Triarylmethyl Radicals Research Articles

The solution conformation of triarylmethyl radicals

Hyperfine coupling tensors to 1H, 2H, and natural abundance 13C were measured using X-band pulsed electron nuclear double resonance (ENDOR) spectroscopy for two triarylmethyl (trityl) radicals used in electron paramagnetic resonance imaging and oximetry: methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[1,2 d:4,5- d′]bis(1,3)dithiol-4-yl) and methyl tris(8-carboxy-2,2,6,6-tetramethyl(- d 3)-benzo[1,2 d:4,5- d′]bis(1,3)dithiol-4-yl). Quantum chemical calculations using density functional theory predict a structure that reproduces the experimentally determined hyperfine tensors. The radicals are propeller-shaped with the three aryl rings nearly mutually orthogonal. The central carbon atom carrying most of the unpaired electron spin density is surrounded by the sulfur atoms in the radical and is completely shielded from solvent. This structure explains features of the electron spin relaxation of these radicals and suggests ways in which the radicals can be chemically modified to improve their characteristics for imaging and oximetry.

Frequency (250 MHz to 9.2 GHz) and viscosity dependence of electron spin relaxation of triarylmethyl radicals at room temperature

Electron spin relaxation times for four triarylmethyl (trityl) radicals at room temperature were measured by long-pulse saturation recovery, inversion recovery, and electron spin echo at 250MHz, 1.5, 3.1, and 9.2GHz in mixtures of water and glycerol. At 250MHz T1 is shorter than at X-band and more strongly dependent on viscosity. The enhanced relaxation at 250MHz is attributed to modulation of electron–proton dipolar coupling by tumbling of the trityl radicals at rates that are comparable to the reciprocal of the resonance frequency. Deuteration of the solvent was used to distinguish relaxation due to solvent protons from the relaxation due to intra-molecular electron–proton interactions at 250MHz. For trityl-CD3, which contains no protons, modulation of dipolar interaction with solvent protons dominates T1. For proton-containing radicals the relative importance of modulation of intra- and inter-molecular proton interactions varies with solution viscosity. The viscosity and frequency dependence of T1 was modeled based on dipolar interaction with a defined number of protons at specified distances from the unpaired electron. At each of the frequencies examined T2 decreases with increasing viscosity consistent with contributions from T1 and from incomplete motional averaging of anisotropic hyperfine interaction.

Direct-detected rapid-scan EPR at 250 MHz

EPR spectra at 250 MHz for a single crystal of lithium phthalocyanine (LiPc) in the absence of oxygen and for a deoxygenated aqueous solution of a Nycomed triarylmethyl (trityl-CD 3) radical were obtained at scan rates between 1.3 × 10 3 and 3.4 × 10 5 G/s. These scan rates are rapid relative to the reciprocals of the electron spin relaxation times (LiPc: T 1=3.5 μs and T 2=2.5 μs; trityl: T 1=12 μs and T 2=11.5 μs) and cause characteristic oscillations in the direct-detected absorption spectra. For a given scan rate, shorter values of T 2 and increased inhomogeneous broadening cause less deep oscillations that damp out more quickly than for longer T 2. There is excellent agreement between experimental and calculated lineshapes and signal amplitudes as a function of radiofrequency magnetic field ( B 1) and scan rate. When B 1 is adjusted for maximum signal amplitude as a function of scan rate, signal intensity for constant number of scans is enhanced by up to a factor of three relative to slow scans. The number of scans that can be averaged in a defined period of time is proportional to the scan rate, which further enhances signal amplitude per unit time. Longer relaxation times cause the maximum signal intensity to occur at slower scan rates. These experiments provide the first systematic characterization of direct-detected rapid-scan EPR signals.

Simultaneous measurement of oxygenation in intracellular and extracellular compartments of lung microvascular endothelial cells.

A new technique is described for simultaneous determination of intra- and extracellular oxygen concentrations [partial pressure of O(2) (pO(2))] in bovine lung microvascular endothelial cells (BLMVECs) using electron paramagnetic resonance (EPR) oximetry. The measurements were performed in BLMVEC suspensions of a 20- micro l volume containing 4,000 cells. The extracellular pO(2) was measured using a trityl EPR probe [triarylmethyl (TAM), 10 micro M], a tricarboxylate anion radical, that stays exclusively in the extracellular space. The intracellular oxygen was measured using a pre-internalized particulate spin probe, lithium 5,9,14,18,23,27,32,36-octa-n-butoxy-naphthalocyanine (LiNc-BuO). Because there is a wide discrepancy in the reported values of cellular oxygenation by and large due to differences in the methods employed, we utilized the dual EPR probe technique to measure the oxygen gradient that apparently exists across the cell membrane. The intra- and extracellular pO(2) values were 139 +/- 2.5 and 157 +/- 3.6 mm Hg, respectively, for cells exposed to room air. A fairly smaller gradient of oxygen was observed in cells exposed to 7.5% oxygen (pO(2) = 57 mm Hg). In summary, this study confirms the feasibility of simultaneous and accurate measurements of intra- and extracellular pO(2) using LiNc-BuO and TAM EPR oximetry probes.

