Spin-echo difference

The HMQC approach has also been employed in the measurements of 3/(H3 P) and 3/(C4 P) coupling constants using a 2D 31P spin-echo difference constant-time experiment [44] to increase sensitivity by a factor of 1.5-2.4 in a 17 kDa DNA-protein complex. 

Several modifications have been proposed for the basic HNN-COSY experiment. For example, frequency separations between amino and aromatic 15N resonances are typically in the range 100-130 ppm and therefore much larger than between imino 15N donor and aromatic 15N acceptor resonances. As has been pointed out by Majumdar and coworkers [33], such 15N frequency separations are too large to be covered effectively by the non-selective 15N pulses of the homonuclear HNN-COSY. They therefore designed a pseudo-heteronuclear H(N)N-COSY experiment, where selective 15N pulses excite the amino and aromatic 15N resonances separately to yield excellent sensitivity [33]. An inconvenience of this experiment is that the resonances corresponding to the amino 15N nuclei are not detected, and a separate spin-echo difference experiment was used to quantify the h2/NN values. A slightly improved version of this pseudo-heteronuclear H(N)N-COSY [35] remedies this problem by the use of phase-coherent 15N pulses such that both amino and aromatic 15N resonances can be detected in a single experiment. 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Fig. 36. 2D- Cy " J-Spin-echo-difference H/ N-CT-HSQC spectra. Only residues with large couplings give signals in the spectrum indicating an antiperiplanar conformation...

The H, relayed spin-echo difference spectra of maltose 1-phosphate and D-glucose 1-phosphate have been reported. Use of this technique allowed observation of the total sub-spectrum of the phosphorylated glucose residue of the disaccharide. ... 

Fig. 3. Spin-echo difference spectra of DTT-activated CF versus DTT-act-ivated CFj having MgAMPPNP tightly bound. Purified CFi (10.4 mg) was activated by incubation in 50 mM DTT for 90 m. The DTT was then removed by Sephadex G-50 fine chromatography. Following accumulation of spectra in 25 mM NaCl and 30 mM Borate pD 9.0, 150 wM MgAMPPNP was added and, after a 60 min incubation, a second set of spectra were obtained. Spin-echo difference spectra MgAMPPNP were recorded at the t values shown.

The spin-echo is an elegant method for the measurement of transverse relaxation time Tj. In practice, this is done by repeating the spin-echo experiment many times, with different delay intervals (2t, 4t, 6t,. . . etc.). The following sequence is used ... 

Nuclei resonating at different chemical shifts will also experience similar refocusing effects. This is illustrated by the accompanying diagram of a two-vector system (acetone and water), the nuclei of which have different chemical shifts but are refocused together by the spin-echo pulse (M, = magnetization vector of acetone methyl protons, M(v = magnetization vector of water protons). 

Another 2D homonuclear shift-correlation experiment that provides the coupling information in a different format is known as SECSY (spin-echo correlation spectroscopy). It is of particular use when the coupled nuclei lie in a narrow chemical shift range and nuclei with large chemical shift differences are not coupled to one another. The experiment differs... 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

In many products, the spin-relaxation properties of the components can be different due to molecular sizes, local viscosity and interaction with other molecules. Macromolecules often exhibit rapid FID decay and short T2 relaxation time due to its large molecular weight and reduced rotational dynamics [18]. Mobile water protons, on the other hand, are often found to have long relaxation times due to their small molecular weight and rapid diffusion. As a result, relaxation properties, such as T2, have been used extensively to quantify water/moisture content, fat contents, etc. [20]. For example, oil content in seeds is determined via the spin-echo technique as described according to international standards [64]. 

DOSY is a technique that may prove successful in the determination of additives in mixtures [279]. Using different field gradients it is possible to distinguish components in a mixture on the basis of their diffusion coefficients. Morris and Johnson [271] have developed diffusion-ordered 2D NMR experiments for the analysis of mixtures. PFG-NMR can thus be used to identify those components in a mixture that have similar (or overlapping) chemical shifts but different diffusional properties. Multivariate curve resolution (MCR) analysis of DOSY data allows generation of pure spectra of the individual components for identification. The pure spin-echo diffusion decays that are obtained for the individual components may be used to determine the diffusion coefficient/distribution [281]. Mixtures of molecules of very similar sizes can readily be analysed by DOSY. Diffusion-ordered spectroscopy [273,282], which does not require prior separation, is a viable competitor for techniques such as HPLC-NMR that are based on chemical separation. 

In an NMR analysis of the effects of /-irradiation induced degradation on a specific polyurethane (PU) elastomer system, Maxwell and co-workers [87] used a combination of both H and 13C NMR techniques, and correlated these with mechanical properties derived from dynamic mechanical analysis (DMA). 1H NMR was used to determine spin-echo decay curves for three samples, which consisted of a control and two samples exposed to different levels of /-irradiation in air. These results were deconvoluted into three T2 components that represented T2 values which could be attributed to an interfacial domain between hard and soft segments of the PU, the PU soft segment, and the sol... 

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