Observation of broad EPR spectra of transient free radicals by FT-EPR

Application of electron spin echo Fourier transform EPR (ESE-FT-EPR) to photo-induced chemical reactions is presented. Main purpose of this study is to observe broad EPR spectra of free radicals having very shortT ₂ ^* by means of the ESE-FT-EPR technique. Details of the experimental procedures are described. In ESE experiments design of the resonator is important to obtain sufficient spectral bandwidth because of use of multiple pulses which decrease the bandwidth. We designed and constructed Loop-Gap-Resonantors (LGR) for light irradiation experiments and their specifications were examined. The phase cycling method is essential to obtain pure ESE signals and proper time resolution by eliminating unwanted FID signals which result from imperfect pulse angles. We applied this technique to observe the photo-induced electron transfer reaction between tetraphenylporphinato zinc(II) (ZnTPP) and duroquinone (DQ) in an ethanol solution, and successfully observed the time resolved EPR spectra of the both Zn(TPP) cation and DQ anion radicals by ESE-FT-EPR of the Hahn echo. The half-height full-width of envelope of EPR spectrum of Zn(TPP)⁺, which is never observed in ordinary FT-EPR, is about 16 MHz. Specificity of spectra and the time resolution are compared among the ESE-FT-, FT- and cw-Time-Resolved-EPR (cw-TREPR) techniques.     

Pulsed 95 GHz high-field EPR heterodyne spectrometer with high spectral and time resolution

Various time resolved EPR methods are applied to different test samples to demonstrate the abilities of pulsed high-field EPR spectroscopy. Two-pulse-echo field swept EPR spectroscopy on a nitroxide radical shows the increased spectral resolution by separating different spin systems by their relaxation properties. Additionally N¹⁴ electron-spin-echo-envelope-modulation (ESEEM) is observed for these systems at fields as high as 3.5 T. Thus, the N¹⁴ hyperfine interaction couplings can be probed by ESEEM and pulsed ENDOR (electron-nuclear-double-resonance) experiments. The sensitivity of pulsed ENDOR experiments is compared with cw-ENDOR. The different linewidths and amplitudes of the two methods are discussed. Transient nutation experiments on light induced triplet states demonstrate the high sensitivity and time resolution of high-field pulsed EPR. The sensitivity and time resolution of our 95 GHz spectrometer are determined and compared with pulsed X-band EPR spectrometer performances.     

Nucleic acid spin labeling study of interaction between polyadenylic acid: Polyuridilic acid duplex and egg phosphatidylcholine vesicles. Involvement of terminal and internal nucleotides and sugar-phosphate backbone

Model DNA-membrane contacts were studied in the system containing polyadenylic acid (polyA): polyuridilic acid (polyU) duplex and egg phosphatidylcholine (PC) vesicles without or in the presence of MgCl₂. PolyU containing O-(1-oxyl-2,2,5,5-tetramethylpyrroline-3-carbonyl) spin label at 2′-OH moiety was used for spin labeling study of nucleic acid-phospholipid interactions. For polyA labeled with the same nitroxyl radical EPR spectra from spin labels attached at terminal and at internal nucleotide units contribute into the EPR spectrum of d.s. polynucleotide without or in the presence of egg PC vesicles and Mg ions. Internal spin labels are more sensitive to the interaction with PC-MLV than terminal ones. The complexation between s.s. polynucleotide and PC-MLV was shown. The values of spin labels’ melting temperature of polyA: SL-polyU duplex at 1 mM MgCl₂ reflect the changes in rotational mobility of terminal spin labels connected with duplex→triplex transition. Arrhenius plots of temperature dependence of EPR spectra parameters prove that ribose moieties of sugarphosphate backbone are involved into the interaction between polynucleotide duplex and phospholipid bilayer. Egg PC vesicles demonstrate stabilizing effect on polyA: polyU duplex at 1 mM of Mg²⁺ and destabilizing effect at 5 mM Mg²⁺.     

Electron spin polarization of doublet state species due to interaction with excited triplet states in single crystals

Doublet states species trapped in crystalline solids show transient spin polarized EPR spectra if the crystal is illuminated by visible or UV light. The spin polarization is accounted for by the interaction of the doublet species with photoexcited triplet states. The mechanism of the process producing the spin polarization is examined and some experimental examples are discussed. The analysis of the time evolution of the transient variation of the EPR signal allows the measurements of the spin lattice relaxation time and in some cases of the diffusion rate of mobile triplet excitations in the crystal lattice.     

Quantitative EPR spectrometry — “State of the art”

The purpose of the present review is to collect some results available in the literature in order to focus the attention of the world EPR community and thus to stimulate their activity on solving the problems of quantitative EPR spectrometry.     

IBM XT/AT data acquisition and processing system for Bruker ER 200D-SRC and ER 420 electron paramagnetic resonance spectrometers

Module giving IBM PC format of the EPR spectra recorded by Bruker ER 420 and ER 200D-SRC spectrometers is described. The files thus stored may be used for post-acquisition spectral processing using any program for this purpose and any IBM compatible PC.     

