ISSN:0021-2148    

DOI:10.1002/ijch.198800034

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractFourier‐Transform Electron Paramagnetic Resonance has measured the rates of homogeneous electron transfer reactions involving duroquinone radical anions. The radicals are generated by laser photolysis of duroquinone in methanol solution containing 10% triethylamine. The duroquinone concentration was varied over a factor of 1000. The rate constant for electron transfer between the radical anion and the neutral duroquinone is 1.5×108M−1s−1and the intrinsic spin relaxation rates,T−11andT−12are 0.32 MHz and 0.44 MHz, respectively. Two‐Dimensional magnetization transfer spectra show that the reaction is a homogeneous electron transfer reaction. The electron transfer rates are measured by two novel pulse sequences designed for more efficient data acquisition in samples with no unresolved inhomogeneous broadening. These two sequences result in a more rapid, accurate determination of the second‐order chemical rate constant since the second‐order rate constant is obtained directly and is not derived from a single measurement of a pseudo‐first‐order rate. The transient duroquinone radical anions studied here appear to have the sameT1,T2and electron transfer rates as stable duroquinone radical anions i

ISSN:0021-2148    

DOI:10.1002/ijch.198800035

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe physical principles underlying steady‐state electron‐nuclear double resonance (ENDOR) and electron‐nuclear‐nuclear triple resonance (TRIPLE) experiments on radicals in solution are discussed. The gain in spectral resolution achievable by these techniques, as compared with standard ESR, allows hyperfine coupling constants to be measured even for large and low‐symmetry radicals. ENDOR/TRIPLE is applied to cation radicals related to bacterial photosynthesis. The measured hyperfine couplings are interpreted by means of the all‐valence electron MO method RHF‐INDO/SP. Information about the spatial structure of chromophores in the reaction centers of photosynthetic bacteria,Rb. sphaeroidesandRps. viridis, is obtained from an analysis of the spin density distribution and from minimization of the total energy. The results are compared with recently available structural data of the primary donor obtained from X‐ray diffraction experiments. The spin density distribution in the primary donor cation radical P+960, which consists of two bacteriochlorophyllbmolecules, is discussed in terms of symmetry and perturbations from the prot

ISSN:0021-2148    

DOI:10.1002/ijch.198800036

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThis paper is concerned with quantification of magnetic resonance signals in terms of physically significant parameters such as amplitudes, frequencies, damping factors (line widths) and phases. The approach considered here is to fit an appropriate model function, in the same domain as that in which the data are collected. Important advantages of this approach are 1) absence or rejection of data points need not impair the quantification and 2) non‐uniform sample distributions aimed at resolution enhancement can be accommodated in a natural fashion. Aspects given attention are singular value decomposition (SVD) of the data matrix, SNR enhancement, linear prediction (LPSVD), state space formalism (HSVD), variable projection method (VARPRO) in various forms, exploitation of prior knowledge, precision of the fitted parameters, experimental design and 2‐D time domain model function fitt

ISSN:0021-2148    

DOI:10.1002/ijch.198800037

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractTwo novel methods for enhancing the effect of small anisotropies on magic angle spinning (MAS) NMR spectra are described. The first method is a simple two‐dimensional (2D) spin echo experiment, which generates rotational sidebands at a fraction of the spinning frequency in the ω1dimension, while maintaining the effective spinning speed in ω2. It is shown that the rotational sidebands in the 2D spectrum are much larger than in a normal one‐dimensional MAS spectrum obtained at the same spinning speed, and the relative intensities of the two‐dimensional sideband array can be analyzed to yield the anisotropy and asymmetry parameter of the coupling tensor. The second method is designed to measure homonuclear dipolar couplings. We show that the effect of a dipolar coupling between members of an isolated spin pair can be enhanced by adjusting the spinning speed such thatnωr= ωΔiso, wherenis an integer, ωris the spinning speed, and ωΔisois the difference between isotropic shifts. When this condition, termed rotational resonance, is satisfied, a broadening or a splitting of the normally sharp resonance lines is observed. In addition, rapid exchange of Zeeman order between the dipolar coupled spins is seen. This method holds promise for the measurement of through space dipolar couplings and thus internuclear distances in polycryst

