Dear All
For those that may be interested in the topic, here is a summary of the
replies to my recent question on HOESY mixing times. Many thanks to
Richard, Jerry, Chris, Tim, Charles, Peter.
Craig.
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It’s difficult to have such guidelines, since the optimal mixing time
depends so much on competing relaxation pathways. 195Pt, for example,
often has a very short T1 because of the strong CSA contribution to the
195Pt relaxation. Therefore, the single-quantum relaxation pathway
often can override the 2Q or zeroQ pathways, so you will have a
difficult time observing the HOE in a transient experiment with longer
mixing times. Have you measured the T1 of your 195Pt, as that always
limits the maximum mixing time.
Have you tried doing a steady-state NOE-difference using selective
excitation of the nearest 1H, and looking directly at the enhancement of
the 195Pt? I have done this to get a limiting value for the amount of
NOE transfer I can expect in a transient experiment.
Just a few thoughts, as I have done this w/ 195Pt with limited
success.
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The NOE generally develops at the rate of 1/T1 of the resonance that is
experiencing the saturation or inversion. The NOE then goes away at the
rate of 1/T1 of the resonance that is experiencing the enhancement (
positive or negative ). So in any NOE experiment ( Noesy or Hoesy ) it
is important to make the mixing period a significant portion of T1 of
the inverted resonance ( e.g. in a proton-metal experiment you would
want to adjust the mixing time to the proton T1 ). Then you need to
adjust the recycle rate of the 2D experiment to allow the metal to
recover and also allow interproton effects to die away. You want to
avoid conditions where the protons are so uniformly saturated that your
measured Hoesy is the result of spin diffusion among several protons.
You also need to allow the metal resonance an opportunity to recover or
you will loose valuable s/n.
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We set/estimate the mixing time equal to the T1 relaxation time of
either 1H or the other nuclei for small molecules. What is the T1 of
195Pt in your sample? 1H T1?
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In general NOE mixing times are dictated by the T1 of the nuclei
involved, so for example, for most 1H-1H NOE experiments on small
molecules it is common to using mixing times in the region of 0.5-1 s as
these correlate with typical proton T1s.
For 195Pt the HOESY experiment may not work too well as the dipolar
relaxation mechanism required for the NOE may not be the dominant one-
for Pt chemical shift anisotropy can be strong and may be the dominant
relaxation process (especially at higher field where CSA is more
pronounced). I haven’t looked to see if 195Pt HOESY has ever been
reported though, but I would not be surprised if this were not
successful!? Likewise, NOEs to most quadrupolar nuclei do not work with
HOESY as dipolar relaxation does not dominate (6/7Li is one exception
where quadrupolar relaxation can be rather inefficient).
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I have a tendency to describe all noe experiments as limited by
spin-lattice relaxation; simplistic, but useful as a general
rule-of-thumb. Take the shortest T1 of all species involved in the
particular noe one is measuring, and typically will find that the
enhancement maximizes somewhere close to 0.6*T1. So I use that value as
a starting place.
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Pt-195 usually has enormous chemical shift anisotropy. As a result,
dipole-dipole relaxation by interaction with neighboring protons does
not contribute to relaxation. Since the NOE is related to the fraction
of relaxation by dipole-dipole interaction with protonstypically small to nonexistent.
Unfortunately, you picked the worse system to detect HOESY
cross-peaks.
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Received on Fri Apr 01 2011 - 05:22:26 MST