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Electron Spin-lattice Relaxation-time and Spectral Diffusion In Gamma-irradiated L-alanine.
No full textWe performed continuous (CW) wave and pulsed ESR experiments to obtain information on the relaxation behavior of the 1-alanine radical in an irradiated single crystal. The analysis of the CW saturation behavior gives a relaxation time of 2.8 micro s. The echo detected saturation recovery was obtained for a number of different experimental conditions. In any case only a portion of the 120 G wide ESR spectrum can be affected by the microwave (MW) pulses, spectral diffusion is active and a multi-exponential decay is therefore obtained. We measured characteristic spectral diffusion times of 1-10 and 20-50 micro s. We found that a long time of about 200 micro s can be measured only by using a train of long selective saturating pulses and short detecting pulses. The stimulated echo decay is bi-exponential, and the characteristic times are very short. A variable temperature investigation in the range 200 to 290 K showed that the decay is governed by the spectral diffusion and by the transverse nuclear spin relaxation time T-2n of the methyl protons
STRUCTURE AND DYNAMICS OF RADICALS IN SOLIDS BY EPR AND ENDOR SPECTROSCOPIES
No full textEPR and ENDOR (Electron Nuclear DOuble Resonance) spectroscopies give complementary information in the study of paramagnetic species in solids. The ENDOR technique allows the determination of hyperfine tensors an order of magnitude smaller with respect to the EPR one. The molecular motions in solids affect the relaxation of both the transverse and longitudinal magnetization of the paramagnetic probe. The measurement of the transverse relaxation time is traditionally available by the EPR Lineshape analysis. Population variations due to the longitudinal relaxation processes can be monitored by the study of the amplitude of the ENDOR spectra
STRUCTURAL INFORMATION ON CHROMIUM(V) COMPLEXES OF 1,2-DIOLS IN SOLUTION, AS DETERMINED BY ISOTROPIC AND ANISOTROPIC H-1 ENDOR SPECTROSCOPY
No full textThe H-1 ENDOR study of complexes formed by 1,2-diols, namely ethylene glycol, (S)-(+)-1,2-propanediol, and (2R,3R)-(-)-2,3-butanediol, has demonstrated the potential of this technique as a structural probe of Cr(V) complexes in solution. The spectra recorded in liquid CD3OD solution demonstrate the H-1 superhyperfine (shf) interaction, which, except for the case of the glycolate complex, is not resolved in the EPR spectra. The comparative analysis provides unambiguous evidence for the presence of two classes of protons in the chelated ethylene bridge of the complexes. The analysis shows also that the single H-1 ENDOR coupling observed for the bis(ethylene glycolato(2-))oxochromate(V) complex is really an average signal due to a fast axial-equatorial interconversion. Crystallike ENDOR information has been obtained on the anisotropic spectra recorded in frozen solution by using both Cr-52 and Cr-53 isotopes. The complementary use of the isotropic and anisotropic couplings allow discrimination among all the protons of the ligands and calculation of their distances from the metal ion. The proton ENDOR analysis has been supported by a theoretical simulation of the spectra
ENDOR and ESEEM study of the radical obtained by gamma irradiation of a single crystal of ammonium tartrate
No full textThe radical obtained by gamma irradiation of a single crystal of ammonium tartrate has been identified by paramagnetic resonance spectroscopies. The proton hyperfine coupling tensors have been obtained from the angular dependence of the ENDOR transition frequencies. ESEEM spectra of the radical have been also obtained. The intensities of the ESEEM lines are related to the depth of the modulation, which has a dramatic angular dependence
EPR AND ENDOR RELAXATION STUDY OF MOLECULAR MOTIONS OF A KETONE UREA INCLUSION COMPOUND
No full textThe radical obtained by gamma irradiation of the 10-nonadecanone/urea inclusion compound has been studied by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. The EPR line widths have been analyzed, and the characteristics and the rates of the molecular motions of the included molecule have been obtained in the temperature range 160-290 K. Two different types of motion have been detected: the wobbling motion of the methylene groups about the carbon-carbon bond modulates the beta proton isotropic hyperfine interactions, while overall uniaxial rotation inside the host channel mainly modulates the anisotropic dipolar interaction of the alpha proton. The internal motion is faster and has higher activation energy than molecular rotation. These results have been confirmed by the analysis of the amplitudes of the ENDOR lines of the gamma and zeta protons, which shows that the internal motion rate is just in the range of the electron spin Larmor frequency, while the rotation rate is lower