Proton electron double resonance imaging of the in vivo distribution and clearance of a triaryl methyl radical in mice.

Proton electron double resonance imaging (PEDRI) measures the spatial distribution of paramagnetic species in biological samples using the Overhauser effect. Triaryl methyl (TAM) free radicals have been developed as a spin probe for PEDRI since they have very long T(1e). Therefore, low RF power levels are sufficient to saturate the electron spin system with resultant high NMR enhancement facilitating application of PEDRI in living animals. In this report, PEDRI studies were performed at 0.02 T. The power-dependent image enhancement was studied using phantoms of TAM in saline, then the distribution and pharmacokinetics of TAM in living mice were measured. Following intravenous administration of 0.7 mmol/kg of TAM, enhancements of up to -34 were observed enabling visualization of its distribution within the body. Maximum uptake of TAM in the vascular compartment was seen 1 min postadministration with half-clearance within 5 min. Maximum uptake in the kidneys occurred at 10 min with half-clearance at 26 min and maximum accumulation in the bladder after 50 min. Thus, TAM is initially compartmentalized in the vasculature and this is followed by rapid uptake and excretion by the kidneys. PEDRI enabled rapid imaging of the distribution and clearance of this paramagnetic probe and this information should facilitate the use of TAM as a label for oximetry and other applications. Magn Reson Med 48:530-534, 2002.

General synthesis of persistent trityl radicals for EPR imaging of biological systems.

In this paper we describe the syntheses of the tetraoxygenated triarylmethyl (trityl) radical 14 and the tetrathiatriarylmethyl (trityl) radicals 15 and 16. The syntheses include new and improved preparations of the key intermediate compounds 1 and 2. The new route to compound 2 is noteworthy for its efficiency and its avoidance of the highly toxic compound phosgene as well as the isolation of the air-sensitive 1,2,4,5-benzenetetrathiol.

Noninvasive in vivo oximetric imaging by radiofrequency FT EPR.

A novel method, called relaxo-oximetry, for rapid spatially resolved in vivo measurements of oxygen concentration using time-domain radiofrequency (RF) electron paramagnetic resonance (EPR) is described. Time-domain data from triaryl methyl (TAM)-based single-electron contrast agents were processed by systematic deletion of the initial time points to arrive at T2*-weighted discrimination of signal amplitudes. In experiments involving phantoms, the line widths [ approximately (T2*)(-1)] increased as a function of oxygen, and the slope (line width/pO(2)) was the same for both absorption- and magnitude-mode line shapes. Line widths derived from T2* weighting and the computed pO(2) values agreed favorably with the measured ones from phantoms of known oxygen tension. In vivo relaxo-oximetry was performed on C3H mice, and it was found that the liver was more hypoxic than the kidneys. For tumors, 2D oxygen maps were generated while the animal breathed room air or Carbogen(R) (95% O(2)/5% CO(2)). Carbogen(R) enhanced oxygen concentration within the tumor, and the pO(2) histograms showed considerable heterogeneity. Clark electrode oxygen measurements on organs and tumors were in good agreement with tissue oxygen measurements done by relaxo-oximetry. Thus, from a single spatial image data set, pO(2) measurements can be done noninvasively by relaxo-oximetry, and 3D imaging can be performed in less than 3 min.

Rapid imaging of free radicals in vivo using hybrid FISP field-cycled PEDRI

A new pulse sequence for rapid imaging of free radicals is presented which combines snapshot imaging methods and conventional field-cycled proton electron double resonance imaging (FC-PEDRI). The new sequence allows the number of EPR irradiation periods to be optimized to obtain an acceptable SNR and spatial resolution of free radical distribution in the final image while reducing the RF power deposition and increasing the temporal resolution. Centric reordered phase encoding has been employed to counter the problem of rapid decay of the Overhauser-enhanced signal. A phase-correction scheme has also been used to correct problems arising from instability of the magnetic field following field-cycling. In vivo experiments were carried out using triaryl methyl free radical contrast agent, injected at a dose of 0.214 mmol kg−1 body weight in anaesthetized adult male Sprague–Dawley rats. Transaxial images through the abdomen were collected using 1, 2, 4 and 8 EPR irradiation periods. Using 4 EPR irradiation periods it was possible to generate free radical distributions of acceptable SNR and resolution. The EPR power deposition is reduced by a factor of 16 and the acquisition time is reduced by a factor of 4 compared to an acquisition using the conventional FC-PEDRI pulse sequence.

