Wei Li 448-7532 wrote:
>
> Hi All,
>
> I got a silly question. Someone here wants his students to do direct
> detection of natural abundant nitrogens on their small molecules. I never
> did that before. We have an ARX-300 with an inverse broadband probe. I am
> not sure if we can detect nitrogen directly since the natural abundance is
> so small. Has anyone had experience to observe natural abundant nitrogens
> directly before?
Over the years we have done a fair bit of nitrogen-15 NMR at natural
abundance, but there are a number of things to look out for. First
of all, as you indicate, is the problem of sensitivity, so you
want to use as concentrated of a solution as possible. Second, 15N
has a negative magnetogyric ratio, and thus if you decouple any
directly attached (1J N-H is typically 90 Hz, at least for amides)
or otherwise near protons, you can end up with enhanced signals,
reduced signals, or no signal at all:). Nitrogens with directly
attached protons in small molecules are typically enhanced by a factor
of around four however. Finally, T1 for 15N can be quite long, esp.
for non protonated N, so use a relaxation delay of two to several seconds,
with 30-45 degree 15N pulses.
With a 10 mm nornal (1H coil outside) BB probe with 1H decoupling
we see signals in an hour to several hours in reasonably concentrated
(say 250 mM, roughly) samples on a 400. The inverse probe will be a disadvantage
in this regard however, so use a standard BB probe if available.
Good luck with your experiments.
Deane D McIntyre,
NMR Manager,
Dept. Biological Sciences, University of Calgary
2500 University Dr NW Calgary Alberta CANADA T2N 1N4
Phone 403-220-4193
Deane_at_deane.bio.ucalgary.ca
http://deane.bio.ucalgary.ca/
Received on Tue Jun 27 2000 - 17:41:31 MST