Electron spin relaxation times and internal motions of radicals in the solid state investigated by ENDOR and pulsed EPR
No full textThe angular dependent ENDOR spectra of the radical formed by gamma-irradiated single crystal of 4-methyl-2,6-di-t-butylphenol have been studied and the full hyperfine tensors of all the t-butyl and the ring protons have been obtained at T = 190 K. We found six different tensors for the t-butyl protons. This result shows that the t-butyl groups are slowly rotating on the ENDOR time scale, whereas each methyl group rotates fastly. The dynamical parameters of the motions have been determined by pulsed EPR experiments. The longitudinal relaxation and the phase memory times have been measured in the temperature range 130-290 K. Three different kinds of motions have been detected and the resulting values of the dynamical parameters have been compared with those obtained for the undamaged molecule by previous NMR studies
Structure and Dynamics of Hydrogen Bonding Guests in Urea Inclusion Compounds
No full textThe radical obtained by gamma-irradiation of the 2-nonadecanone/urea (2-NDOU) and nonadecanoic acid/urea (NDAU) inclusion compounds have been studied by EPR spectroscopy. The spectra have first-order, fast-motion line shapes with anisotropic linewidths. For both compounds the spectra show the presence of two similar species. They originate from the two possible arrangements of the molecules inside the host channels, i.e. head-to-head or head-to-tail. The relative abundance of the two conformations is obtained from the intensities of their EPR signals and is accounted for in terms of the balance between guest-to-guest and guest-to-host hydrogen bonding. The transverse relaxation rate constants for the different hyperfine components have been obtained by computer simulation of the spectra. The relaxation originates from the librational motion of the beta methylene group and from the hindered rotation of the radical inside the host channel. Evidence of pretransitional effects is shown by the spin relaxation rates above the order-disorder transition in NDAU
Preferential Orientation of Fulleropyrrolidine Bisadducts in E7 Liquid Crystal: a Time Resolved EPR Study
No full textStructure-based calculations of the optical spectra of the ligth-harvesting peridin-chlorophyll-protein complexes from Amphydiniumcarterae and Heterocapsa pygmaea
No full textThe molecular structure of the light-harvesting complex peridinin-chlorophyll-protein from the dinoflagellate Amphidinium carterae (A-PCP) provides the positions and orientations of the eight peridinin (Per) and two chlorophyll a (Chl) molecules in the complex whose apoprotein is 32 kD. We made structure-based calculations of the distinctive optical properties (absorption and CD spectral of A-PCP and of the complex containing a ratio of four peridinin and one chlorophyll per complex (apoprotein similar to 15 kD) obtained from the related species Heterocapsa pygmaea (H-PCP). The latter structure has not been determined but can be inferred from that of A-PCP. A point-monopole approximation was used to represent the low-energy transition of peridinin in the blue region of the spectrum and that of chlorophyll in the Soret region. Vibronic interactions are taken into account for peridinin because of the strong vibrational progression exhibited by the spectrum of the latter. From the calculations, we are able to simulate the absorption and CD spectra for H-PCP and A-PCP by using, in addition to the atomic coordinates taken from the A-PCP structure, one and only one set of parameters, adjusted for the small unit of four Per and one Chi common to both systems. In particular, the four peridinin site energies were assigned values in the range 18 500-19 500 cm(-1), and those for the B-x and B-y transitions of chlorophyll a were given the common value 23 100 cm(-1). The transition moments for peridinin were in the range 10.6-12.4 D, and those of the chlorophyll B-x and B-y transitions were 9.0 and 1.0 D, respectively. Each resolved vibronic transition was given the same Gaussian line width of 550 cm(-1) Excitonic coupling among the different chromophores of the small cluster unit of the complex is not sufficient to describe the A-PCP optical properties. Intercluster interactions are necessary in order to reproduce the CD spectrum. The H-PCP spectrum, being practically identical to the former, is reproduced only if such interactions are maintained, meaning that the solution unit is a dimer of the monomeric polypeptide as previously inferred from the biochemical properties
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