Phase ROESY

PSP pseudo-stationary phase ROESY rotation frame nuclear Overhauser... 

In order to combat this, the rotating frame Overhauser effect spectroscopy (ROESY) techniques can be employed. An in-depth discussion of how this technique works is outside the remit of this book but suffice to say, in the ROESY methods (1- and 2-D), NOE data is acquired as if in a weak r.f. field rather than in a large, static magnetic field and this assures that all NOEs are present and positive, irrespective of tumbling rate and magnet size. It is possible that some TOCSY correlations can break through in ROESY spectra but these will have opposite phase to the genuine ROESY correlations and so should therefore not be a problem - unless they should overlap accidentally with them. A 2-D ROESY spectrum of the naphthalene compound is shown below (Spectrum 8.6). 

The disposition of the bridgehead proton with respect to the protons in position-3 and -6 was studied by ROESY 2-D NMR in four sets, 288-291 of 16 compounds bearing different substituents. The compounds were obtained via solid-phase syntheses by tandem cyclization <2004TL6333>. 

The data from H NMR studies of 63, which included double quantum filtered phase sensitive correlated spectroscopy (DQF-COSY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments (Figure 12), are collected in Table 17. 

ID-ROESY studies performed on the complex between (+)-BrPh and /3-CD in solution did not allow one to explain the NOE effect observed on the protons of the maleate counteranion (70). X-ray crystallographic studies performed on the monocrystals obtained from a 1 1 aqueous solution of (+)-BrPh maleate and /3-CD (Fig. 8) provide a plausible explanation for the contradiction maintained in Ref. 70. In particular, as shown in Fig. 8, (+)-BrPh forms with /3-CD, at least in the solid state, not a 1 1 complex but a complex with 1 2 stoichiometry. In this complex the (+)-BrPh molecule is sandwiched between two molecules of /3-CD. The 4-bromophenyl moiety of (+)-BrPh enters the cavity of one of the /3-CD molecules, whereas the cavity of another /3-CD molecule is occupied by the maleate counteranion. Thus, X-ray crystallography may provide useful information on the supra-molecular structure of the selector-selectand complexes and in this way complement well ID-ROESY data. However, the aforementioned possible differences between the structure of the complexes in solution and in the solid phase must be considered. 

Fig. 5. Pulse sequences of NOESY and ROESY with spin-lock purge pulses for water suppression. (A) NOESY pulse sequence. The spin-lock pulses are typically of length 0.5 ms and 2 ms, and r = 1/SW, where SW is the spectral width in the acquisition dimension. Phase cycle (pi = x,—x) 4>2 = 4 x,x,—x,—x) ...

Compared to other multidimensional experiments the exchange experiments are fairly simple and, thus, easy to optimize. Experiments are robust with regard to the pulse imperfections and miscalibration. All artifacts except coherence transfer can be removed with standard phase cycling of RF pulses and receiver. The coherence transfer can be removed by appropriate pulse sequences, preferably with T-ROESY. 

More serious are the coherence transfer cross peaks in ROESY spectra because the coherence peaks are in phase with the genuine cross-relaxation peaks and thus may modulate intensity of the genuine peaks. To emphasize the effect of coherence transfer peaks (now TOCSY peaks) we do the ROESY experiment with Tm = 300 ms and with a spin-lock field of 5 kHz (fig. 4(C)). Besides positive diagonal peaks (thick contours), several pairs... 

During the structural elucidation of the first flavone-xanthone dimer (swertifranche-side) through a series of ID or 2D NMR techniques including COSY, phase-sensitive ROESY, reversed-detected HMQC, HMBC, and selective INEPT experiments, H and C signals appeared as two peaks indicating two conformers and classically, the coalescence of the split signal was observed as the temperature was increased. 

Occasionally, COSY-type artifacts appear in NOESY and ROESY spectra but these are easy to identify by their anti-phase multiplet structure. 