250 MHz crossed‐loop resonator for pulsed electron paramagnetic resonance

AbstractThe crossed‐loop resonator (CLR) uses two orthogonal lumped‐element resonators [one to excite the spins and one to detect the electron paramagnetic resonance (EPR)] to isolate the signal from the microwave source. It eliminates the need for a circulator. The high isolation provided by the CLR reduces the energy stored in the resonator that detects the signal, thereby reducing the intensity of the resonator ring down after the pulse, which decreases the instrument dead time. Overcoupling and synchronous switching of the Q's of the two resonators between high and low states were used to further reduce dead time and maximize the EPR signal following a pulse. Each section of a 250 MHz resonator that accommodates 1 inch diameter samples had a critically coupled Q of 950 when Q‐switching was not installed. With Q‐switching, free induction decays and electron spin echoes for 0.2 mM aqueous solutions of triarylmethyl radicals were obtained with dead times of a few hundred nanoseconds. Typically 50–60 dB isolation was achieved with various samples. © 2002 Wiley Periodicals, Inc. Concepts in Magnetic Resonance (Magn Reson Engineering) 15: 37–46, 2002.

Overhauser enhanced magnetic resonance imaging for tumor oximetry: coregistration of tumor anatomy and tissue oxygen concentration.

An efficient noninvasive method for in vivo imaging of tumor oxygenation by using a low-field magnetic resonance scanner and a paramagnetic contrast agent is described. The methodology is based on Overhauser enhanced magnetic resonance imaging (OMRI), a functional imaging technique. OMRI experiments were performed on tumor-bearing mice (squamous cell carcinoma) by i.v. administration of the contrast agent Oxo63 (a highly derivatized triarylmethyl radical) at nontoxic doses in the range of 2-7 mmol/kg either as a bolus or as a continuous infusion. Spatially resolved pO(2) (oxygen concentration) images from OMRI experiments of tumor-bearing mice exhibited heterogeneous oxygenation profiles and revealed regions of hypoxia in tumors (<10 mmHg; 1 mmHg = 133 Pa). Oxygenation of tumors was enhanced on carbogen (95% O(2)/5% CO(2)) inhalation. The pO(2) measurements from OMRI were found to be in agreement with those obtained by independent polarographic measurements using a pO(2) Eppendorf electrode. This work illustrates that anatomically coregistered pO(2) maps of tumors can be readily obtained by combining the good anatomical resolution of water proton-based MRI, and the superior pO(2) sensitivity of EPR. OMRI affords the opportunity to perform noninvasive and repeated pO(2) measurements of the same animal with useful spatial (approximately 1 mm) and temporal (2 min) resolution, making this method a powerful imaging modality for small animal research to understand tumor physiology and potentially for human applications.

Frequency dependence of EPR signal intensity, 248 MHz to 1.4 GHz.

The electron paramagnetic resonance pulsed free induction decay (FID) of a degassed solution of a triaryl methyl radical, methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2-d:4,5-d']bis(1,3)dithiol-4-yl) tripotassium salt, 0.2 mM in H2O, was measured at VHF (247.5 MHz) and L-band (1.40 GHz). The calculated and observed FID signal amplitudes (in millivolts) agreed within 1 and 6%, and the ratio of the normalized FID signals at the two frequencies agreed within 5%. The FID decay time constant was 2.7 micros at both frequencies.

