主题：NEOSY和ROESY究竟有什么区别阿？

1H/1H-NOESY (NOE correlation spectroscopy) detects intra- and intermolecular spatial proximity (via direct coupling) among protons and allows to establish the corresponding dipolar network most efficiently. The same experiment (EXCSY) may be used to detect chemical and dynamic exchange among protons.  Ref.: J. Jeener, B.H. Meier, P. Bachmann, R.R. Ernst, J. Chem. Phys. 71, 4546-4563 1979         D.J. States, R.A. Haberkorn, D.J. Ruben, J. Magn. Reson. 48, 286-292 1982 Note:·    ·  NOEs are dependent on molecular size and tumbling rates characterized by correlation times. For small molecules NOEs are positive (with a maximum of +0.385); for large (bio-)molecules NOEs are negative (with a maximum of –1.0). For molecules of intermediate size NOEs may be close to zero even for closely spaced protons. The ROESY experiment is used in such situations. ·    ·  so-called transient NOEs are measured, in contrast to the stronger steady-state NOEs obtained with the 1D NOE experiment.

1H/1H-ROESY (ROE correlation spectroscopy) detects intra- and intermolecular spatial proximity (via direct coupling) among protons and allows to establish the corresponding dipolar network most efficiently. The same experiment may be used to detect chemical and dynamic exchange among protons.  Ref.: A. A. Bothner-By, R.L. Stephens, J.-M. Lee, C.D. Warren, R.W. Jeanloz, J. Am. Chem. Soc.        106, 811-813 1984         A. Bax, D.G. Davis, J. Magn. Reson. 63, 207-213 1985 Note: ·    ·  in contrast to the NOESY experiment with the NOE built up under the influence of the strong static magnetic field, ROEs are built up in the much weaker “rotating field”. As a consequence ROEs are positive throughout (no zero-crossing) irrespective of the molecular size, but yield weaker effects for large molecules (<0.7 compared to –1.0 for NOEs) ·    ·  so-called transient ROEs are measured, in contrast to the stronger steady-state NOEs obtained with the 1D NOE experiment.

NOESY

NOESY is one of the most useful techniques as it allows to correlate nuclei through space (distance smaller than 5Å). By measuring cross peak intensity, distance information can be extracted.

The pulse sequence starts as usual with a 90` pulse followed by an evolution time t1. This delay is varied systematically as usual to provide chemical shift information in the F1 domain. Then a 90` pulse transmit some of the magnetization to the Z axis and during the following mixing period, the non-equilibria Z component will exchange magnetization through relaxation (dipole-dipole mechanism). This exchange of magnetization is known as NOE (Nuclear Overhauser Effect). After some time (shorter than the relaxation time T1), the transverse magnetization is restored and detected. If relaxation exchange (or chemical exchange) have taken place during the mixing time, cross peaks will be observed in the spectra.

The phase cycling ensure proper detection of NOESY signal. In small/medium size molecules, the mixing time can be selected to be about 80% of the relaxation time. For larger molecules, shorter mixing time should be used to avoid "spin-diffusion" problems.

In NOESY experiment, the distinction between cross peaks originating from NOE effect and those originating from chemical or conformational exchange is not easy. In small molecules, having long correlation time, the phase of the peak can be used as evidence:

With Diagonal signal phased "up"
NOE cross peak is phased "down"
Chemical Exchange cross peak is phased "Up"
For large molecule, having short correlation time, the phase of the diagonal, the NOE cross peak and the exchange cross peak is the same. It is therefore impossible to distinguish NOE from chemical exchange. In this case, the ROESY (NOE in the rotating frame) pulse sequence should be used.



ROESY

The ROESY sequence is very similar to the NOESY in that it provides information concerning distance between nuclei. This technique is based on NOE in the Rotating Frame. This pulse sequence is almost identical as the one used for TOCSY. To avoid TOCSY artefacts, the power used to achieve spin-lock is reduced. Also to further reduce the chances of Hartmann-Hahn match, the Spin-Lock offset is shifted to one end of the spectra (instead of the center). The HOHAHA signals are very large compare to the ROE cross peaks. It is therefore essential to take as many precautions as necessary to cancel those signals. Also If HOHAHA peaks are present, relayed cross peaks can be observed (HOHAHA-ROESY or ROESY-HOHAHA).

Contrary to NOE that can be positive (for small molecule), negative (for large molecule) or null (if the correlation time happen to cancel the NOE), the ROE (NOE in the rotating frame) is always positive. Alternation in sign of the ROE effect allows to distinguish the "three-spin effect" from true small ROE).

The peak phase behavior from the ROESY experiment is as follows:

If Diagonal peaks are phased "up" or positive
ROE cross peaks will be phased "down" or negative
HOHAHA peaks will be phased "up" or positive
Exchange peaks will be phased "up" or positive

Although ROESY has the advantage compared to NOESY that enhancements for molecules of all weights are positive, and therefore there is no molecular weight at which ROEs are nearly zero, ROESY has the disadvantage that TOCSY artifact peaks can occur. This problem does not arise in a NOESY.