[Question from NMRWiki Q&A forum] Diagnosing 1D selective NOE issues

[nmrlearner]

Diagnosing 1D selective NOE issues

I'm trying to diagnose a possible issue performing a selective 1D NOE (selnogp) on a Bruker Avance II 500Mhz with TopSpin 2.1.

I have been testing a variety of samples in D20 or D20-H20. I'm observing "strange" looking line shapes, and I was curious if you had any insight on what factors could be at play here. I'm not sure if this is a normal issue due to multiplet structures, or a possible configuration issue with our gradients.

For example, attached are two control experiments (First: 0.1% H20 in 99.9% D20 || Second: 0.1% EtOH in 99.9% D20). For the short (= 1s), it seems like the humps disappear. The literature suggests that I should be able to achieve decent results even with short (~50ms) mixing times.

Do you have any suggestions about this? Below is a list of things I've already tried.

1. Tried autoshim, didn't change much
2. Tried adjusting relaxation delay (d1) from 1 to 5 s, didn't see a big effect
3. Tried lb, but it ended up smearing the humps into smooth but non-symmetric lineshapes
4. Tried using other systems (EtOH in D20, peptides)
5. Tried re-phasing--there's no way to make the "hump" disappear

Below I've got





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