Preparation d'une serie de complexes Cu-salicylaldimine dans lesquels le nitroxyle marqueur d... more Preparation d'une serie de complexes Cu-salicylaldimine dans lesquels le nitroxyle marqueur de spin est relie au carbone α du pont ethylene ou trimethylene par une liaison amide
A programmable timing unit (PTU) has been designed and constructed having 1 ns resolution and a g... more A programmable timing unit (PTU) has been designed and constructed having 1 ns resolution and a generalized matrix-based architecture. Automated measurement and calibration of commercially available programmable delay lines achieve the 1 ns resolution. The matrix architecture allows generalized flexibility in establishing the connections between timing circuits called event markers and output functions that control equipment external to the PTU. The microprocessor-based PTU communicates with a PC through a standard RS-232C serial interface. The PC contains software developed to control specific timing sequences produced by the PTU for control of experimental apparatus. While the PTU was developed for use in an electron paramagnetic resonance laboratory, the architecture is extremely generalized and use in other applications would require only the development of specific application software. The software developed so far has been done in Microsoft Visual BASIC, but other software tools could be used without modification of the PTU hardware.
to polarize the ligand differently in the two oxidation states. Acknowledgment. We thank Dr. John... more to polarize the ligand differently in the two oxidation states. Acknowledgment. We thank Dr. John Petersen of Clemson of this pro,ect by the PSC-BHE grant Crabtree and Morris.20*21 The relative effectiveness of the rhodium and iridium catalysts and a comparison with the reactions22 might be related to the ability of the metal ions reactivity of Wilkinson's catalyst, [RhC1(PPh3)31 3 in these University for the gift of [Rh(",),py] (C104)3, We gratefully acknowledge program of t h e c i t y University of New York.
Single-crystal EPR spectra have been obtained for two spin-labeled silver porphyrins doped into z... more Single-crystal EPR spectra have been obtained for two spin-labeled silver porphyrins doped into zinc tetraphenylporphyrin. For each of the complexes four conformations were observed. The dependence of the electron-electron spinspin splittings on the orientation of the crystal in the magnetic field was analyzed to obtain the isotro ic exchange and anisotropic dipolar constant, J , ranged from-35 X lo4 to +7 X lo4 cm-I. There was no correlation between the values of r and J. Comparison of the values of J obtained for the series of vanadyl, copper, and silver complexes of the spin-labeled porphyrins in fluid solution and frozen solution and those doped into ZnTPP indicated good agreement among the results obtained in the three media, Exchange increased in the order VIVO < Cu(I1) < Ag(I1) and was greater for the trans isomers than for the cis isomers contributions to the interaction. The interspin distance, r, ranged from 9.5 to 13.8 BI. Values of the exchange coupling
indicates that the magnitude of J correlates with the extent of metal electron delocalization. In... more indicates that the magnitude of J correlates with the extent of metal electron delocalization. In systems of this complexity, one-parameter fits are not to be expected and many additional complexes will have to be examined before a quantitative description of exchange interactions emerges. Acknowledgment. This work was supported in part by NIH Grant GM 21 156. The Q-band spectrometer was purchased with partial support from NSF Grant CHE 78-16195. The assistance of K. T. Trevor and J. M. Neri in the preparation 20, 3354-3362 of the starting porphyrin (IV) is gratefully a~knowledged.~ Discussions with Professor J. M. Hornback contributed to some of the syntheses. Elemental analyses were performed by Spang Microanalytical Laboratory. B.M.S. thanks Shivaji University for a study leave.
Electron4ectron coupling constants, J, have been obtained from solution EPR spectra for a series ... more Electron4ectron coupling constants, J, have been obtained from solution EPR spectra for a series of spin-labeled pyridines coordinated to vanadyl bis(trifluoroacetylacetonate), vanadyl bis(hexafluoroacetylacetonate), and copper bis(hexafluor0acetylacetonate). The value of J was larger for a ligand with a urea linkage between the pyridine ring and the nitroxyl ring than for a ligand with an amide linkage. The addition of a CH2 group between the pyridine ring and the amide or urea linkage led to a decrease in the value of J. The impact of the additional CH2 group was greater for the urea linkage than for the amide linkage and was greater for the 4-substituted pyridine ligands than for the 3-substituted pyridine ligands. The observed dependence of J on linkage is attributed to a winteraction pathway in the urea ligands.
