Accurate measurements of small J coupling constants under inhomogeneous fields via intermolecular multiple-quantum coherences

Two new NMR pulse sequences, based on intermolecular multiple-quantum coherences (iMQCs), were developed to obtain apparent J coupling constants with a scaling factor from one to infinity relative to the conventional J coupling constants. Here the apparent J coupling constants were defined as apparent peak separations in unit of Hz in a reconstructed spectrum for a coupled spin system. Except for the adjustable scaling factor for apparent J coupling constants, the sequences hold the advantage of high acquisition efficiency, and retain the spectral information such as chemical shifts, multiplet patterns, and relative peak areas under inhomogeneous fields. For spin systems with small scalar coupling constants, well-resolved J-spectra can be achieved by selecting a proper scaling factor. Theoretical predictions are in good agreement with simulation results and experimental observations.     

High-resolution NMR spectra in inhomogeneous fields utilizing the CRAZED sequence without coherence selection gradients

Coherence selection gradients have been considered as indispensable for high-resolution NMR spectroscopy in inhomogeneous fields utilizing the CRAZED-type sequences. However, our experimental results demonstrate that these gradients can be omitted if an appropriate phase cycling is applied. The measured linewidth of reconstructing 1D high-resolution spectral peaks does not depend on the dipolar correlation distance determined by the coherence selection gradients, but is only affected by diffusion and T(2) relaxation. This finding suggests the need to reconsider the mechanism for the iMQC-based high-resolution spectroscopy.     

液体NMR中平板间多量子相干受限扩散行为的有限差分模拟

将平板间单量子相干受限扩散理论表述推广到多量子相干,并结合积算符矩阵、Bloch方程和有限差分方法进行模拟. 通过模拟找出平板间受限扩散信号衰减随平板间距变化的规律,并与实际体系比较. 结果表明：平板间n量子相干的自旋回波信号衰减曲线与单量子类似,且其产生同样衍射图样所需的脉冲梯度场强度仅为单量子的1/|n|,可用于测量微小的平板间距. 本文的模拟方法可进一步推广到复杂体系的研究.     

Isolating quantum coherences in structural imaging using intermolecular double-quantum coherence MRI

Intermolecular multiple-quantum coherence (iMQC) MR imaging provides a fundamentally different contrast mechanism. It allows probing tissue microstructure by tuning the direction and strength of the correlation gradient. However, iMQC images of a specific quantum-coherence can easily be contaminated by leakage signals from undesired quantum coherences (zero, single, and triple quantum coherence in this work). Using a modified double-quantum CRAZED imaging sequence, we show that signals originating from various coherence orders (M=0, 1, 2, 3) can be predicted in k-space and effectively isolated by means of a four-step phase cycling scheme and judicious choice of flip angles. Finally, preliminary data suggest the method to be able to provide information on trabecular bone architecture such as regional mean trabecular plate separation.     

双Z脉冲实现多量子相干选择检测

本文提出一种改进的多量子相干选择检测(滤波)方法。滤波Z脉冲分别在准备期和混合期末。它能有效地防止单量子相干的"漏进"并避免使用数字相移器。在一般谱仪上容易实现,用密度算符理论对AMX体系进行理论计算,实验结果与理论预期相符。     

The 2D {P} Spin-Echo-Difference Constant-Time [C, H]-HMQC Experiment for Simultaneous Determination of JH3′P and JC4′P in C-Labeled Nucleic Acids and Their Protein Complexes

A two-dimensional {³¹P} spin-echo-difference constant-time [¹³C, ¹H]-HMQC experiment (2D {³¹P}-sedct-[¹³C, ¹H]-HMQC) is introduced for measurements of ³J_C4′P and ³J_H3′P scalar couplings in large ¹³C-labeled nucleic acids and in DNA–protein complexes. This experiment makes use of the fact that ¹H–¹³C multiple-quantum coherences in macromolecules relax more slowly than the corresponding ¹³C single-quantum coherences. ³J_C4′P and ³J_H3′P are related via Karplus-type functions with the phosphodiester torsion angles β and ε, respectively, and their experimental assessment therefore contributes to further improved quality of NMR solution structures. Data are presented for a uniformly ¹³C, ¹⁵N-labeled 14-base-pair DNA duplex, both free in solution and in a 17-kDa protein–DNA complex.     

二维多量子MAS NMR的计算机模拟研究

最近Frydman等人报道了联合MAS及多量子方法获取半整数四极核各向同性谱的新途径,本文以自旋3/2的四极核为例,采用步进式时序积分手续,数值计算了魔角样品旋转条件下的传播子,从而真实地模拟了非对易含时Hamiltonian情况下的密度矩阵演化;对Fryman等人采用的双脉冲序列实验应采取的最佳条件进行了详尽的理论分析;并对Na₂C₂O₄样品的2D-MQ-MAS实验进行拟谱。     

Magnetic resonance microscopic imaging based on high-order intermolecular multiple-quantum coherences

Most imaging studies using intermolecular multiple-quantum coherences (iMQCs) have focused on the two-spin dipolar interactions--zero and double quantum coherences. Here, we report the results of various experimental studies to assess the feasibility of magnetic resonance microscopy with high-order iMQCs in model systems at 7 and 14 T. Experimental results demonstrated that the iMQC microscopic images with high coherence orders are readily observable at high field and have unique contrast depending on the sample microstructure and coherence order.     

The theoretic design of NMR pulses program of arbitrary N-qubit Grover's algorithm and the NMR experiment proof

Grover's quantum searching algorithm is most widely studied in the current quantum computation research, and has been implemented experimentally by NMR (Nuclear Magnetic Resonance) technique. In this article, we design arbitrary N-qubit NMR pulses program of Grover's algorithm based on the multiple-quantum operator algebra theory and demonstrate 2-qubit pulses program experimentally. The result also proves the validity of the multiple-quantum operator algebra theory.     

Double-quantum excitation in the NMR of spinning solids by pulse-assisted rotational resonance

We describe a new technique for double-quantum excitation in magic-angle-spinning NMR of powdered solids. The technique is designed to efficiently excite double-quantum coherence in the vicinity of a rotational resonance condition. The offset from rotational resonance allows the double-quantum filtered signals to be observed with high resolution and sensitivity. The method uses rotational excitation of zero-quantum coherence, assisted by radiofrequency pulse cycles. The zero-quantum coherence is converted into double-quantum coherence by a frequency-selective inversion sequence. Experiments on [(13)C(2), (15)N]-glycine demonstrate a double-quantum filtering efficiency of approximately 41% at a sample rotation frequency of 8.300 kHz, which is 1.600 kHz away from the n = 1 rotational resonance. We achieve 32% double-quantum filtering efficiency at a spinning frequency of 9.250 kHz, which is 2.550 kHz away from rotational resonance.