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51.
A sensitive 3D NMR pulse scheme, (H)C(CA)NH-COSY, is presented for the assignment of (13)C(delta)(1) Ile chemical shifts in large perdeuterated, methyl-protonated proteins. The nonlinearity of branched amino acids, such as Ile, significantly degrades the quality of TOCSY schemes which transfer magnetization from methyl carbons to the backbone (13)C(alpha) positions, and in applications to high molecular weight proteins (correlation times on the order of 40-50 ns), this compromises the sensitivity of spectra used for methyl assignment. The experiment presented utilizes COSY-based transfer steps and refocuses undesirable (13)C-(13)C scalar couplings that degrade the efficiency of TOCSY transfers. The (H)C(CA)NH-COSY scheme is tested on an (15)N,(13)C,(2)H-[Leu, Val, Ile (delta 1 only)]-methyl-protonated maltose binding protein (MBP)/beta-cyclodextrin complex at 5 degrees C (molecular tumbling time 46 +/- 2 ns), facilitating the assignment of (13)C(delta 1) chemical shifts for 18 of the 19 Ile residues for which backbone assignments were previously obtained. Both sensitivity and resolution of the resulting spectra are shown to be significantly better than those for a similar TOCSY-based approach. 相似文献
52.
Aboelella NW Lewis EA Reynolds AM Brennessel WW Cramer CJ Tolman WB 《Journal of the American Chemical Society》2002,124(36):10660-10661
The X-ray structure of a 1:1 Cu/O(2) adduct revealed side-on (eta(2)) O(2) coordination. Density functional calculations corroborated the structure, indicated a significant contribution of a Cu(III)-(O(2)(2-)) resonance form, and provided insights into the key bonding interactions. Reaction of a 1:1 adduct supported by a slightly different beta-diketiminate ligand with Cu(I) reagents resulted in the formation of novel asymmetric bis(mu-oxo) complexes that were identified by EPR, UV-vis, and Raman spectroscopy, as well as by an X-ray structure in one instance. 相似文献
53.
An (15)N NMR R(1rho) relaxation experiment is presented for the measurement of millisecond time scale exchange processes in proteins. On- and off-resonance R(1rho) relaxation profiles are recorded one residue at a time using a series of one-dimensional experiments in concert with selective Hartmann-Hahn polarization transfers. The experiment can be performed using low spin-lock field strengths (values as low as 25 Hz have been tested), with excellent alignment of magnetization along the effective field achieved. Additionally, suppression of the effects of cross-correlated relaxation between dipolar and chemical shift anisotropy interactions and (1)H-(15)N scalar coupled evolution is straightforward to implement, independent of the strength of the (15)N spin-locking field. The methodology is applied to study the folding of a G48M mutant of the Fyn SH3 domain that has been characterized previously by CPMG dispersion experiments. It is demonstrated through experiment that off-resonance R(1rho) data measured at a single magnetic field and one or more spin-lock field strengths, with amplitudes on the order of the rate of exchange, allow a complete characterization of a two-site exchange process. This is possible even in the case of slow exchange on the NMR time scale, where complementary approaches involving CPMG-based experiments fail. Advantages of this methodology in relation to other approaches are described. 相似文献
54.
Ramanan Krishnamoorti William W. Graessley Achim Zirkel Dieter Richter Nikos Hadjichristidis Lewis J. Fetters David J. Lohse 《Journal of Polymer Science.Polymer Physics》2002,40(16):1768-1776
The unperturbed chain dimensions (〈R2〉o/M) of cis/trans‐1,4‐polyisoprene, a near‐atactic poly(methyl methacrylate), and atactic polyolefins were measured as a function of temperature in the melt state via small‐angle neutron scattering (SANS). The polyolefinic materials were derived from polydienes or polystyrene via hydrogenation or deuteration and represent structures not encountered commercially. The parent polymers were prepared via lithium‐based anionic polymerizations in cyclohexane with, in some cases, a polymer microstructure modifier present. The polyolefins retained the near‐monodisperse molecular weight distributions exhibited by the precursor materials. The melt SANS‐based chain dimension data allowed the evaluation of the temperature coefficients [dln 〈R2〉o/dT(κ)] for these polymers. The evaluated polymers obeyed the packing length (p)‐based expressions of the plateau modulus, G = kT/np3 (MPa), and the entanglement molecular weight, Me = ρNanp3 (g mol?1), where nt denotes the number (~21) of entanglement strands in a cube with the dimensions of the reptation tube diameter (dt) and ρ is the chain density. The product np3 is the displaced volume (Ve) of an entanglement that is also expressible as pd or kT/G. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1768–1776, 2002 相似文献
55.
Rapid and reversible G-quadruplex hairpin dimer formation is observed for bis(oligonucleotide) conjugates possessing stilbenediether (Sd) linkers connecting two short poly(G) sequences. 相似文献
56.