A series (1a-1h) of crystalline bis-pyrrolate-imine copper(II) complexes was isolated in good yie... more A series (1a-1h) of crystalline bis-pyrrolate-imine copper(II) complexes was isolated in good yield (typically 60-90%) by reaction of the Schiff base 2-N-(R)-pyrrolecarbaldimine (where R is an alkyl group of variable steric capacity) with a copper(II) salt and base in methanol or water. For new complexes 1f-1h, pyrrolate-imine coordination is inferred by loss of ν(N-H) and a shift (ca. 40 cm)
Biomedical EPR Part A focuses on applications of EPR spectroscopy in the areas of free radicals, ... more Biomedical EPR Part A focuses on applications of EPR spectroscopy in the areas of free radicals, metals, medicine, and physiology. The book celebrates the 70th birthday of Prof. James S. Hyde, Medical College of Wisconsin, and his contributions to this field. Chapters are written to provide introductory material for new-comers to the field which lead into up-to-date reviews that provide perspective on the wide range of questions that can be addressed by EPR. Key Features: Free Radicals in Medicine Radicals in vivo and in Model Systems, and their Study by Spin Trapping In vivo EPR, including Oximetry and Imaging Time Domain EPR at Radio Frequencies EPR of Copper Complexes: Motion and Frequency Dependence Time Domain EPR and Electron Spin Echo Envelope Modulation
Relaxation times were measured at X-band (9.5 GHz) at concentrations up to 20.8 mM for two nitrox... more Relaxation times were measured at X-band (9.5 GHz) at concentrations up to 20.8 mM for two nitroxides that are widely used in Overhauser dynamic nuclear polarization (ODNP) experiments. Carboxy proxyl (CP, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy) in water and tempol (TP, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) in toluene have been studied with oxygen removed. For comparison, solutions of CP were also studied in air-saturated solution, which is the typical preparation for ODNP experiments. T 2 and T 2HE (HE = Heisenberg exchange) were measured by electron spin echo. The spin magnetization recovery time constant from inversion recovery experiments, which we denote as T 1e *, includes contributions from T 1e and T 1HE. In the absence of oxygen, values of T 1e * for both radicals decrease with increasing concentration up to ~ 1 mM, then increase again towards 10 mM. The concentration dependence results from changes in the relative contributions from T 1e and T 1HE. In air-saturated solutions of CP T 1e * decreases with concentration to about 200 ns at 1 mM, and then remains independent of further concentration increases. T 1e and T 1N were also measured with a digital saturation recovery spectrometer. Using the combined results from spin echo, inversion recovery, and saturation recovery we could extract the values of T 1e , T 1HE , T 1N , T 2e , and T 2HE for both radicals in this fast tumbling regime. Applied Magnetic Resonance Certain commercial instruments and software are identified to specify the experimental study adequately. This does not imply endorsement by NIST or that the instruments and software are the best available for the purpose.
Site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy offer a... more Site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide α-amino acid and its N-(9fluorenylmethoxycarbonyl) (Fmoc) derivative, has been synthesized that exhibit slow enough spin echo dephasing to permit accurate distance measurements by pulse EPR at temperatures up to 125 K in 1:1 water:glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR to the study of structure and conformation for peptides and proteins.
