 53 ps and tau(2) approximately 5 ns were observed in addition to rapid fluorescence decay. Picosecond time-resolved fluorescence spectra of C153/PAAm/H2O solution were also measured. In the time-resolved fluorescence spectra of C153/PAAm/H2O, a peak shift from 490 to 515 nm was measured, which can be assigned to the solvation dynamics of polymer fine particles. The fluorescence peak shift was related to the solvation response function and two time constants were determined (tau(3) approximately 50 ps and tau(4) approximately 467 ps). Therefore, the tau(1) component observed in the femtosecond time-resolved fluorescence measurement was assigned to the solvation dynamics that was observed only in the presence of polymer fine particles. Rotational diffusion measurements were also carried out on the basis of the picosecond time-resolved fluorescence spectra. In the C153/PAAm/H2O solution, anisotropy decay having two different time constants was also derived (tau(6) approximately 76 ps and tau(7) approximately 676 ps), indicating the presence of two different microscopic molecular environments around the polymer surface. Using the Stokes-Einstein-Debye (SED) equation, microscopic viscosity around the polymer surface was evaluated. For the area that gave a rotational diffusion time of tau(6) approximately 76 ps, the calculated viscosity is approximately 1.1 cP and for tau(7) approximately 676 ps, it is approximately 10 cP. The calculated viscosity values clearly revealed that there are two different molecular environments around the polyacrylamide fine particles.     

Spin‐labeling of polymeric micelles and application in probing micelle swelling using EPR spectroscopy

Polymer structure and conformational dynamics are essential to polymer macroscopic properties, but are challenging to probe. We report here a synthetic pathway to chemically add a nitroxide moiety onto block polymers in a mild, aqueous environment and demonstrate its use in a series of polymeric micelles using Electron Paramagnetic Resonance (EPR) spectroscopy. The micelles were characterized with several analytical approaches and EPR findings were in general consistent with other approaches. Upon exposure to organic solvents, the line shape changes reflected the micelle swelling and EPR spectral simulations revealed structural information of the swelled micelles. The label introduced via our method can be cleaved and replaced with other probes to report different information site‐specifically. The mild conditions facilitate the future use of EPR in solving biopolymer problems. In combination with other labeling approaches, one can perform polymer spin labeling with different chemistry, so that various information about polymers can be obtained site‐specifically. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1770–1782     

Further EPR studies of molecular motions in polymer/plasticizer mixtures

Molecular motions in mixtures of the side chain polymer—poly(vinyl acetate) and dibuthyl phthalate were studied as a functionof polymer concentration and temperature using the technique of paramagnetic resonance (EPR). When the small spherical probe tempol (TPL) was used, we were able to approximate the observed EPR spectrum with a simulation using a single rotational correlation time τ. The peviously developed Grest–Cohen all-temperature model matched the Arrhenius polts. The EPR spectra from a cigar-shaped cholestane (COL) probe could not be adequately matched by single τ simulation when the polymer was at temperatures somewhat above the glass to rubber transition temperature (T_g). Points corresponding to these temperatures were left of the Arrhenius plot and a discontinuity was observed where the gap in the data occurred. As the concentration of plasticizer was increased, we found that the discontinuity became less steep, but the τ at which the gap occurs was always ≈ 10^?8. The spectra observed at the temperature region of the gap were approximately 50–50 composites of experimental spectra observed at ± K. In both the TPL and COL cases, there was evidence of the existence of multiple correlation times. Preliminary studies of other polymers, both with and without side chains, also indicated the existence of the gap when COL is used as the probe. © 1993 John Wiley & Sons, Inc.