P. F. Crain James A. McCloskey Arthur F. Lewis Karl H. Schram Leroy B. Townsend 《Journal of heterocyclic chemistry》1973,10(5):843-847
The mass spectrum of pyrazomycin has been obtained and reveals a fragmentation pattern with several important peaks which are not normally found in the mass spectra of C-nucleosides. It has now been established, using model compounds, that these unusual fragment ions are a direct result of the juxtaposition of the exocyclic hydroxy and carboxamido groups of the aglycon. It appears that a facile elimination of ammonia and ethanol from o-hydroxycarboxamides and o-hydroxyethylesters, respectively, may be a general fragmentation reaction for aromatic heterocycles. A mass spectrum of the TMS derivative of pyrazomycin has also been obtained and factors which may result in exceptions to the empirical B+30 (M-103) rule for C-nucleosides are discussed. 相似文献
57.
Abstract— Absorbance changes were monitored from 250 to 650 nm during the first microsecond after photolysis of detergent suspensions of bovine rhodopsin at 20°C. Global analysis of the resulting data produced difference spectra for bathorhodopsin, BSI and lumirhodopsin which give the change in absorbance of the aromatic amino acid side chains in these photointermediates relative to rhodopsin. These spectra show that the significant bleaching of absorbance near 280 nm, which has been seen previously for the lumirhodopsin, metarhodopsin I and metarhodopsin II intermediates, extends to times as early as bathorhodopsin. Because no corresponding absorbance increase is observed in the 250-275 nm region, the earliest bleaching of the 280 nm absorbance in rhodopsin is attributed to disruption of a hyperchromic interaction affecting Trp265 . Partial decay of this 280 nm bleaching as bathorhodopsin converts to BSI takes place maximally near 290 nm, where Trp265 has been shown to absorb, and could be due to the ring of the retinylidene chromophore resuming a position at the BSI stage that reestablishes the hyperchromic interaction with Trp265 . A subsequent change in the 250-300 nm region, which has no counterpart in the visible chromophore bands, indicates the possible presence of a protein-localized process as lumirhodopsin is formed. 相似文献
58.
Tugarinov V Hwang PM Ollerenshaw JE Kay LE 《Journal of the American Chemical Society》2003,125(34):10420-10428
A comparison of HSQC and HMQC pulse schemes for recording (1)H[bond](13)C correlation maps of protonated methyl groups in highly deuterated proteins is presented. It is shown that HMQC correlation maps can be as much as a factor of 3 more sensitive than their HSQC counterparts and that the sensitivity gains result from a TROSY effect that involves cancellation of intra-methyl dipolar relaxation interactions. (1)H[bond](13)C correlation spectra are recorded on U-[(15)N,(2)H], Ile delta 1-[(13)C,(1)H] samples of (i) malate synthase G, a 723 residue protein, at 37 and 5 degrees C, and of (ii) the protease ClpP, comprising 14 identical subunits, each with 193 residues (305 kDa), at 5 degrees C. The high quality of HMQC spectra obtained in short measuring times strongly suggests that methyl groups will be useful probes of structure and dynamics in supramolecular complexes. 相似文献
59.
Umar Hayat Philip N. Bartlett George H. Dodd Michael H. Lewis 《Journal of polymer science. Part A, Polymer chemistry》1988,26(1):201-206
The surface compositions of various polymeric films, grown electrochemically on platinum foils, have been investigated by energy-dispersive x-ray analysis in conjunction with scanning electron microscopy (SEM/EDS). Comparison of the relative area ratios of peaks for the C and N Kemission lines show that the EDS may be used to study the surface composition of polymers. The evidence presented strongly suggests that there is limited structural degradation and the elemental composition is not changed under the electron beam at relatively low accelerating voltages. This technique statistically samples the repeat units of the polymer. For samples grown in both aqueous and nonaqueous solutions. SEM/EDS provides evidence for extensive contamination with oxygen. 相似文献
60.
In the previous paper in this issue we have demonstrated that it is possible to measure the five different relaxation rates of a deuteron in (13)CH(2)D methyl groups of (13)C-labeled, fractionally deuterated proteins. The extensive set of data acquired in these experiments provides an opportunity to investigate side-chain dynamics in proteins at a level of detail that heretofore was not possible. The data, acquired on the B1 domain of peptostreptococcal protein L, include 16 (9) relaxation measurements at 4 (2) different magnetic field strengths, 25 degrees C (5 degrees C). These data are shown to be self-consistent and are analyzed using a spectral density mapping procedure which allows extraction of values of the spectral density function at a number of frequencies with no assumptions about the underlying dynamics. Dynamics data from 31 of 35 methyls in the protein for which data could be obtained were well-fitted using the two-parameter Lipari-Szabo model (Lipari, G.; Szabo, A. J. Am. Chem. Soc. 1982, 104, 4546). The data from the remaining 4 methyls can be fitted using a three-parameter version of the Lipari-Szabo model that takes into account, in a simple manner, additional nanosecond time-scale local dynamics. This interpretation is supported by analysis of a molecular dynamics trajectory where spectral density profiles calculated for side-chain methyl sites reflect the influence of slower (nanosecond) time-scale motions involving jumps between rotameric wells. A discussion of the minimum number of relaxation measurements that are necessary to extract the full complement of dynamics information is presented along with an interpretation of the extracted dynamics parameters. 相似文献