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41.
Magnetic relaxation in solids may be complicated by the creation and loss of dipolar order at finite rates. In tissues the molecular and spin dynamics may be significantly different because of the relatively high concentration of water. We have applied a modified Jeneer-Broekaert pulse sequence to measure dipolar relaxation rates in both dry and hydrated protein systems that may serve as magnetic models for tissue. In lyophilized and dry serum albumin, the dipolar relaxation time, T(1D) is on the order of 1 ms and is consistent with earlier reports. When hydrated by deuterium oxide, the dipolar relaxation times measured were on the order of tens of microseconds. When paramagnetic centers are included in the protein, the Jeneer-Broekaert echo decay times became the order of the decay time for transverse magnetization, i.e., the order of 10 micros or less. In the hydrated or paramagnetic systems, the dipolar relaxation times are too short to require inclusion in the quantitative analysis of magnetization transfer experiments. 相似文献
42.
Harry W. Gibson William S. Bryant Sang‐Hun Lee 《Journal of polymer science. Part A, Polymer chemistry》2001,39(12):1978-1993
Poly[(methyl acrylate)‐rotaxa‐(30‐crown‐10)] ( 5 ) and poly[(methyl methacrylate)‐rotaxa‐(30‐crown‐10)] ( 6 ) were synthesized by azobisisobutyronitrile‐initiated free‐radical bulk polymerizations of the respective monomers in the presence of 30‐crown‐10 ( 1 ; equimolar; 5 times the monomer mass). For 5 , 3.8 mass % (0.81 mol % with respect to the monomer) of the crown was incorporated versus 1.7 mass % (0.39 mol % with respect to the monomer) for 6 . Control reactions with 18‐crown‐6, which is to small to be threaded, showed that chain transfer to the crown ethers was detectable only for the acrylate but was relatively negligible and spectroscopically distinct. The threading yields were much higher with these systems than with polystyrene, most likely because of the greater compatibility of the crown ether with these polar monomers and polymers and the consequent ability to carry out the polymerizations homogeneously in the absence of added solvent; however, the threading process was still essentially statistical. Therefore, the polymerization of methacrylate monomers 8a – 8c based on tetraarylmethane moieties connected via diethyleneoxy or triethyleneoxy spacers was examined in the presence of 1 in the belief that the supramolecular semirotaxane monomer 9 formed statistically in situ could be captured more efficiently and produce higher threading yields, presumably of side‐chain polyrotaxanes, than the simple (meth)acrylate monomers. Azobisisobutyronitrile‐initiated polymerizations either neat or in toluene produced polyrotaxanes 10 with up to about 1.6 mass % and 2 mol % threaded crown ether, presumably trapped on the pendant stoppered side chains. Although primarily statistical in nature, the latter rotaxane syntheses afforded on a molar basis 3–7 times more efficient incorporation of 1 than styrene (0.33 mol %), methyl acrylate (0.81 mol %), or methyl methacrylate (0.39 mol %) monomers for the preparation of main‐chain polypseudorotaxanes and indeed even surpassed the 60‐crown‐20/polyacrylonitrile system (1.5 mol %). This was presumed to be due to the fact that the loss of the crown ether, once it was threaded onto the monomer to form 9 and the latter was polymerized, was either retarded (by the tetraphenylmethyl stopper in 10a ) or prevented completely [by tris(p‐t‐butylphenyl)phenylmethyl stoppers in 10b and 10c ]. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1978–1993, 2001 相似文献
43.