Optimization of field-cycled PEDRI for in vivo imaging of free radicals

A numerical model of the behaviour of the magnetization in a field-cycled dynamic nuclear polarization (DNP) experiment is presented, with the aim of optimizing pulse sequence parameters in field-cycled proton–electron double-resonance free radical imaging. The model is used to predict the observed enhancement of the NMR signal as a function of the magnetic field strength, EPR irradiation frequency and pulse sequence timing, as well as the properties of the sample including the NMR and EPR relaxation times. The model allowed optimization of parameters in the field-cycled DNP experiment, in particular the EPR irradiation frequency, to find the value which would give the largest difference between NMR signals recorded with and without EPR irradiation. Experiments to verify the model were carried out using aqueous solutions of TEMPOL, which exhibits three hyperfine lines in its EPR spectrum and triarylmethyl (TAM), which has a single, narrow line. It was found that the model predicted very well the variation in DNP enhancement with EPR irradiation power for both samples. The behaviour of the NMR signal with EPR irradiation frequency in studies using TEMPOL was also accurately modelled, with the optimum frequency lying between 60 and 80 MHz, depending on the EPR irradiation power. The optimum frequency obtained from the model also agreed with the experimental data obtained using the TAM free radical, but with this sample the theoretical curves tended to deviate from the experimental data at irradiation frequencies below 70 MHz.

Electron spin relaxation of triarylmethyl radicals in fluid solution.

Electron spin relaxation times of a Nycomed triarylmethyl radical (sym-trityl) in water, 1:1 water:glycerol, and 1:9 water:glycerol were measured at L-band, S-band, and X-band by pulsed EPR methods. In H(2)O solution, T(1) is 17+/-1 micros at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T(1) in 1:1 water:glycerol is characteristic of domination by a Raman process between 20 and 80 K. The increased spin-lattice relaxation rates at higher temperatures, including room temperature, are attributed to a local vibrational mode that modulates spin-orbit coupling. In H(2)O solution, T(2) is 11+/-1 micros at X-band, increasing to 13+/-1 micros at L-band. For more viscous solvent mixtures, T(2) is much shorter than T(1) and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T(2). In water and 1:1 water:glycerol solutions continuous wave EPR linewidths are not relaxation determined, but become relaxation determined in the higher viscosity 1:9 water:glycerol solutions. The Lorentzian component of the 250-MHz linewidths as a function of viscosity is in good agreement with T(2)-determined contributions to the linewidths at higher frequencies.

Electron Spin Relaxation Time Measurements Using Radiofrequency Longitudinally Detected ESR and Application in Oximetry

Longitudinally detected ESR (LODESR) involves transverse ESR irradiation with a modulated source and observing oscillations in the spin magnetization parallel to the main magnetic field. In this study, radiofrequency-LODESR was used for oximetry by measuring the relaxation times of the electron. T1e and T2e were measured by investigating LODESR signal magnitude as a function of detection frequency. We have also predicted theoretically and verified experimentally the LODESR signal phase dependence on detection frequency and relaxation times. These methods are valid even for inhomogeneous lines provided that T1e>>T2e. We have also developed a new method for measuring T1e, valid for inhomogeneous spectra, for all values of T1e and T2e, based on measuring the spectral area as a function of detection frequency. We have measured T1e and T2e for lithium phthalocyanine crystals, for the nitroxide TEMPOL, and for the single line agent Triarylmethyl (TAM). Furthermore, we have collected spectra from aqueous solutions of TEMPOL and TAM at different oxygen concentrations and confirmed that T1e values are reduced with increased oxygen concentration. We have also measured the spin-lattice electronic relaxation time for degassed aqueous solutions of the same agents at different agent concentrations. T1e decreases as a function of concentration for TAM while it remains independent of free radical concentration for TEMPOL, a major advantage for oxygen mapping. This method, combined with the ability of LODESR to provide images of exogenous free radicals in vivo, presents an attractive alternative to the conventional transverse ESR linewidth based oximetry methods.

Cyclic Voltammetric Characteristics of Tri- and Diarylcarbenium Salts Having Some Methoxyphenyl Groups

Abstract Triarylcarbenium salts (Φa3C+, ΦaΦb2C+, Φb3C+, ΦfΦb2C+, ΦeΦb2C+, PhΦb2C+, and FcC+ΦbPh [Φa, Φb, Φe, Φf, Ph, Fc = 2,4,6-(MeO)3C6H2, 2,6-(MeO)2C6H3, 2-(MeO)C6H4, 4-(MeO)C6H4, C6H5, Fe(η5-C5H5)(η5-C5H4)]) showed a reversible redox wave with one-electron transfer at a Pt electrode in 1,2-dichloroethane, while the reduction of diarylcarbenium salts (Φa2C+H, FcC+ΦaH, FcC+ΦbH, FcC+ΦcH, FcC+ΦeH, and FcC+PhH [Φc = 2,5-(MeO)2C6H3]) proceeded irreversibly. A wide range of cathodic shifts in the redox potentials (198—-377 mV for triarylcarbenium salts) and in the reduction peak potentials (-23—-311 mV for monoaryl(ferrocenyl)carbenium salts) could be observed as the number of o- and p-methoxy groups increased. Probably, the electron donation from the o- and p-methoxy substituents to the central carbon caused the wide negative shift in the redox potentials. These triarylcarbenium salts again showed reversible redox waves in their relatively low concentration region at an ITO electrode in an aqueous acidic medium. In the higher concentration region, an anodic wave was varied to a sharp peak, indicating the adsorption of triarylmethyl radicals. It is very interesting that the reversible redox behavior of these triarylcarbenium salts was observable even in an aqueous medium.

