Many thanks to all who answered. I'm used to looking at protein spectra!!
Looking at my spectra more carefully, some of the negative NOEs/ROEs (ie those that are opposite in sign to the diagonal) are due to COSY.
The methyl NOEs, which are positive in both ROESY and NOESY, are true NOE crosspeaks.
It is possible there is a ring that flips... so the negative NOE/ROE crosspeak from one ring to the other could then be result of exchange.
Also, it is observed (and i see it in a NOESY I ran of cyclosporin) that at 800, I could be in the regime where NOEs are positive, but given the rapid rotation of the methyl groups, NOEs to them are negative.
Really what it needs it a more thorough setup and check of mixing times etc.... establishing a conformation for the molecule wasn't the goal- just a quick characterization- but now that the sample has been used for something else the conformation has become more interesting. So time for a new sample and rigorous experiment setup.
Tara
*********************************************
Dr. Tara Sprules
QANUC
Quebec/Eastern Canada High Field NMR Facility
www.nmrlab.mcgill.ca
phone: (514) 398-1721
fax: (514) 398-8254
3420 University St., Rm 023
McGill University
Montreal, QC, H3A 2A7
*********************************************
________________________________
From: Tara Sprules, Dr. <tara.sprules_at_mcgill.ca>
Sent: Friday, June 21, 2019 12:51 PM
To: ammrl
Subject: AMMRL: small molecule noesy/roesy at 800
Hi all,
I'm trying to understand some NOESY and ROESY data on a small molecule. Mostly I just run the spectra and hand them off...
I've gone back and looked at data for a few samples and I see positive NOE/ROE (ie same sign as the diagonal) peaks for some smallish molecules I've run at 800. Most of the crosspeaks are same sign as the diagonal. But crosspeaks with methyls are negative (opposite from diagonal).
Is this some effect of being at such high field, and looking at some handy plots I've found, I could be approaching zero NOE, depending on the molecule and mixing time?
I will run at lower field, but while I wait for the sample back and time thought I'd check with the collective wisdom out there. (concerns with overlap and low concentrations are why data was on the 800 with cryoprobe).
Thanks for any input!
Tara
*********************************************
Dr. Tara Sprules
QANUC
Quebec/Eastern Canada High Field NMR Facility
www.nmrlab.mcgill.ca
phone: (514) 398-1721
fax: (514) 398-8254
3420 University St., Rm 023
McGill University
Montreal, QC, H3A 2A7
*********************************************
Received on Fri Jun 21 2019 - 10:32:45 MST