ROESY spin-lock power

The easiest way to reduce the amplitude of TOCSY cross peaks in the ROESY spectra is to record a spectrum with minimal spin-lock power [23]. The other possibility is to modulate the frequency of the spin-lock field [25]. However, the most convenient way is to apply a series of 180° pulses instead of a single continuous-wave pulse during the mixing time, as is done in the T-ROESY experiment. Figure 4(D) shows the T-ROESY spectrum of cyclo(Pro-Gly) recorded with = 300 ms. Although the... 

In the following, three different experiments are discussed, where short, high-power spin-lock pulses are used to purge the spectrum from undesired resonances. The experiments are (i) the HSQC experiment [5], (ii) experiments with C half-filter elements [6], and (iii) NOESY and ROESY experiments for the observation of water-protein NOEs [7]. In the first two experiments, spin-lock purge pulses are used to suppress the signals from... 

The 2D ROE or ROESY experiment is an experiment to measure cross-relaxation in the rotating frame (Fig. 8.2B). After an initial 90° pulse and the variable evolution period t, a low power or soft spin-lock sequence (SL) is applied for a time during which magnetization transfer in the rotating frame occurs due to cross relaxation. Since scalar connectivities can also develop during spin lock, as... 

In TOCSY experiments, the problem of overheating the sample is more serious than in ROESY experiments because of the large irradiation energy required by the spin-lock pulse. Each individual component of the pulse train must have enough power to irradiate the whole spectral window of interest. Spin-lock sequences different from the MLEV17 sequence, that may alleviate the heating problem,... 

The ROESY used to be a bit difficult to set up because the low power spin-lock RF had to come from a different source than the hard pulse RF. Now rapid solid-state power switching is so routine that all H RF comes from the same source, with no variation of phase or frequency. ROESY tends to replace the NOESY experiment for NOE measurements, especially for small molecules where T2 is relatively long. Because the NOE builds up about twice as fast in the x -y plane as it does on the z axis, ROESY mixing times are set to about half of what would be the NOESY mixing time. There is one additional parameter to set up the power level ( Z i field strength ) of the spin lock pulse. This is typically... 

ROESY (rotating frame Overhauser effect spectroscopy) is a variant of NOESY, in which the transfer of magnetization occurs on the spin-lock axis in the x-y plane rather than on the z axis (Fig. B.9). A continuous low-power radio frequency pulse provides the mixing by effectively reducing the field strength (Bq in the laboratory frame on the z axis to B in... 

The experimental method for obtaining ROESY is essentially the same as that for HOHAHA, application of a spin-lock pulse sequence for mixing at the end of the evolution period. HOHAHA effects can interfere with ROESY measurements but are minimized by using lower rf power and offsetting the pulse frequency to interfere with the Hartmann-Hahn condition. 

Figure 8.45. The 2D ROESY sequence. The mixing time, tm. is defined by the duration of the low-power spin-lock pulse.

Figure 8.48. Practical mixing schemes for the ROESY experiment, (a) a single, low-power pulse, (b) a pulsed spin-lock comprising a repeated sequence of a small tip angle pulse followed by a short delay, and (c) the Tr-ROESY alternating-phase spin lock.

Figure 6 Rotating frame NOE pulse sequences. The 1D experiment requires two sequences, represented by (A) and (B). (A) is the reference experiment in which a 90 non-selective pulse is applied on all the spins, followed by a spin-lock along the y-direction for a time and the state of the spin system is detected. (B) The control experiment in which a selective 180 pulse, inverts the magnetization of the spin from which the NOE is to be observed before the 90j pulse and the experiment is continued as (A). The 1D NOE spectrum is the difference between the spectra obtained with the sequence (A) and (B). (C) The 2D ROESY sequence. The times and fs are the evolution and detection periods and is the mixing time. SL refers to the low power spin-locking RF field.

At 500 MHz, moderate-sized (more than six residues) oligosaccharides lie within the spin-diffusion limit. However, for smaller molecules, as the value of the function (OqT (where (Oq is the Larmor frequency of protons, and is the correlation time of the molecule) approaches 1 then the value of the NOE tends towards 0. Cross-peak intensities of NOESY spectra of smaller oligosaccharides (2-5 residues) may thus become too small to measure accurately. In such cases, the rotating frame Overhauser effect spectroscopy (ROESY, originally referred to as CAMELSPIN) experiment is commonly used to measure NOE values. To reduce the appearance of HOHAHA-like cross-peaks, a low power spin-lock field should be used, and the transmitter carrier offset to the low-field end of spectrum. The offset dependency of cross-peak intensities should also be removed by 90° pulses at either end of spin-lock period. 

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