Trityl-nitroxides show substantial promise as polarizing agents in solid state dynamic nuclear po... more Trityl-nitroxides show substantial promise as polarizing agents in solid state dynamic nuclear polarization. To optimize performance it is important to understand the impact of spin-spin interactions on relaxation times of the diradicals. CW spectra and electron spin relaxation were measured for two trityl-nitroxides that differ in the substituents on the amide linker and have different strengths of the exchange interaction J. Analysis of the EPR spectra in terms of overlapping AB spin-spin splitting patterns explains the impact of J on various regions of the spectra. Even modest values of J are large relative to the separation between trityl and nitroxide resonances for some nitrogen nuclear spin state. Two conformations for each diradical were observed in CW spectra in fluid solution at X-band and Q-band. For one diradical J = 15 G (83%) and 5 G (17%) at 293 K, and J = 27 G (67%) and 3 G (33%) with interspin distances of 16 Å and 12 Å, respectively, at 80 K. For the second diradical the exchange interaction is stronger: the two conformations in fluid solution at 293 K had J = 113 G (67%) and 59 G (33%) and at 80 K the value of J was 43 G and there were two conformations with interspin distances of 13 and 11.5 Å. The observation of two conformations for each diradical, with different values of J, demonstrates the dependence of their exchange interactions on through-bond orbital interactions. X-band values of spin relaxation rates 1/T1 and 1/Tm at 80 to 120 K for the trityl-nitroxides are similar to values for nitroxide mono-radicals, and faster than for trityl radicals. These observations show that even for a relatively small value of J, the nitroxide is very effective in enhancing the relaxation of the more slowly relaxing trityl.
Synthesis of bis-spiro-oxetane and bis-spiro-tetrahydrofuran pyrroline nitroxide radicals relies ... more Synthesis of bis-spiro-oxetane and bis-spiro-tetrahydrofuran pyrroline nitroxide radicals relies on the Mitsunobu reaction-mediated double cyclizations of N-Boc protected pyrroline tetraols. Structures of the nitroxide radicals are supported by X-ray crystallography. In a trehalose/sucrose matrix at room temperature, the bis-spiro-oxetane nitroxide radical possesses electron spin coherence time, Tm ≈ 0.7 μs. The observed enhanced Tm is most likely associated with strong hydrogen bonding of oxetane moieties to the trehalose/sucrose matrix.
Electron spin relaxation times T 1 and T m of Tb 3+ and Tm 3+ in 1:1 water:ethanol and of Tb 3+ d... more Electron spin relaxation times T 1 and T m of Tb 3+ and Tm 3+ in 1:1 water:ethanol and of Tb 3+ doped (2%) in crystalline La 2 (oxalate) 3 decahydrate were measured between about 4.2 and 10 K. Both cations are non-Kramers ions and have J = 6 ground states. Echo-detected spectra are compared with CW spectra and with field-stepped direct-detected EPR spectra. Due to the strong temperature dependence of T 1 , measurements were not made above 10 K. Between about 4.2 and 6 K T 1 is strongly concentration dependent between 1 and ~50 mM. T 1 values at 4.2 K are in the μs range which is orders of magnitude faster than for 3d transition metals. Phase memory times, T m , are less than 500 ns, which is short relative to values observed for 3d transition metals and organic radicals at 4 K. T m is longer in the oxalate lattice which is attributed to the lower proton concentration in oxalate than in the organic solvent, which decreases nuclear spin diffusion. The rigidity of the crystalline lattice also may contribute to longer T m .