ISSN:0021-2148    

DOI:10.1002/ijch.198800038

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractIn solid state magic angle spinning experiments on a polycrystalline sample containing dilute heteronuclear spin pairs, the usual spectrum is insensitive to small heteronuclear couplings between the spins, and to the relative orientations of the various interaction tensors. Applying a radio frequency of an intensity chosen to satisfy a rotary resonance condition can restore the heteronuclear splittings in the spectrum. Theory and numerical simulations of this effect are presented. The sensitivity of the spectra to the intensity of the radio frequency field and to the relative orientations of tensors is explored. To fully explain the experimental results, it is necessary to postulate a small phase modulation induced by the motion of the sample through a radio frequency field of non‐uniform directio

ISSN:0021-2148    

DOI:10.1002/ijch.198800039

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractA theoretical analysis is given of deuterium 2D exchange spectroscopy using Zeeman‐ and quadrupole‐order experiments in liquid crystalline solutions. For the analysis, a phenomenological vector model for spinI= 1 nuclei under the effect of the quadrupole Hamiltonian and rf pulses is employed, which makes it easy to follow the evolution of the various coherences during the pulse sequences. In both experiments several types of cross‐peaks are obtained; however, only part of them are relevant to the exchange process. It is shown that proper combination of the signals from the Zeeman‐ and quadrupole‐order experiments can lead to 2D exchange spectra with “pure” exchange cross‐peaks as the only off‐diagonal signals. These exchange cross‐peaks can provide estimates for the exchange rates and indicate the relative signs of the quadrupole interactions in the exchanging sites. The method is applied to two compounds which undergo ring inversion, i.e., cyclohex‐ ane‐d12and 1,4‐dioxane‐d8, dissolved in liquid crystalline solvents. In the Appendices procedures for phase cycling are discussed, and peak intensities for a general two‐site 2D

ISSN:0021-2148    

DOI:10.1002/ijch.198800040

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe form of the NMR spectra obtained from liquid crystalline samples is discussed. It is shown how orientational order parameters can be derived from quadrupolar splittings and dipolar couplings, and their use in investigating possible forms for potentials of mean torque is illustrated for rigid solute molecules. For non‐rigid molecules, the observed partially‐averaged quadrupolar and dipolar couplings depend upon the nature of the internal rotations about bonds, as well as on the orientational order; it is shown how both of these factors can be investigated by NMR for both mesogenic molecules and for non‐rigid solutes. Finally, it is shown how deuterium NMR is being used to study field‐induced orientational order in the isotropic phase of mesogenic mo

ISSN:0021-2148    

DOI:10.1002/ijch.198800041

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractThe principles of homonuclear magnetization transfer via isotropic and non‐isotropic mixing schemes are discussed. Practical aspects of such experiments concern the minimization of the generation of multiple quantum coherence, minimization of rf power requirements, suppression of NOEs during mixing and minimization of relaxation losses. The analogy between these experiments and heteronuclear decoupling is briefly discussed. The efficiencies of several mixing schemes are compared and it is demonstrated experimentally that the resolution and sensitivity of the homonuclear cross polarization methods can be superior to the conventional COSY experimen

ISSN:0021-2148    

DOI:10.1002/ijch.198800042

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY

摘要:

AbstractPhoto‐CIDNP is a useful technique to identify surface residues in proteins and to assign resonances in their1H NMR spectra by the spectral simplification that is obtained. In spite of this simplification, conventional photo‐CIDNP spectra can be quite complex due to spectral overlap. For this reason two‐dimensional (2D) methods are welcome. We present combinations of photo‐CIDNP with 2DJ‐correlated spectroscopy (CIDNP‐COSY) and with 2D NOE spectroscopy (CIDNP‐NOESY). In these experiments we insert a saturation pulse sequence and a short laser‐irradiation period at the beginning of the preparation period to generate CIDNP, leaving the remaining of the pulse scheme essentially unaltered. The CIDNP‐COSY and CIDNP‐NOESY spectra of a hen egg‐white (HEW) lysozyme are presented. The CIDNP‐NOESY spectrum of lysozyme is compared with the results from 1D cross‐polarisation experiments on the same protein. CIDNP‐NOESY spectra of lysozyme with and without

ISSN:0021-2148    

DOI:10.1002/ijch.198800043

出版商:WILEY‐VCH Verlag    

年代:1988

数据来源: WILEY