Hello,
A student here is using our Bruker DRX-500 instrument to run 1D ROE
experiments to study the binding of a host compound to cyclodextrin. This
is a very common experiment for these systems. The pulse sequence uses a
shaped pulse to selectively invert the desired resonance and gradient pulses
to avoid having to take a difference spectrum (Bruker pulse program,
selrogp.2) It worked fine for the first sample, but for a second sample
involving a modified cyclodextrin, we can not selectively irradiate the
desired resonances.
The problem is the two resonances located at 7.1 and 7.3 ppm. For example,
if I set O1 = 3565 Hz and SPOFF= 0 Hz, I expect the peak at 3565 Hz (7.1
ppm) to be inverted. Instead the peak at 3605 Hz (7.2 ppm) is inverted and
the peak at 3565 Hz is positive. I don't think this is a phasing problem
because of the correct sign of the other roe signals in the spectrum. In
another experiment, I set O1 = 3605 Hz and SPOFF= 0 Hz, and the peak at 3605
Hz is inverted along with half of the doublet at 3565 Hz. I have found the
same problems using a simple one pulse selective experiment. Again the
experiment works for a different sample and for resonances located in
different areas of the spectrum.
Note that the peaks at 7.1 and 7.3 ppm are closer together (30 Hz
difference) than the corresponding peaks in the previous sample (85 Hz
difference). However, I did change the shaped pulse length and strength to
irradiate a smaller frequency range for this new sample. I can also invert
both peaks if I make the length shorter (frequency range larger).
We would appreciate any ideas as to what is causing this problem.
Thanks,
Stephanie Mabry
--------------------------------------------
Stephanie Mabry, Ph.D., Instrument Manager
University of Maryland, Baltimore County
Dept. of Chemistry and Biochemistry
1000 Hilltop Circle
Baltimore, MD 21250
office (410)455-1031
fax (410)455-2608
mabry_at_umbc.edu
Received on Tue Dec 04 2001 - 17:46:10 MST