A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in 13 C at the central carbon (1... more A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in 13 C at the central carbon (13 C 1-PTMTC) was characterized in phosphate buffered saline solution (pH = 7.2) (PBS) at ambient temperature. Samples immobilized in 1:1 PBS:glycerol or in 9:1 trehalose:sucrose were studied as a function of temperature. Isotope enrichment at C 1 creates a trityl that can be used to accurately measure microscopic viscosity. Understanding of the impact of the 13 C hyperfine interaction on electron spin relaxation is important for application of this trityl in oximetry and distance measurements. The anisotropic 13 C 1 hyperfine couplings (A x = A y = 24 ± 2 MHz, A z = 200 ± 1 MHz) are larger than for the related 13 C 1-perdeuterated Finland trityl (13 C 1-dFT) and the g anisotropy (g x = 2.0013, g y = 2.0016, g z = 2.0042) is slightly larger than for 13 C 1-dFT. The tumbling correlation times (τ R) for 13 C 1-PTMTC are 0.20 ± 0.02 ns in PBS and 0.40 ± 0.05 ns in 3:1 PBS:glycerol, which are shorter than for 13 C 1-dFT in the same solutions. T 1 for 13 C 1-PTMTC is 3.5 ± 0.5 μs in PBS and 5.3 ± 0.4 μs in 3:1 PBS:glycerol, which are shorter than for 13 C 1-dFT due to faster tumbling, larger anisotropy of the 13 C 1 hyperfine, and about 30% larger contribution from the local mode. In immobilized samples T 1 for 13 C 1-PTMTC is similar to that for 13 C 1-dFT and other trityls without chlorine or 13 C 1 substituents, indicating that the 13 C 1 and Cl substituents on the phenyl rings have little impact on T 1. The temperature dependence of T 1 was modeled with contributions from the direct, Raman, and local mode processes. Broadening of CW linewidths of about 0.6 G in fluid solution and about 2 G in rigid lattice is attributed to unresolved 35,37 Cl hyperfine couplings.
Soluble stable radicals are used as spin probes and spin labels for in vitro and in vivo Electron... more Soluble stable radicals are used as spin probes and spin labels for in vitro and in vivo Electron Paramagnetic Resonance (EPR) spectroscopy and imaging applications. We report the synthesis and characterization of a perchlorinated triarylmethyl radical enriched 99% at the central carbon, 13C1-PTMTC. The anisotropy of the hyperfine splitting with the 13C1 (Ax=26, Ay=25, Az=199.5 MHz) and the g (gx=2.0015, gy=2.0015, gz=2.0040) are responsible for a strong effect of the radical tumbling rate on the EPR spectrum. The rotational correlation time can be determine by spectral simulation or via the linewidth after calibration. As spin probe 13C1-PTMTC can be used to measure media microviscosity with high sensitivity. Bound to a macromolecule as spin label, 13C1-PTMTC could be used to study local mobility and molecular interactions.
Preparation d'une serie de complexes Cu-salicylaldimine dans lesquels le nitroxyle marqueur d... more Preparation d'une serie de complexes Cu-salicylaldimine dans lesquels le nitroxyle marqueur de spin est relie au carbone α du pont ethylene ou trimethylene par une liaison amide
A programmable timing unit (PTU) has been designed and constructed having 1 ns resolution and a g... more A programmable timing unit (PTU) has been designed and constructed having 1 ns resolution and a generalized matrix-based architecture. Automated measurement and calibration of commercially available programmable delay lines achieve the 1 ns resolution. The matrix architecture allows generalized flexibility in establishing the connections between timing circuits called event markers and output functions that control equipment external to the PTU. The microprocessor-based PTU communicates with a PC through a standard RS-232C serial interface. The PC contains software developed to control specific timing sequences produced by the PTU for control of experimental apparatus. While the PTU was developed for use in an electron paramagnetic resonance laboratory, the architecture is extremely generalized and use in other applications would require only the development of specific application software. The software developed so far has been done in Microsoft Visual BASIC, but other software tools could be used without modification of the PTU hardware.
to polarize the ligand differently in the two oxidation states. Acknowledgment. We thank Dr. John... more to polarize the ligand differently in the two oxidation states. Acknowledgment. We thank Dr. John Petersen of Clemson of this pro,ect by the PSC-BHE grant Crabtree and Morris.20*21 The relative effectiveness of the rhodium and iridium catalysts and a comparison with the reactions22 might be related to the ability of the metal ions reactivity of Wilkinson's catalyst, [RhC1(PPh3)31 3 in these University for the gift of [Rh(",),py] (C104)3, We gratefully acknowledge program of t h e c i t y University of New York.