Fluorocarbanion chemistry. Tris(4-nitro-2,3,5,6-tetrafluorophenyl) methane and companions

The titled compound ( 2) is prepared by oxidation of tris(4-amino-2,3,5,6-tetrafluorophenyl) methane with 98% H 2O 2 and trifluoroacetic anhydride. Compound 2, a strong carbon acid, forms long-persisting deep blue solutions of the anion on reaction with alkalis, amines, and alcohols. Oxidation of the anion of 2 with KMnO 4 provides the corresponding triarylmethyl radical. Reaction of 2 with LiOCH 3 in methanol gives the stable blue lithium salt. The 4-cyano analog of 2 has also been prepared.

Formation of triphenylmethyl radicals upon alkaline hydrolysis of diphenyl sulfophthalide

Triphenylmethyl radicals (TPMR) with ano-positioned sulfonate group are generated by alkaline hydrolysis of diphenyl sulfophthalide in DMSO. Electronic and ESR spectra of these radicals are characterized. It is suggested that the radicals result from one-electron transfer reactions. Triarylmethyl radicals are also formed in alkaline hydrolysis of polyarylenesulfophthalides.

High-power Low-Q RF ESR Resonator for Pulse Techniques

Abstract A new type RF ESR resonator for pulse techniques was developed. The resonator is an LC circuit that consists of a single turn coil (inner diameter, 45 mm) and ceramic capacitors. A two-turn coil-shaped polyethylene tube is located inside of the loop and an aqueous sodium chloride solution is perfused into the tube at a rate of 6 cm3/min. Its resonant frequency is approximately 300 MHz, with a Q of less than 70. Using this resonator, pulsed-longitudinally detected ESR measurements of a phantom (inner diameter, 20 mm) containing a triaryl methyl spin probe were performed. The Q value was sufficiently low for irradiation over wide frequency band in the pulsed radiowave where the pulse width was 200 ns. The apparatus can handle a sufficient high power (maximum, 100 W) adjusted according to the sample size.

Synthesis and X-ray Molecular Structure of the First Stable Organic Radical Lacking Resonance Stabilization

Carbon-centered radicals constitute an important and extensively studied group of reactive intermediates.1 However, despite extensive research the experimental structures of only four types of stable organic radicals have been reported: (1) triarylmethyl radicals 1, with X ) NO2, halogen;3 (2) a per-pyridiniumsubstituted allyl radical, 2;4 (3) the cyclopentadienyl radical, 3;5 (4) the dodecamethylcarba-closo-dodecaboranyl radical (CB11Me12). All of these radicals are strongly stabilized thermodynamically by effective delocalization of the unpaired electron and kinetically by the bulky substituents.2-6 Bulky alkyl and silyl substituents can reduce the reactivity of simple localized carboncentered radicals to the extend that they become “persistent”.1a,b However, there are no examples of such radicals being a pure crystal.

Thermolysis of Benzoenyne-Allenes To Form Biradicals and Subsequent Intramolecular Trapping with a Tetraarylallene To Generate Two Triarylmethyl Radical Centers.

Thermolysis of the benzoenyne-allene 20 in 1,4-cyclohexadiene (1,4-CHD) at 75 degrees C produced the cycloaromatized adduct 23 in 22% yield. The reaction presumably proceeds through a cascade sequence involving an initial Myers cyclization reaction to form the biradical 21. The subsequent trapping of the aryl radical center in 21 with the tetraarylallenic moiety intramolecularly affords 22, having two stabilized triarylmethyl radical centers. Hydrogen-atom abstraction from 1,4-CHD by 22, manifesting its radical character, then produces 23. Similarly, thermolysis of benzoenyne-allenes 25 in 1,4-CHD furnished fluoranthenes 27 in essentially quantitative yields. The presence of the fused five-membered ring in 20 and 25 is necessary to direct the initial biradical-forming step toward the Myers cyclization reaction. Without the five-membered ring as in the cases of 31, the C2-C6 cyclization reaction became the preferred pathway, leading to benzofluorenes 34.