I need to measure the 4-bond coupling into a group of unassigned quaternary carbons. I would like to use the 1,n-ADEQUATE but it is commonly displayed as a DQ format which is not comparable to the standard 2D axis.
I read about a refocused version of the 1,1-ADEQUATE. This refocusing makes it possible to directly compare the spectra of traditional non-DQ formats so I was wondering if the same refocusing period could be used in the 1,n experiment?
If not, how do you optimize the indirect DQ formated axis (in Bruker SW and O2P) of the 1,n-ADEQUATE experiment if you know the expected range of your signals from a standard HMBC?
asked Jul 29 '11 at 08:05
In case you've not gotten an answer, I can offer the following:
The version of the 1,1-ADEQUATE experiment in the Bruker pulse sequence library (versions of TopSpin 2.1 and 3.0.x at least) is refocused so that responses are observed in the F1 frequency domain at the 13C chemical shift of the carbon that is coupled (via 1Jcc or nJcc) to the proton-carbon pair in question at the intrinsic 13C chemical shift of the coupled carbon. The Bruker pulse sequence name is: adeq11etgprdsp. There is a review published late last year (G.E. Martin, "Using 1,1- and 1,n-ADEQUATE 2D NMR Data in Structure Elucidation Protocols," Ann. Rep. NMR Spectrosc., G.A. Webb, Ed., vol. 74, 2011, pp 215-291 and several papers that contain 1,n-ADEQUATE spectra: see S. W. Myer and M. Kock, J. Nat. Prod., 71, 1524 (2008); G. E. Martin, B.D. Hilton, and K.A. Blinov, Magn. Reson. Chem., 49, 641 (2011); G.E. Martin, B.D. Hilton, and K.A. Blinov,* J. Nat. Prod.*, 74, 2400 (2011)). Fundamentally, you can use the 1,1-ADEQUATE pulse sequence and simply optimize the delay for the 1Jcc coupling for the nJcc coupling that you want to observe. I've found that optimization in the range of 5-7 Hz works best for strychnine and retrorsine (see the two papers cited). You may also want to consider using generalized indirect covariance processing as a means of improving the sensitivity of your data once you have it in hand. I would recommend coprocessing the 1,n-ADEQUATE spectrum with a multiplicity-edited GHSQC spectrum that was acquired with high F1 digital resolution (e.g. ni=512). You should use a 3 sec d1 delay. I would recommend starting with ni=128 and probably ns(nt)=128 for a 5-10 mg sample using a 500 MHz instrument equipped with an inverse cryoprobe. Strychnine is a good model compound to start with -- there are data in the published literature that you can compare to.
I hope that the above is helpful.
answered Feb 26 '12 at 14:27