Single-crystal EPR spectra have been obtained for two spin-labeled silver porphyrins doped into z... more Single-crystal EPR spectra have been obtained for two spin-labeled silver porphyrins doped into zinc tetraphenylporphyrin. For each of the complexes four conformations were observed. The dependence of the electron-electron spinspin splittings on the orientation of the crystal in the magnetic field was analyzed to obtain the isotro ic exchange and anisotropic dipolar constant, J , ranged from-35 X lo4 to +7 X lo4 cm-I. There was no correlation between the values of r and J. Comparison of the values of J obtained for the series of vanadyl, copper, and silver complexes of the spin-labeled porphyrins in fluid solution and frozen solution and those doped into ZnTPP indicated good agreement among the results obtained in the three media, Exchange increased in the order VIVO < Cu(I1) < Ag(I1) and was greater for the trans isomers than for the cis isomers contributions to the interaction. The interspin distance, r, ranged from 9.5 to 13.8 BI. Values of the exchange coupling
indicates that the magnitude of J correlates with the extent of metal electron delocalization. In... more indicates that the magnitude of J correlates with the extent of metal electron delocalization. In systems of this complexity, one-parameter fits are not to be expected and many additional complexes will have to be examined before a quantitative description of exchange interactions emerges. Acknowledgment. This work was supported in part by NIH Grant GM 21 156. The Q-band spectrometer was purchased with partial support from NSF Grant CHE 78-16195. The assistance of K. T. Trevor and J. M. Neri in the preparation 20, 3354-3362 of the starting porphyrin (IV) is gratefully a~knowledged.~ Discussions with Professor J. M. Hornback contributed to some of the syntheses. Elemental analyses were performed by Spang Microanalytical Laboratory. B.M.S. thanks Shivaji University for a study leave.
Electron4ectron coupling constants, J, have been obtained from solution EPR spectra for a series ... more Electron4ectron coupling constants, J, have been obtained from solution EPR spectra for a series of spin-labeled pyridines coordinated to vanadyl bis(trifluoroacetylacetonate), vanadyl bis(hexafluoroacetylacetonate), and copper bis(hexafluor0acetylacetonate). The value of J was larger for a ligand with a urea linkage between the pyridine ring and the nitroxyl ring than for a ligand with an amide linkage. The addition of a CH2 group between the pyridine ring and the amide or urea linkage led to a decrease in the value of J. The impact of the additional CH2 group was greater for the urea linkage than for the amide linkage and was greater for the 4-substituted pyridine ligands than for the 3-substituted pyridine ligands. The observed dependence of J on linkage is attributed to a winteraction pathway in the urea ligands.
A series (1a-1h) of crystalline bis-pyrrolate-imine copper(II) complexes was isolated in good yie... more A series (1a-1h) of crystalline bis-pyrrolate-imine copper(II) complexes was isolated in good yield (typically 60-90%) by reaction of the Schiff base 2-N-(R)-pyrrolecarbaldimine (where R is an alkyl group of variable steric capacity) with a copper(II) salt and base in methanol or water. For new complexes 1f-1h, pyrrolate-imine coordination is inferred by loss of ν(N-H) and a shift (ca. 40 cm)
Biomedical EPR Part A focuses on applications of EPR spectroscopy in the areas of free radicals, ... more Biomedical EPR Part A focuses on applications of EPR spectroscopy in the areas of free radicals, metals, medicine, and physiology. The book celebrates the 70th birthday of Prof. James S. Hyde, Medical College of Wisconsin, and his contributions to this field. Chapters are written to provide introductory material for new-comers to the field which lead into up-to-date reviews that provide perspective on the wide range of questions that can be addressed by EPR. Key Features: Free Radicals in Medicine Radicals in vivo and in Model Systems, and their Study by Spin Trapping In vivo EPR, including Oximetry and Imaging Time Domain EPR at Radio Frequencies EPR of Copper Complexes: Motion and Frequency Dependence Time Domain EPR and Electron Spin Echo Envelope Modulation
Relaxation times were measured at X-band (9.5 GHz) at concentrations up to 20.8 mM for two nitrox... more Relaxation times were measured at X-band (9.5 GHz) at concentrations up to 20.8 mM for two nitroxides that are widely used in Overhauser dynamic nuclear polarization (ODNP) experiments. Carboxy proxyl (CP, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy) in water and tempol (TP, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) in toluene have been studied with oxygen removed. For comparison, solutions of CP were also studied in air-saturated solution, which is the typical preparation for ODNP experiments. T 2 and T 2HE (HE = Heisenberg exchange) were measured by electron spin echo. The spin magnetization recovery time constant from inversion recovery experiments, which we denote as T 1e *, includes contributions from T 1e and T 1HE. In the absence of oxygen, values of T 1e * for both radicals decrease with increasing concentration up to ~ 1 mM, then increase again towards 10 mM. The concentration dependence results from changes in the relative contributions from T 1e and T 1HE. In air-saturated solutions of CP T 1e * decreases with concentration to about 200 ns at 1 mM, and then remains independent of further concentration increases. T 1e and T 1N were also measured with a digital saturation recovery spectrometer. Using the combined results from spin echo, inversion recovery, and saturation recovery we could extract the values of T 1e , T 1HE , T 1N , T 2e , and T 2HE for both radicals in this fast tumbling regime. Applied Magnetic Resonance Certain commercial instruments and software are identified to specify the experimental study adequately. This does not imply endorsement by NIST or that the instruments and software are the best available for the purpose.
