Homonuclear experiments

Heteronuclear nOe experiments are similar to homonuclear experiments, discussed earlier. Normally, protons are irradiated and the enhancement of the heteronucleus is recorded, since if the reverse were done, the... 

Availability of a sample isotopically enriched with 15N and 13C offers the NMR spectro-scopist additional tools for overcoming the problems encountered in homonuclear experiments. In an obvious extension of the homonuclear experiments, the carbon or nitrogen dimension can be used to improve the resolution in NOESY spectra. However, recent trends clearly favor the through-bond approach where the presence of a cross peak unambiguously proves the existence of a chemical bond between the nuclei involved. Let us review briefly the experiments available for labeled oligonucleotides. 

Another way of avoiding overlaps seen in two-dimensional spectra is to introduce the third dimension. This has been illustrated with the 3D NOESY-HOHAHA [21-25], 3D HOHAHA-NOESY [25-27], NOESY-COSY [28, 29], COSY-NOESY [28] and ROESY-TOCSY [23] experiments. A principal drawback, associated with homonuclear 3D techniques, is the low digital resolution achievable along the first and second spectral axes. This limitation can to certain extent be removed by implementing band selective pulses into the 3D homonuclear experiments [21, 26, 28-31]. 

In a move in the opposite direction, the overlaps resulting from concatenation of different polarization transfer mechanisms in combined 2D experiments can be eliminated by reducing the dimensionality of an experiment. Similarly to the successful transformation of basic 2D NMR techniques into their ID counterparts [32-34], a conversion of combined 2D NMR techniques into their ID analogs is feasible and has been explored by several groups [35-40]. From a different perspective this process can be seen as a twofold reduction of the dimensionality in a 3D experiment. Equally, concatenation of three polarization transfer steps in a single ID experiment represents transformation of a possible 4D homonuclear experiment into its ID analog. 

ID ll spectrum, and crosspeaks are arranged symmetrically around the diagonal. There is only one radio frequency channel in a homonuclear experiment, the H channel, so the center of the spectral window (set by the exact frequency of pulses and of the reference frequency in the receiver) is the same in If and F (Varian tof, Bruker ol). The spectral widths should be set to the same value in both dimensions, leading to a square data matrix. Heteronuclear experiments have no diagonal, and two separate radio frequency channels are used (transmitter for F2, decoupler for F ) with two independently set spectral windows (Varian tof and dof, sw, and swl, Bruker ol and o2, sw(If), and sw(I )). Heteronuclear experiments can be further subdivided into direct (HETCOR) and inverse (HSQC, HMQC, HMBC) experiments. Direct experiments detect the X nucleus (e.g., 13C) in the directly detected dimension (Ff) using a direct probe (13C coil on the inside, closest to the sample, H coil on the outside), and inverse experiments detect XH in the To dimension using an inverse probe (XH coil on the inside, 13C coil outside). 

Another way of getting substantially the same result is based upon a homonuclear experiment to saturate selectively the chosen resonance under proton decoupled conditions. (202) The spectrum is again acquired under fully coupled conditions and thus is a complete coupled spectrum except that the multiplet from the saturated site is absent. This spectrum is subtracted from a normal fully coupled one to yield a trace that contains only the multiplet from the desired carbon. Full fine structure is displayed. Figure 27 shows the application of this method to the spectrum of the monosaccharide derivative... 

Consider a basis sequence that consists of N square pulses (see Fig. 3). In homonuclear experiments, where all rf pulses are applied at the same frequency the spin system is conveniently analyzed in the corresponding rotating frame, where the Hamiltonian... 

Today, spin diffusion is often used in a quite general way to describe multispin polarization-transfer processes, whether or not the process can actually be described by a diffusion equation. In this overview, we will interpret spin diffusion in this broad sense. If applied in the context of two-dimensional homonuclear experiments, it becomes synonymous with total through-space correlation spectroscopy (TOSSY) [2], the dipolar equivalent of the liquid-state TOCSY [3] experiment. 

Beyond its general use as a means of sensitivity enhancement of spin-V2 nuclei with low magnetogyric ratios, the specific heteronuclear NOE has occasionally been used as a tool in structural studies [77] and is capable of providing a unique source of stractural information in favourable circumstances. Techniques for its observation largely parallel those for homonuclear experiments in the form of ID steady-state difference and 2D transient experiments, so there is nothing fundamentally new to understand here. The main limitation with these approaches is the low sensitivity associated with the observation of the low-y spin, meaning heteronuclear NOEs tend to be far less used than their homonuclear proton counterparts. The most widespread applications have involved the NOE, and more recently the H- Li NOE [78], in which... 

In this teaching version of NMR-SIM the NMR Wizard may only be used for setting up homonuclear experiments, any attempt to set up a heteronuclear experiment will generate an error. Consequently this version of the NMR Wizard is only really suitable for nuclei with high receptivity to the NMR experiment such as IR, and possibly Shaped pulses are not supported in any form. In addition to selecting the experimental parameters, the NMR-Wizard can also select the appropriate processing parameters but if necessary this function can be switched off and the parameters can be entered manually. 

Fig.(l). The principles of usual 2D homonuclear experiments, (bold lines represent typical correlations obtainable through the corresponding experiments)... 

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