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Leif Schrder Christian Schmitz Peter Bachert 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,171(2):87
Proton NMR resonances of the endogenous metabolites creatine and phosphocreatine ((P)Cr), taurine (Tau), and carnosine (Cs, β-alanyl-l-histidine) were studied with regard to residual dipolar couplings and molecular mobility. We present an analysis of the direct 1H–1H interaction that provides information on motional reorientation of subgroups in these molecules in vivo. For this purpose, localized 1H NMR experiments were performed on m. gastrocnemius of healthy volunteers using a 1.5-T clinical whole-body MR scanner. We evaluated the observable dipolar coupling strength SD0 (S = order parameter) of the (P)Cr-methyl triplet and the Tau-methylene doublet by means of the apparent line splitting. These were compared to the dipolar coupling strength of the (P)Cr-methylene doublet. In contrast to the aliphatic protons of (P)Cr and Tau, the aromatic H2 (δ = 8 ppm) and H4 (δ = 7 ppm) protons of the imidazole ring of Cs exhibit second-order spectra at 1.5 T. This effect is the consequence of incomplete transition from Zeeman to Paschen-Back regime and allows a determination of SD0 from H2 and H4 of Cs as an alternative to evaluating the multiplet splitting which can be measured directly in high-resolution 1H NMR spectra. Experimental data showed striking differences in the mobility of the metabolites when the dipolar coupling constant D0 (calculated with the internuclear distance known from molecular geometry in the case of complete absence of molecular dynamics and motion) is used for comparison. The aliphatic signals involve very small order parameters S ≈ (1.4 − 3) × 10−4 indicating rapid reorientation of the corresponding subgroups in these metabolites. In contrast, analysis of the Cs resonances yielded S ≈ (113 − 137) × 10−4. Thus, the immobilization of the Cs imidazole ring owing to an anisotropic cellular substructure in human m. gastrocnemius is much more effective than for (P)Cr and Tau subgroups. Furthermore, 1H NMR experiments on aqueous model solutions of histidine and N-acetyl-l-aspartate (NAA) enabled the assignment of an additional signal component at δ = 8 ppm of Cs in vivo to the amide group at the peptide bond. The visibility of this proton could result from hydrogen bonding which would agree with the anticipated stronger motional restriction of Cs. Referring to the observation that all dipolar-coupled multiplets resolved in localized in vivo 1H NMR spectra of human m. gastrocnemius collapse simultaneously when the fibre structure is tilted towards the magic angle (θ ≈ 55°), a common model for molecular confinement in muscle tissue is proposed on the basis of an interaction of the studied metabolites with myocellular membrane phospholipids. 相似文献
3.
Phase modulation in dipolar-coupled A2 spin systems: effect of maximum state mixing in H NMR in vivo
Leif Schrder Christian Schmitz Peter Bachert 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,171(2):69
Coupling constants of nuclear spin systems can be determined from phase modulation of multiplet resonances. Strongly coupled systems such as citrate in prostatic tissue exhibit a more complex modulation than AX connectivities, because of substantial mixing of quantum states. An extreme limit is the coupling of n isochronous spins (An system). It is observable only for directly connected spins like the methylene protons of creatine and phosphocreatine which experience residual dipolar coupling in intact muscle tissue in vivo. We will demonstrate that phase modulation of this “pseudo-strong” system is quite simple compared to those of AB systems. Theory predicts that the spin-echo experiment yields conditions as in the case of weak interactions, in particular, the phase modulation depends linearly on the line splitting and the echo time. 相似文献
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R. Klein R. Grieser L. Hoog G. Huber I. Klaft P. Merz T. Kühl S. Schröder M. Grieser D. Habs W. Petrich D. Schwalm 《Zeitschrift für Physik A Hadrons and Nuclei》1992,342(4):455-461
We have performed for the first time precision spectroscopy on a coasting fast7Li+ ion beam in a storage ring. The ion beam moving with 6.4% speed of light was first electron cooled and then merged with two counterpropagating laser beams acting on two different hyperfine transitions sharing a common upper level (λ-system). One laser was frequency locked to thea 3 127J2 hfs frequency component established as a secondary frequency standard at 514 nm. The second laser was tuned over theλ-resonance, which was recorded relative to127J2 hfs components. This experiment is sensitive to the time dilation in fast moving frames and will lead to new limits for the verification of special relatively. The present status of the experiment and perspectives in accuracy are discussed. 相似文献
7.
Leif Kjr Jrgensen 《Journal of Graph Theory》1989,13(1):75-85
A probabilistic result of Bollobás and Catlin concerning the largest integer p so that a subdivision of Kp is contained in a random graph is generalized to a result concerning the largest integer p so that a subdivision of Ap is contained in a random graph for some sequence A1, A2,… of graphs such that Ai+1 contains a subdivision of Ai. A similar result is proved for subdivisions with odd paths or cycles. The result is applied to disprove a conjecture of Chartrand, Geller, and Hedetniemi. The maximum number of edges in a graph without a subdivision of Kp, p = 4, 5, with odd paths or cycles is determined. 相似文献
8.
R. Lacaze A. Morel B. Petersson J. Schröper 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):509-523
We present an investigation of the 2D attractive Hubbard model, considered as an effective model relevant to superconductivity
in strongly interacting electron systems. We use both hybrid Monte-Carlo simulations and existing hopping parameter expansions
to explore the low temperature domain. The increase of the static S-wave pair correlation with decreasing temperature, which depends weakly on the band filling in the explored temperature range,
is analyzed in terms of an expected Kosterlitz-Thouless superconducting transition. Using both our data and previously published
results, we show that the evidence for this transition is weak: If it exists, its temperature is very low. The number of unpaired
electrons remains nearly constant with temperature at fixed attractive potential strength. In contrast, the static magnetic
susceptibility decreases fast with temperature, and cannot be related only to pair formation. We introduce a method by which
the Padé approximants of the existing series for the susceptibility give sensible results down to rather low temperature region,
as shown by comparison with our numerical data.
Received: 30 October 1996 / Revised: 23 October 1997 / Accepted: 29 January 1998 相似文献
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Juraj Bunta Martin Dahlberg Leif Eriksson Nikolai Korolev Aatto Laaksonen Raimo Lohikoski Alexander Lyubartsev Miroslav Pinak Patric Schyman 《International journal of quantum chemistry》2007,107(2):279-291
This work highlights four different topics in modeling of DNA: (i) the importance of water and ions together with the structure and function of DNA; the hydration structure around the ions appears to be the determining factor in the ion coordination to DNA, as demonstrated in the results of our MD simulations; (ii) how MD simulations can be used to simulate single molecule manipulation experiments as a complement to reveal the structural dynamics of the studied biomolecules; (iii) how damaged DNA can be studied in computer simulations; and (iv) how repair of damaged DNA can be studied theoretically. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献