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Friday, May 15, 2009

The Selective 1D Gradient NOESY

Many students run very long 2D NMR experiments to find one particular piece of information. This is especially true for 2D NOESY experiments where perhaps only a single NOE correlation is sought. Depending on the amount of compound available, 2D NOESY measurements often take hours or tens of hours to acquire and can cost alot of money in instrument user fees. Many people do not realize that there are selective 1D analogs to the non selective 2D experiments. These 1D experiments rely on shaped pulses for selective excitation and take only a small fraction of the time required to run the comparable non selective 2D experiment. Previously I have posted entries in this BLOG on the very useful 1D selective gradient TOCSY experiment. The figure below demonstrates the use of the 1D selective gradient NOESY experiment for bis(phenylthio)methane compared to the standard proton NMR spectrum. In this case, the methylene protons were selectively irradiated and the NOE's were observed on the phenyl rings. The experiment took less than 5 minutes to acquire.

8 comments:

Manoj said...

we always put gradient value in percentage How to calculate those value in Tesla/meters?

Thanks

Glenn Facey said...

Dear Manoj,

Thank you for your comment. You must calibrate the gradient strengths on your spectrometer to know the strength in Tesla/m. One way of doing this is described here:

http://u-of-o-nmr-facility.blogspot.com/2008/03/gradient-calibration-1d-mri.html

Glenn

Anonymous said...

Glenn, hello!

There is an interesting pulse program in the topspin library selnorgzs. It contains adiabatic shape pulse at the end of the sequence. I understand that is for elimination of COSY artifacts in the resulting spectrum. It is very useful for small molecules. Have you ever calibrated this adiabatic pulse.

Glenn Facey said...

Anonymous,
I think you are referring to the selnogpzs sequence. I have not calibrated this the adiabatic pulse but I believe you can do it within the shape tool based on the hard pulse duration and power level.
Glenn

~H2O+ said...

Dear Glenn,

This is a very quick method to extract NOE information in short time.

However, I have several questions about this method:

1. Shall I have to calibrate the shaped pulse power sp1 to perfect phasable 270 deg (signal phased downward) for every sample using pulse program selzg?

2. I have performed an experiment selnogp on (+)-menthol, I noticed that the relative intensity of the positive NOE signals are very small (very poor SNR) compared with the peaks being selective excited while their signal cannot be phase properly (dispersion signal) And I found this seems not the case in your sample. How could I solve this problem?

And I would like to thank you for your work as I am new to NMR, your blog is very useful.

CF

Glenn Facey said...

~H2O+,

In response to your questions...

1. Shall I have to calibrate the shaped pulse power sp1 to perfect phasable 270 deg (signal phased downward) for every sample using pulse program selzg?

If you run the 1D selective gradient NOESY experiment through Bruker's button selective NMR button panel, "butselnmr", then there is a guided automatic shaped pulse calibration routine available. I would encourage you to use "butselnmr". Otherwise the shaped pulse should be calibrated by another means like selzg. I do not find it necessary to calibrate the selective pulse on every sample.

2. I have performed an experiment selnogp on (+)-menthol, I noticed that the relative intensity of the positive NOE signals are very small (very poor SNR) compared with the peaks being selective excited while their signal cannot be phase properly (dispersion signal) And I found this seems not the case in your sample. How could I solve this problem?

The NOE signals are typically a few percent of the selectively excited signal. Apart from distance, the intensity of the transient NOE signal depends on the mixing time used in the sequence (d8). For small molecules, I would start with a mixing time of at least 500 msec (I have sometimes used mixing times of several seconds). I think that you will get the maximum transient NOE signal if you choose d8 to be equal to T1. Your "phasing" problem may be due to non equilibrium conditions. Set d1 to be 5*T1 and use dummy scans. Also you should process your data with about 1 Hz of line broadening. Methyl group singlets are usually the worst for this as they often have very long T1's.

Glenn

Gloria Ivonne Hernández Bolio said...

Hi! Iwould like to know if there is an equivalent button (butselnmr) for Varian software to perform an automatized calibration of the shaped pulse. Thanks in advance.

Glenn Facey said...

Gloria,
This experiment can easily be done on a Varian instrument however I am not able to give you specific instructions.
Glenn