Chemical Exchange Saturation Transfer (CEST) is a technique where one resonance, in
slow exchange with a second resonance, is saturated with a selective low power pulse followed by a hard non-selective 90° pulse. The intensity of the second resonance is then diminished due to the transfer of saturation from the first resonance as the result of
chemical exchange. The figure below demonstrates this for a 25 mM solution of salicylic acid in H
2O/D
2O buffered at pH 7.
The left-hand panel of the figure is a stacked plot of extracted spectra collected in a pseudo 2D acquisition as a function of saturation frequency. The saturation frequency was varied from an initial value of 20 ppm to a final value of -20 ppm in steps of 0.2 ppm. The spectra are plotted such that only the water resonance is on scale. One can see that the intensity of the water resonance dips when a saturation frequency of ~14 ppm is applied, corresponding to the resonance frequency of the –COOH and –OH protons of the salicylic acid (which appear to be in
fast or intermediate exchange with one another). The water resonance of course also dips to zero when a saturation frequency of ~4.7 ppm is used, corresponding to a simple
presaturation of the water. The right-hand panel of the figure is a plot of the integral of the water resonance as a function of saturation frequency, showing again a dip at ~14 ppm.
CEST is used in
MRI to provide image contrast where a chemical exchange agent is introduced and images are collected with and without saturation of the exchange agent. The difference provides an image enhanced by the presence of the chemical exchange agent.
Thank you to
Dr. Mojmir Suchy of
Prof. Adam Shuhendler’s group at the
University of Ottawa for arousing my interest in the use of CEST for MRI and preparing the sample used in this post.
11 comments:
So I guess one could use this in order to see non-exchangeable protons below the water signal. If one presaturates the water line then these other signals are also killed,. But if the presaturation is done on one of the exchangeable protons then the main water peak will also get suppressed but the signals underneath it not. I am trying to think of a nice sample one could use to demonstrate this...
Anonymous,
Yes, you are correct however I suspect it may not be possible to suppress the water signal enough by saturating the other exchanging signal even at very long saturation times. The water signal would decrease to a plateau - not to zero. See this link.
http://u-of-o-nmr-facility.blogspot.ca/2011/08/saturation-transfer-and-exchange.html
Glenn
Thank you for the nice explanation and illustration of CEST. In a CEST experiment, do you vary the frequency of the hard pulse too or only the saturation pulse? what are typical saturation pulse durations?
Anonymous,
The frequency of the hard pulse is constant while the frequency of the saturation pulse is varied. Typical saturation times are on the order of seconds.
Glenn
HI Glen,
I am new to CEST. Is the long saturation pulse a single one or made of several smaller ones? Which shape is preferred for saturation? If it is made of smaller pulses, then what is the width of each pulse?
Arindam
Arindam,
I used a single long low power rectangular pulse for saturation.
Glenn
Dear Glenn,
Thanks for explaining nicely, if possible could you please share the pulse program used for this experiment.
Could you also comment on which time scales this experiment would be helpful.
thanks in advance,
Anonymous,
Since my retirement, I no longer have access to the pulse programs I have modified over the years but this is just a simple presaturation sequence with a variable frequency list for the presaturation pulse.
The time scales over which the experiment is sensitive are those for fast or intermediate exchange on the NMR time scale as divined by the reciprocal frequency difference between the water and the resonances of interest. See this link regarding the NMR time scale http://u-of-o-nmr-facility.blogspot.com/2008/08/nmr-time-scale.html .
Glenn
That last comment should read:
“ The time scales over which the experiment is sensitive are those for slow or intermediate exchange on the NMR time scale as divined by the reciprocal frequency difference between the water and the resonances of interest. See this link regarding the NMR time scale http://u-of-o-nmr-facility.blogspot.com/2008/08/nmr-time-scale.html .”
Glenn
Hi Glenn,
In several places I have seen that CEST is typically used with protons that are "slowly exchanging." However, if I have two states that are rapidly equilibrating ("fast exchange") but I know the true chemical shifts of one of those states, can I presaturate using that true chemical shift value and observe how the signal average responds?
Thanks
Anonymous,
If the exchange is fast such that there is sharp averaged resonance, irradiating at the frequency (which would be that in the absence of exchange) will not affect the averaged resonance.
Glenn
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