Site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy offer a... more Site-directed spin labeling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide α-amino acid and its N-(9fluorenylmethoxycarbonyl) (Fmoc) derivative, has been synthesized that exhibit slow enough spin echo dephasing to permit accurate distance measurements by pulse EPR at temperatures up to 125 K in 1:1 water:glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR to the study of structure and conformation for peptides and proteins.
Trityl-nitroxides show substantial promise as polarizing agents in solid state dynamic nuclear po... more Trityl-nitroxides show substantial promise as polarizing agents in solid state dynamic nuclear polarization. To optimize performance it is important to understand the impact of spin-spin interactions on relaxation times of the diradicals. CW spectra and electron spin relaxation were measured for two trityl-nitroxides that differ in the substituents on the amide linker and have different strengths of the exchange interaction J. Analysis of the EPR spectra in terms of overlapping AB spin-spin splitting patterns explains the impact of J on various regions of the spectra. Even modest values of J are large relative to the separation between trityl and nitroxide resonances for some nitrogen nuclear spin state. Two conformations for each diradical were observed in CW spectra in fluid solution at X-band and Q-band. For one diradical J = 15 G (83%) and 5 G (17%) at 293 K, and J = 27 G (67%) and 3 G (33%) with interspin distances of 16 Å and 12 Å, respectively, at 80 K. For the second diradical the exchange interaction is stronger: the two conformations in fluid solution at 293 K had J = 113 G (67%) and 59 G (33%) and at 80 K the value of J was 43 G and there were two conformations with interspin distances of 13 and 11.5 Å. The observation of two conformations for each diradical, with different values of J, demonstrates the dependence of their exchange interactions on through-bond orbital interactions. X-band values of spin relaxation rates 1/T1 and 1/Tm at 80 to 120 K for the trityl-nitroxides are similar to values for nitroxide mono-radicals, and faster than for trityl radicals. These observations show that even for a relatively small value of J, the nitroxide is very effective in enhancing the relaxation of the more slowly relaxing trityl.
Synthesis of bis-spiro-oxetane and bis-spiro-tetrahydrofuran pyrroline nitroxide radicals relies ... more Synthesis of bis-spiro-oxetane and bis-spiro-tetrahydrofuran pyrroline nitroxide radicals relies on the Mitsunobu reaction-mediated double cyclizations of N-Boc protected pyrroline tetraols. Structures of the nitroxide radicals are supported by X-ray crystallography. In a trehalose/sucrose matrix at room temperature, the bis-spiro-oxetane nitroxide radical possesses electron spin coherence time, Tm ≈ 0.7 μs. The observed enhanced Tm is most likely associated with strong hydrogen bonding of oxetane moieties to the trehalose/sucrose matrix.