The EPR dipolar broadening of irradiated solids as detected using FID and ESE following selective hole burning

A novel electron spin echo method employing detection of FID and transient echo shape following selective hole burning in the EPR spectrum has been applied to study γ-irradiated malonic acid. The dipolar lineshapes of stabilized free radicals are separated into bulk and pairwise contributions; both of them have proved to be Lorentzian.     

Photo-CIDNP study of the splitting of the dinucleotidecis,syn-thymine dimer with reduced flavin as a sensitizer: Evidence for a thymine radical anion intermediate

Photochemically induced dynamic nuclear polarization (photo-CIDNP) studies were performed on the splitting ofcis,syn 2′-deoxythymidylyl-(3′→5′)-2′-deoxythymidine cyclobutane dimer (cs-dTp dT), using reduced flavin as a sensitizer. This system serves as a model for the light-induced repair mechanism of thymine dimers in DNA by the enzyme photolyase. The CIDNP spectrum shows enhanced absorption of the two C6-H protons of the corresponding monomer dTpdT, which demonstrates that a thymine radical anion is involved in the splitting of the dimer. This is supported by a similar CIDNP spectrum that is obtained with the electron-donorN _α-acetyl tryptophan as a sensitizer. This result suggests that the light-induced splitting of thymine dimers in DNA by photolyase also proceedsvia the thymine radical anion. The small difference in intensity of the two CIDNP signals belonging to the C6-H protons shows that the unpaired electron in the monomer radical has a slight preference for the thymine moiety at the 5′ terminus.     

The present state of quantitative EPR spectrometry: The results from an international experiment

The present work collects the results obtained from an international experiment carried out during 1991–1992 on quantitative EPR spectrometry. The data from 12 labs in Europe, Asia and America are compared with those obtained from weighing procedure for the same samples. The results from the different labs are characterized with a big variance not permitting to apply a standard statistical procedure. Nevertheless, these data are very helpful to point out the main group of problems in the present state of quantitative EPR spectrometry.     

Very-high-field EPR and its applications

A kind of review of recent developments of the very-high-field EPR technique and applications is presented, the largest part of attention being attracted to studies of molecular structures and molecular and chemical dynamics of free radicals, biradicals and some paramagnetic complexes. The very recent results on thermal spin polarization effects on high-field/low-temperature EPR lineshape are overviewed. These effects are proved to provide a novel approach to study systems withS>1/2, particularly radical pairs, radical clusters and biradicals.     

ESR and ESR microscopy in geosciences and radiation dosimetry

ESR and its microscopic imaging (microscopy) have found new applications in interdisciplinary fields related with geosciences and with radiation dosimetry. Geological, archaeological and forensic dating has been made through paleo-dosimetry of natural radiation and chemical reactions of both radical formation and valency changes. ESR microscopy, especially the simple scanning method has a potentiality to be used in various fields. New applications of ESR are briefly reviewed with some examples.     

Statistical analysis of DNA duplex structures in solution derived by high resolution NMR

An initial statistical analysis has been performed on the helical parameters for the solution structures of three DNA duplexes recently solved in this laboratory by proton NMR. Local conformations in these structures belong to the B family of forms; nevertheless they display a strong sequence-dependent heterogeneity akin to that found in single crystals and by theoretical calculations. However, average helical parameters as well as their variations are quite different for short DNA fragments in solution and in crystal. Average helical twist in three NMR-refined oligonucleotides is 34.6°, in remarkable agreement with independent solution-state data, while helical twist is 36° for DNA in crystals. Other characteristic features of solution DNA conformations are negative slide, systematically open minor groove (for almost all sequences), and decreased helical rise. The latter, rather unexpected finding, is correlated with a surprisingly strong non-flatness of Watson-Crick base pairs. Deviations of base pairs from planarity proved to be a significant source of conformational variability; of particular importance is base stagger, which is often missed in structural analysis of DNA. Several new structural parameters have been introduced for dinucleotide steps, characterizing non-planar geometries of constituent base pairs; these parameters show a significant degree of correlation with traditional step parameters (twist, tillt roll, shift, slide, rise). Many sequence-dependent features are observed in solution structures; variation of roll and slide parameters occurs according to “Calladine’s rules”, while variation of helical twist appears to oppose them. However, a larger set of solution structures is needed to complete the analysis of sequence dependence of DNA conformation.     

Interactions between polyamines and nucleotides studied by31P and1H NMR

The formation of complexes between poly- and mono-amines and some polyphosphate molecules such as ATP, ADP and tripolyphosphate was studied by³¹P and¹H NMR. Evidence for formation of complexes was obtained by¹H NMR, while the values of dissociation constants of the complexes between nucleotides and spermine and spermidine, calculated from the shift of the³¹P NMR, were in mM range.     

Very high field EPR study ofcis- andtrans-polyacetylene

Some neutralcis- andtrans-polyacetylene samples have been studied at 30–0.7 mm wave band EPR. A higher spectral resolution allows one to determine the principal values of anisotropicg-tensor of localized paramagnetic centers in polyacetylene, attributed to pinned neutral solitons. The Very High Frequency EPR allows us to observe the increase of a number of such centers and the appearance of soliton diffusion along the polymer chains after thecis-trans isomerization.     

