Dear Qi,
15 N NMR of aliphatic amines is hard to measure with either direct or indirect
methods. Amine protons easily exchange with other labile protons. DMSO and
Acetone almost always contain some water.
The next problem comes if your NH signals are broad you might not be able to
transfer the magnetization to 15N and back in HSQC or HMBC. In simple words
the same proton has to stay on the same nitrogen for the entire pulse sequence.
If it exchanges or undergoes some other dynamics it will not show up.
Last issue is the coupling constants as pointed out by Gerd 2J and 3J(N,H)
couplings can be small, some very small like 1Hz or even less. This means that
your delay in the HMBC now is 0.5 seconds or more.
THe INEPT is going to have the same issue if the signals from which you start
the transfer is broad.
This would only leave direct observe. Which as also mentioned is a time consuming
exercise. The standard sample for signal to noise testing is 90% formamide. From
this you can extrapolate the times it will take to collect data. Additonally 15N
can have very long relaxation times. And to make matters worse due to the negative
gamma the NOE is also negative. This means decoupling can make your signal disappear
if you have partial or small NOE buildup.
For good quantitative 15N data you need tu use inverse gated decoupling and long
relaxation delays.
Sorry to be the bearer of such bad news but that is life.
With best regards
Clemens
On 4/2/2025 11:11 AM, Qi Wu via groups.io wrote:
Hi Dr. Gemmecker,
I have some related questions on 15N. I have Bruker and PA BBO probe (with X
on the inner coil I assume). I'm looking to quantify levels of primary, secondary
and tertiary amine for polyamine （aliphatic） and its reaction
products. I've tried the standard 1H-15N 2D NMR experiments, but the HSQC didn't
work well, and got some data from HMBC.
Do you have any suggestions for 2D to assign the various amines (Bruker pre
set experiments, pulse programs, parameters)?
Also if 2D is not quantitative, is it reliable to quantify 1, 2 amines using 1H
with the labile Hs, and using solvents such as DMSO and acetone?
Thank you and best regards,
Qi
--
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Clemens Anklin Ph.D. Vice President
Bruker BioSpin NMR Applications & Training
15 Fortune Drive NEW Phone: 978-313-5NMR(5667)
Billerica MA 01821 bluesky: _at_canklin.bsky.social
web: http://www.bruker.com
e-mail: clemens.anklin_at_bruker.com
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Received on Wed Apr 02 2025 - 17:24:35 MST