Electron spin relaxation times T 1 and T m of Tb 3+ and Tm 3+ in 1:1 water:ethanol and of Tb 3+ d... more Electron spin relaxation times T 1 and T m of Tb 3+ and Tm 3+ in 1:1 water:ethanol and of Tb 3+ doped (2%) in crystalline La 2 (oxalate) 3 decahydrate were measured between about 4.2 and 10 K. Both cations are non-Kramers ions and have J = 6 ground states. Echo-detected spectra are compared with CW spectra and with field-stepped direct-detected EPR spectra. Due to the strong temperature dependence of T 1 , measurements were not made above 10 K. Between about 4.2 and 6 K T 1 is strongly concentration dependent between 1 and ~50 mM. T 1 values at 4.2 K are in the μs range which is orders of magnitude faster than for 3d transition metals. Phase memory times, T m , are less than 500 ns, which is short relative to values observed for 3d transition metals and organic radicals at 4 K. T m is longer in the oxalate lattice which is attributed to the lower proton concentration in oxalate than in the organic solvent, which decreases nuclear spin diffusion. The rigidity of the crystalline lattice also may contribute to longer T m .
A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in 13 C at the central carbon (1... more A perchlorotriarylmethyl tricarboxylic acid radical 99% enriched in 13 C at the central carbon (13 C 1-PTMTC) was characterized in phosphate buffered saline solution (pH = 7.2) (PBS) at ambient temperature. Samples immobilized in 1:1 PBS:glycerol or in 9:1 trehalose:sucrose were studied as a function of temperature. Isotope enrichment at C 1 creates a trityl that can be used to accurately measure microscopic viscosity. Understanding of the impact of the 13 C hyperfine interaction on electron spin relaxation is important for application of this trityl in oximetry and distance measurements. The anisotropic 13 C 1 hyperfine couplings (A x = A y = 24 ± 2 MHz, A z = 200 ± 1 MHz) are larger than for the related 13 C 1-perdeuterated Finland trityl (13 C 1-dFT) and the g anisotropy (g x = 2.0013, g y = 2.0016, g z = 2.0042) is slightly larger than for 13 C 1-dFT. The tumbling correlation times (τ R) for 13 C 1-PTMTC are 0.20 ± 0.02 ns in PBS and 0.40 ± 0.05 ns in 3:1 PBS:glycerol, which are shorter than for 13 C 1-dFT in the same solutions. T 1 for 13 C 1-PTMTC is 3.5 ± 0.5 μs in PBS and 5.3 ± 0.4 μs in 3:1 PBS:glycerol, which are shorter than for 13 C 1-dFT due to faster tumbling, larger anisotropy of the 13 C 1 hyperfine, and about 30% larger contribution from the local mode. In immobilized samples T 1 for 13 C 1-PTMTC is similar to that for 13 C 1-dFT and other trityls without chlorine or 13 C 1 substituents, indicating that the 13 C 1 and Cl substituents on the phenyl rings have little impact on T 1. The temperature dependence of T 1 was modeled with contributions from the direct, Raman, and local mode processes. Broadening of CW linewidths of about 0.6 G in fluid solution and about 2 G in rigid lattice is attributed to unresolved 35,37 Cl hyperfine couplings.
Soluble stable radicals are used as spin probes and spin labels for in vitro and in vivo Electron... more Soluble stable radicals are used as spin probes and spin labels for in vitro and in vivo Electron Paramagnetic Resonance (EPR) spectroscopy and imaging applications. We report the synthesis and characterization of a perchlorinated triarylmethyl radical enriched 99% at the central carbon, 13C1-PTMTC. The anisotropy of the hyperfine splitting with the 13C1 (Ax=26, Ay=25, Az=199.5 MHz) and the g (gx=2.0015, gy=2.0015, gz=2.0040) are responsible for a strong effect of the radical tumbling rate on the EPR spectrum. The rotational correlation time can be determine by spectral simulation or via the linewidth after calibration. As spin probe 13C1-PTMTC can be used to measure media microviscosity with high sensitivity. Bound to a macromolecule as spin label, 13C1-PTMTC could be used to study local mobility and molecular interactions.
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Papers by Gareth Eaton