Complete (1)H NMR spectral fingerprint of huperzine A

Complete analysis of the (1)H NMR spectrum of huperzine A, 1-amino-13-ethylidene-11-methyl-6-aza-tricyclo[7.3.1.0(2, 7)]trideca-2(7),3,10-trien-5-one, a Lycopodium alkaloid and anti-Alzheimer drug lead containing an ABCD(E)(MN)(OP)X(3)Y(3)-type system of 15 nonexchangeable proton spins, is reported for the first time, and earlier assignments are corrected. The complete (1)H parameter set of 11 chemical shifts clarifies the diastereotopism of both methylene groups, and provides a total of 38 observed H,H-couplings including 31 long-range ((4-6)J) connectivities. The NMR data is consistent with the comparatively rigid alicyclic backbone predicted by molecular mechanics calculations, and forms the basis for (1)H NMR fingerprint analysis for the purpose of dereplication, purity analysis, and elucidation of structural analogs.     

SESAME-HSQC for simultaneous measurement of NH and CH scalar and residual dipolar couplings

We present a novel pulse sequence, SESAME-HSQC, for the simultaneous measurement of several NH and CH scalar and residual dipolar couplings in double labeled proteins. The proposed Spin-statE Selective All Multiplicity Edited (SESAME)-HSQC combines gradient selected and sensitivity enhanced (15)N- and constant-time (13)C-HSQC experiments with the recently introduced spin-state selective method (Nolis et al., J. Magn. Reson. 180 (2006) 39-50) for measuring couplings simultaneously at amide and aliphatic regions. Excellent resolution and high sensitivity is warranted by removing all coupling interactions during the indirectly detected t(1) period, and by employing pulsed field gradients for coherence selection and utilizing coherence order selective spin-state selection. The scalar and residual dipolar couplings can be readily measured from a two-dimensional (15)N/(13)C-HSQC spectrum without additional spectral crowding. SESAME-HSQC can be used for epitope mapping by observing chemical shift changes in both amide and aliphatic regions. Simultaneously, potential conversion in protein conformation can be probed by analyzing changes in residual dipolar couplings induced by ligand binding. The pulse sequence is experimentally verified with a sample of (15)N/(13)C enriched human ubiquitin. The internuclear vector directions determined from the residual dipolar couplings are found to be in excellent correlation with those predicted from ubiquitin's refined solution structure.     

高散射介质中光子扩散成像的线性平移不变性

运用微扰理论探讨了光子扩散成像的线性平移不变性,阐明了光子扩散成像用于生物组织这一高散射介质结构与功能成你的信号形成机理.光子扩散成像所得的图像是介质内部异常结构通过光子扩散成像系统的响应.光子扩散成像系统的作用可用一个脉冲响应函数来表征.     

An Investigation of Poly(dimethylsiloxane) Chain Dynamics and Order in Its Inclusion Compound with γ‐Cyclodextrin by Fast‐MAS Solid‐State NMR Spectroscopy

The dynamics of poly(dimethylsiloxane) in its inclusion compound with γ‐cyclodextrin are elucidated using modern fast‐MAS solid‐state NMR techniques. Measurements of methyl ¹H–¹H and ¹H–¹³C dipolar coupling constants indicate that the polymer undergoes a uniform motion, rendering all methyl groups equivalent. The dynamics of the Si—C bond is characterized by either a dynamic order parameter of S = 0.72, or, assuming a stably rotating helical structure, an inclination angle of 73° relative to the rotation axis.     

Computation of the stabilization parameter for the finite element solution of advective–diffusive problems

In a previous paper a general procedure for deriving stabilized finite element schemes for advective type problems based on invoking higher order balance laws over finite size domains was presented. This provides an expression for the element stabilization parameter in terms of the solution residual and its first derivatives in a kind of iterative or adaptative manner. Details of the application of this procedure to 1D and 2D advective–diffusive problems are given. Some examples of applications showing the potential of the new approach are presented. © 1997 John Wiley & Sons, Ltd.     

Generating Majorana modes in one-dimensional p-wave superconductor with second-neighbor couplings by periodic kicking

Generating many Majorana modes (MMs) within one single system is a constructive method for experimental observation of MMs in condensed-matter systems. In this paper, we show that by adding periodic δ-function kicks on the chemical potential, a large number of MMs can be generated in the one-dimensional p-wave superconductor system with second-neighbor couplings. The Floquet theory has been used to analyze the stroboscopic properties and the winding number can be used to characterize the topological properties. We have analyzed the influence of the kicking parameters and second-neighbor couplings on the topological phase transitions.     

Specific Heat of a Two-Layer Magnetic Superlattice

The specific heats of both a two-layer ferromagnetic superlattice and a two-layer ferrimagnetic one are studied. It is found that the spin quantum numbers, the interlayer and intralayer exchange couplings, the anisotropy, the applied magnetic field, and the temperature all affect the specific heat of these superlattices. For both the ferromagnetic and ferrimagnetic superlattices, the specific heat decreases with increasing the spin quantum number, the absolute value of interlayer exchange coupling, intralayer exchange coupling, and anisotropy, while it increases with increasing temperature at low temperatures. When an applied magnetic field is enhanced, the specific heat decreases in the twolayer ferromagnetic superlattice, while it is almost unchanged in the two-layer ferrimagnetic superlattice at low field range at low temperatures.     

Efficient stereoselective synthesis of benzoxazines via copper-catalyzed three-component coupling reactions

A convenient one-pot synthesis of 1,4-benzoxazines via three-component coupling and subsequent O-cyclization is reported. The present reaction provides an efficient protocol to functionalized 1,4-benzoxazine derivatives in good to high yields from aldehydes, amines and alkynes. Furthermore, the O-annulation process is completely regio- and stereoselective, only the six-membered rings and its Z-isomers were obtained.     

Spatial structure of peptides determined by residual dipolar couplings analysis

The gated decoupled (13)C NMR spectra of a dipeptide (Glu-Trp) and a tetrapeptide (NAc-Ser-Phe-Val-Gly-OMe) were recorded in D(2)O and in a lyotropic alignment medium (pentaethylene glycol monododecyl ether/n-hexanol). The residual dipolar couplings were extracted as the differences between the observed couplings for the magnetic nuclei dissolved in the latter and former media. Using a computational optimization, the spatial structures of the compounds were calculated starting from their respective low energy conformations obtained on a semiempirical basis. The uniformity of each conformation was confirmed by the solid-state (13)C NMR spectra of powder samples. Differences between the starting structures and final ones, optimized when employing residual dipolar couplings, are discussed.