Multiple quantum coherence transfer HMQC

J splittings cannot be directly resolved. In addition to the obvious advantage of providing a map of chemical bonds between the spins, /-based transfers do not require spin-locking and are not disturbed by molecular motions. The major drawback of polarization transfer through J coupling is that the delays involved in the pulse sequences, such as insensitive nuclei enhanced by polarization transfer (INEPT) [233] or heteronuclear multiple-quantum coherence (HMQC)... 

FIGURE 12.16 Pulse sequence for the triple resonance 3D NMR experiment HNCO. H and N denote H and 15N, C denotes 13C=0, and K denotes 13C . Pulses at times 1, 2, and 3 constitute an INEPT sequence that transfers coherence from H to. V, where it precesses during q. Pulses at times 6, 7, and 8 represent an HMQC sequence that creates multiple quantum coherence in C (where it precesses during and transfers coherence back to N. Pulses 10 and 11 are an inverse INEPT sequence that transfers coherence back to H for detection during f3.The other 180° pulses refocus heteronuclear spin couplings. Note that coherence is not transferred to spin K. 

The inverse detection heteronuclear multiple quantum coherence (HMQC) experiment is another approach to two-dimensional NMR techniques, which consists of a transfer of chemical shift and coupling information from relatively insensitive nuclei such as and some metals, to more sensitive nuclei such as H. The advantage of this method is a substantial increase in the sensitivity obtained, due to the greater natural abundance of H (Kingery et al., 2001). 

NMR is the tool most widely used to identify the structure of triterpenes. Different one-dimension and two-dimension techniques are usually used to study the structures of new compounds. Correlation via H-H coupling with square symmetry ( H- H COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC), distortionless enhancement by polarisation transfer (DEPT), incredible natural abundance double quantum transfer experiment (INADEQUATE) and nuclear Overhauser effect spectroscopy (NOESY) allow us to examine the proton and carbon chemical shift, carbon types, coupling constants, carbon-carbon and proton-carbon connectivities, and establish the relative stereochemistry of the chiral centres. 

The reduced dimensionahty approach turned out to be beneficial for coupling constants measurements. Kozminski et al. described a C -coupled-MQ-HNCO experiment for a robust evaluation of J(Ni,C i i ) couplings that can be used for the secondary structure identification. The sequence includes nested HMQC coherence transfer blocks generating multiple-quantum coherence. The evolution of the coherence is recorded... 

In heteronuclear correlation experiments, magnetization transfer between protons and heteronuclei can be via either heteronuclear single quantum coherence (HSQC) or heteronuclear multiple quantum coherence (HMQC) pathways. The HSQC sequence gives rise to narrower lines, but uses more pulses and requires a longer phase cycle than the HMQC. Thus, HSQC is used for 2D experiments where the highest resolution is required and HMQC is preferred for 3D sequences in which the experimental time is limited. 

Of the two isotopes of nitrogen, N with a spin quantum number of 1/2 is the most widely used. It is very insensitive (1 x 10" ), and has a low natural abundance of 0.37%. While some studies have used the direct detection of N-labelled compounds, the indirect detection of N through H by techniques such as INEPT (insensitive nuclei enhanced by polarization transfer) and HMQC (heteronuclear multiple quantum coherence), which considerably enhance sensitivity, have increased the applicability of N in biological systems. 

With respect to the pulse sequence layout, the HMBC experiment is essentially a HMQC experiment incorporating a low-pass filter to suppress the one-bond correlation peaks. The low-pass filter, consists of a delay d2 = 1/(2 U(C, H)) and a 90° pulse, which transfers the U(C, H) coherence into a multiple quantum state. In a second period coherences which are generated by JCC, H) evolution are also transferred to a multiple quantum state by a 90° l C pulse but with a different phase in relation to the first 90° pulse of the low-pass filter. A combination of appropriate receiver phase cycling and pulse phase cycling enables the exclusive detection of J(C, H) correlation peaks in the 2D experiment. 

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