H/ H-ROESY (-NOESY) Cross peak.s and diagonal peaks appear in absorption. The diagonal peaks are the most intense and are best suited for phase adjustments. They should be phased for negative absorption in ROESY spectra and in NOESY spectra measured for small molecules, giving cross peaks in positive absorption in both cases. For NOESY spectra of large molecules (e.g. biomolecules), both diagonal and cross peaks should be phased to positive absorption. 

PTFE polytetrafluoroethylene PUFA polyunsaturated fatty acid PV peroxide value PVDF polyvinylidene difluoride PVP polyvinylpyrrolidone PVPP polyvinylpolypyrolidone RAS retronasal aroma stimulator RDA recommended dietary allowance RF radio frequency RFI relative fluorescence intensity RI retention index RNU relative nitrogen utilization ROESY rotational nuclear Overhauser enhancement spectroscopy RP-HPLC reversed-phase HPLC RPER relative protein efficiency ratio RS resistant starch RT retention time RVP relative vapor pressure S sieman (unit of conductance)... 

Fig. 8.2. Some of the most common 2D pulse sequences that can be employed using a proper choice of parameters to record 2D spectra of paramagnetic molecules (A) NOESY, (B) ROESY, (C) COSY, (D) ISECR COSY, (E) zero-quantum (double quantum) COSY, (F) TOCSY, (G) HMQC, (H) HSQC. Sequences (A), (B) and (F) are also used to obtain EXSY spectra. SL indicates a soft spin-lock sequence, while MLEV17 indicates a train of spin-locking hard pulses that optimizes the development of J/j coupling. In the reverse heteronuclear experiment (G) the upper and lower levels refer to H and heteronucleus, respectively. The phase cycles are not indicated. For clarity of discussion, all initial pulses can be thought to be applied along the y axis, in such a way that the coherence after the first 90° pulse is always along x. ...

The pulse sequence for ICP experiments appears simple a 90° proton pulse is followed immediately by a spin lock radio-frequency (rf) field of strength B that is phase shifted by 90° relative to the first pulse. By a spin-lock field is meant a strong rf field B that is on resonance with the given nucleus it keeps magnetization in a spin-locked orientation parallel to the B direction where the decay of magnetization is governed by T p. At present the strong continuous B field is replaced by multipulse sequences that are well known from other spin-lock experiments such as TOCSY, ROESY etc. Simultaneously,... 

In both ROESY mixing and TOCSY mixing, the transfer is from in-phase net magnetization on the spin-lock axis to in-phase net magnetization on the spin-lock axis for example, ly ->1 (TOCSY) or ly - - Iy (ROESY). This is analogous to the NOESY transfer, It -> — for small molecules and in contrast to the INEPT transfer 21 -> 21 1 , which is antiphase to antiphase. 

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... 

Bax and co-workers demonstrated that a homonuclear Hartmann-Hahn transfer of net magnetization can be obtained by the application of a spin-lock field, using CW irradiation (Bax and Davis, 1985a Davis and Bax, 1985) or by the DB-1 sequence that consists of a series of phase-alternated spin-lock pulses (Davis and Bax, 1985). The homonuclear Hartmann-Hahn effect caused by CW irradiation was discovered when artifacts in ROESY experiments were analyzed (Bax and Davis, 1985a). CW irradiation can be regarded as a homonuclear analog of spin-lock experiments for heteronuclear cross-polarization (Hartmann and Hahn,... 

Since the pulse sequence is the same for EXSY and NOESY, NOESY (or ROESY) cross peaks might be mistaken for EXSY cross peaks. They can be distinguished in the phase-sensitive experiment, since EXSY and ROESY peaks have opposite phases, as do EXSY and NOESY peaks in the fast motion regime. For example, two resolved OH or NH resonances may exhibit EXSY cross peaks from slow proton exchange. These peaks could be mistakenly taken to be NOESY peaks and interpreted incorrectly in terms of stereochemistry. 

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