EPR study of carbonate derived and ozonide radicals in carbonated apatites synthesized from aqueous solutions

In this study, a series of Na⁺ and¹²CO ₃ ^2? containing apatites synthesized by a hydrolysis of octo-calciumphosphate (OCP) and dried at 25°C until constant weight, were examined with EPR after X-irradiation. A variety of different paramagnetic radicals was formed, giving rise to composite and hence complex EPR powder spectra. The spectra were successfully decomposed into their basic components using a multivariate statistical technique. The different components could be identified unambiguously. In this way, it was found that an O^?, an O ₃ ^? , a CO ₃ ^? , two types of CO ₂ ^? and two types of CO ₃ ^3? ions were formed upon X-irradiation. Also resonances from atomic hydrogen were observed. The most striking feature is the presence of the ozonide ion, which is only rarely observed in irradiated hydroxyapatites. A comparison is made between the EPR spectra of apatite samples prepared by hydrolysis of OCP on the one hand, and those of samples prepared by hydrolyzing monetite, and the tooth enamel spectrum on the other hand.     

EPR investigation of Fe(III) complex with protoporphyrin IX in polyorganosiloxane matrices

A possibility of using polyorganosiloxanes with incorporated metal complexes as models for active sites of metalloenzymes is discussed. Poly(3-aminopropyl)- and poly(3-aminopropyl, methyl)siloxanes containing incorporated hemin were obtained in the reaction of hydrolytic polycondensation. Results obtained by diffuse electron reflection and EPR spectroscopy demonstrate an influence of the surrounding matrix on the hemin coordination center, namely on the rhombic distortion of the ligand field.     

Inosine — Adenosine base pairing in d-GGTACIAGTACC: Duplex and hairpin with a two-base loop

An oligonucleotide dodecamer d-GGTACIAGTACC containing two inosine-adenosine mismatched base-pairs has been studied by¹H and³¹P NMR spectroscopy. Unique assignments of¹H and³¹P spins have been achieved by using a recently proposed two-dimensionalheteronuclearlong rangecorrelation (2D-HELCO) technique. The 2D nuclear Overhauser enhancement spectroscopy (NOESY) spectrum recorded in a mixed solvent of 90% H₂O+10%²H₂O has been used to assign all the exchangeable imino and amino proton resonances. NMR results indicate that at concentrations above 5 mM, the molecule adopts a duplex structure where the inosine imino protons are hydrogen bonded. Cross-strand NOEs between the imino proton of I6 and H2 proton of A7 and that between H2 of I6 and H2 of A7 provide evidence for inosine-adenosine base-pairing, with both bases I6 and A7 inanti conformation. The observed NOE connectivitiesd1,d2 andd3 from almost the middle of the molecule to the two ends are consistent with a right handed B-DNA conformation. However, the molecule adopts a more extended conformation in the mismatch region to accomodate the bulky purine-purine (IA) base pairs. NMR experiments at lower concentrations indicate the coexistence of a monomeric hairpin with a two base loop, consisting of I6 and A7 units, in equilibrium with the duplex. 2D-ROESY experiment at such a concentration shows that the conformation of the stem of the hairpin does not deviate much from the corresponding region of the duplex and four nucleotides C5, I6, A7 and G8 undergo significant structural changes during the formation of hairpin loop.     

Low-frequency detected EPR: Principles and applications

The experimental works described are performed by the authors over last two decades by means of the LFD EPR technique. The essence of this method is low-frequency detection of the longitudinal spin magnetization while the magnetic resonance is excited by a strong microwave field. The first kind of LFD EPR is the enchanced longitudinal susceptibility effect (ELSE) which has been elaborated and applied to study spin thermodynamics in solids since 1972. Various applications of ELSE are described such as direct measuring of the spin-spin interaction temperatureT _ss in the course of resonance saturation, spin-lattice and cross relaxation, dynamic nuclear polarization etc. Another version of LFD EPR was employed to study electron spin-lattice relaxation of paramagnetic centers in high-temperature superconductors (HTSC). Recent experimental data are presented on the electron spin-lattice relaxation of Cu²⁺ ions in YBa₂Cu₃O_6+x at various temperatures andx values.     

Valence state of paramagnetic centers in fullerenes

Charge distribution on individual carbon atoms of a fullerene molecule C_n depends on the number of atoms it comprises,n. Only C₆₀ has the same charge on all its carbon atoms while other molecules containing fromn=32 ton=84 molecules are characterized by a variety of charge distributions. A neutral C₆₀ molecule is diamagnetic and does not show EPR signal. Charged C₆₀ ^v molecules, wherev is the valence taking values fromv=-6 tov=+1, show EPR signals for oddv and forv=2 when a fullerene molecule is in the triple state. EPR lines of isolated fullerene ions are narrow in the whole temperature range whereas EPR lines of Me_xC₆₀ ^v in ionic compounds Me_xC₆₀ are wide and their width decreases with temperature. A model of paramagnetic centers and an empirical formula relatingg-factor to valence have been given.