Department of Chemistry, University of Alberta

December 1998

NMR News 98-09
 News and tips for users of the Varian NMR systems in the Department.
Editor: Albin.Otter@ualberta.ca     http://nmr.chem.ualberta.ca

There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.
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Contents

FAQ 98-09.1 GCOSY and GTOCSY: what's the difference?

 

FAQ 98-09.1 GCOSY and GTOCSY: what's the difference?
Both techniques belong into the category of homonuclear 2D techniques as only protons are involved. The correlations (= off-diagonal peaks) are created in both cases by couplings between protons (not by close proximity in space which is common to NOESY, (T)ROESY).
The main difference is that, when analyzing the data, the GCOSY requires the crossing of the diagonal which is equivalent to the 1D spectrum. This means that if two peaks overlap in the 1D they will also overlap on the diagonal and the assignment becomes ambiguous. In GTOCSY on the other hand, this is not required. As long as there is an uninterrupted chain of couplings (NOT long range, just an uninterrupted sequence of Js) the information will be visible in off-diagonal peaks as shown in the example below. GTOCSY works particularly well for molecules which have oligomer or polymer character. Perfect examples are peptides/proteins and carbohydrates. It is comparable to chromatography: the spin systems of the individual building blocks of the peptide or the carbohydrate are separated as shown here for a trisaccharide:

  
GCOSY-GTOCSY.gif (33552 bytes) < this image contains links

 

 

 

 

 

 

 

 

 

< this image contains links
GTOCSY is as easy to run as GCOSY within the EZ NMR system. However, there is one parameter which needs some adjustment: the length of the spin locking time controlled by the parameter mix. The value of mix depends on the size of the coupling constants and how far correlations are desired in a spin system. At very short mixing times a GTOCSY is essentially no different than a GCOSY. With increasing mix correlations further and further away will be observed. A detailed help file dealing with the the selection of mix and GTOCSY in general has been added to the system and is available here.

References for GTOCSY experiment

Gradient-enhanced spectroscopy
R.E. Hurd, Journal of Magnetic Resonance 87, 422-428 (1990)

Field gradient techniques in NMR spectroscopy
L.E. Kay, Current Opinion in Structural Biology 5, 674-681 (1995)

Gradient-enhanced homonuclear 2D NMR techniques applied to oligosaccharides containing manno-hexoses provide improved
correlations for protons coupled by small 3J
A. Otter, O. Hindsgaul and D.R. Bundle, Carbohydrate Research 275, 381-389 (1995)

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