Rufat Agalarov Martin Byrdin Fabrice Rappaport Gaozhong Shen Donald A. Bryant Art Van Der Est John H. Golbeck 《Photochemistry and photobiology》2008,84(6):1371-1380
Continuous wave (CW) and transient electron paramagnetic resonance studies have implied that when PsaF is removed genetically, the double reduction of A1A is facile, the lifetime of A1A? is shorter and the ratio of fast to slow kinetic phases increases in PS I complexes isolated with Triton X‐100 (Van der Est, A., A. I. Valieva, Y. E. Kandrashkin, G. Shen, D. A. Bryant and J. H. Golbeck [2004] Biochemistry 43 , 1264–1275). Changes in the lifetimes of A1A? and A1B? are characteristic of mutants involving the quinone binding sites, but changes in the relative amplitudes of A1A? and A1B? are characteristic of mutants involving the primary electron acceptors, A0A and A0B. Here, we measured the fast and slow phases of electron transfer from A1B? and A1A? to FX in psaF and psaE psaF null mutants using time‐resolved CW and pump‐probe optical absorption spectroscopy. The lifetime of the fast kinetic phase was found to be unaltered, but the lifetime of the slow kinetic phase was shorter in the psaF null mutant and even more so in the psaE psaF null mutant. Concomitantly, the amplitude of the fast kinetic phase increased by a factor of 1.8 and 2.0 in the psaF and psaE psaF null mutants, respectively, at the expense of the slow kinetic phase. The change in ratio of the fast to slow kinetic phases is explained as either a redirection of electron transfer through A1B at the expense of A1A, or a shortening of the lifetime of A1A? to become identical to that of A1B?. The constant lifetime and the characteristics of the near‐UV spectrum of the fast kinetic phase favor the former explanation. A unified hypothesis is presented of a displacement of the A‐jk(1) α‐helix and switchback loop, which would weaken the H‐bond from Leu722 to A1A, accounting for the acceleration of the slow kinetic phase, as well as weaken the H‐bond from Tyr696 to A0A, accounting for the bias of electron transfer in favor of the PsaB branch of cofactors. 相似文献
44.
A single turn solenoid, also called a loop-gap resonator, is a device that is efficient for radio frequency spectroscopy on relatively large samples. Thus, the device provides an effective means for magnetic imaging where the single turn solenoid may serve both as the transmitter and receiver coil. The device is readily constructed and provides very efficient use of radio frequency (RF) power for imaging extremities such as breasts, arms, feet, and hands. The resulting magnetic images are acquired in short times with good anatomical resolution and considerable reduction of the RF power delivered to the patient. 相似文献
45.
Darryn E. Bryant Saad El-Zanati Charles Vanden Eynden Dean G. Hoffman 《Graphs and Combinatorics》2001,17(1):55-59
Let S
k
denote the complete bipartite graph K
1,
k
and let Q
n
denote the n-cube. We prove that the obvious necessary conditions for the existence of an S
k
-decomposition of Q
n
are sufficient.
Received: July 21, 1999 Final version received: May 16, 2000 相似文献
46.
47.
W. G. Campbell L. H. Smith St. A. Bryant und A. Breazzano 《Fresenius' Journal of Analytical Chemistry》1939,118(1-2):63-65
Ohne Zusammenfassung 相似文献
48.
49.
W. G. Campbell J. C. Mc Gowan St. A. Bryant A. J. Bailey A. Orup R. I. Thieme H. Bergström K. G. Trobeck P. W. Moryganow J. I. Wlassjuk N. K. Koshewnikowa G. S. Ssurta G. Jayme H. Pfretzschner R. Steinmann M. G. van Beckum G. J. Ritter J. H. Barbour C. F. Cross und E. J. Bevan 《Fresenius' Journal of Analytical Chemistry》1940,119(1-2):67-71
Ohne Zusammenfassung 相似文献
50.
Zheng YR Zhao Z Wang M Ghosh K Pollock JB Cook TR Stang PJ 《Journal of the American Chemical Society》2010,132(47):16873-16882
A novel approach toward the construction of multicomponent two-dimensional (2-D) and three-dimensional (3-D) metallosupramolecules is reported. Simply by mixing carboxylate and pyridyl ligands with cis-Pt(PEt(3))(2)(OTf)(2) in a proper ratio, coordination-driven self-assembly occurs, allowing for the selective generation of discrete multicomponent structures via charge separation on the metal centers. Using this method, a variety of 2-D rectangles and 3-D prisms were prepared under mild conditions. Moreover, multicomponent self-assembly can also be achieved by supramolecule-to-supramolecule transformations. The products were characterized by (31)P and (1)H multinuclear NMR spectroscopy, electrospray ionization mass spectrometry, and pulsed-field-gradient spin echo NMR techniques together with computational simulations. 相似文献