Hello all,
I am running NMR for an aqueous peptide sample in 90% H2O/ 10% D2O. The 1D 1H spectrum collected from a WATERGATE 3-9-19 sequence looked great, with the intensity of peptide peaks being comparable to that of the solvent peak.
However, when I ran 2D experiments such as COSY and TOCSY after copying the acquisition parameters from the 1D 1H experiment, I got much worse spectrum, with peptide cross peaks much less intense than solvent peaks.
Do you know the possible reason and how can I improve the 2D spectra?
Thanks,
Dear Yun Shi,
I only can speculate: with the sensitivity enhanced TOCSY you have to be aware that for a small molecule like your peptide you will (dependent on the duration of mixing) experience substantial loss of signal due to diffusion between de- and rephasing gradient. The sequence actually acts like a DOSY. For small molecules we never use enhancement.
In general I would say that you are a lucky man...to me it looks as if you have all expected cross-signals in the TOCSY you show :-)
Cheers
Alfred
As a solution of your problem I would suggest you try "excitation sculpting" as water suppression method. It's one of the most robust and simple to setup methods if your prosol table filled properly. It's provide excellent results in most cases. You can try pulse programs from standard Bruker library: zgesgp, cosydfesgpph, dipsi2esgpph, noesyesgpph for 1D, COSY, TOCSY and NOESY experiments respectively. Also you can try WET water suppression method highly recomended by Bruker.
Best regards.
Hi all,
I tried a higher concentration sample running with "excitation sculpting".
In my case, the solvent suppression from excitation sculpting is comparable to that from watergate, but the solvent peak in excitation sculpting can be phased, resulting in much less dispersion (narrower peak) in both 1D and 2D spectra.
Only when I increase the P16 parameter in the gradient channel (from 1 ms to e.g. 4 ms) did I obtain a better suppression for excitation sculpting.
However, the major problem now is that all my cross peaks are in "dispersion-mode", even though the pulse programs (e.g. cosydfesgpph) are "phase sensitive".
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