That's exactly right. J(w) should decrease with increasing frequency
w. If you have no chemical exchange then variability in J(0) greater
than the noise is saying something about the data. However a squared
dependence with field strength in J(0) is an indication of fast
chemical exchange. For more info have a look at the Farrow et al.,
1995 reduced spectral density mapping reference. The Butterwick et
al., 2004 reference could also be useful, although that one requires
the collection of the longitudinal and transverse cross-correlated
cross-relaxation rates.
Essentially relax doesn't currently implement these techniques.
However they could be implemented within a relax script using relax's
data analysis tools and functions to make life easier. If the script
is clean and meets relax's coding conventions, it could be
incorporated into relax.
Cheers,
Edward
On 12/13/06, Chris MacRaild <[EMAIL PROTECTED]> wrote:
Hi Sébastien,
The field dependence you note is expected for J(wN) and J(wH), because
the frequencies wN and wH are of course field dependent, and J(w)
decreases with increasing frequency. Field dependence in J(0) is not
expected, however. In fact I often use the consistency of J(0) across
datasets as a check on the quality of the data - any variation in J(0)
across datasets, irrespective of field, is an indication of
inconsistencies in the data.
If you are seeing inconsistency in J(0), then there are any number of
possible problems that might contribute. Changes in sample temperature
in the various magnets and sample instability (aggregation or
proteolytic degredation, eg) both tend to affect J(0) by changing the
apparent rotational diffusion of the molecule; imperfect water
supression causes errors in relaxation measurements, etc ...
Hope this helps,
Chris
On Tue, 2006-12-12 at 11:34 -0500, Sebastien Morin wrote:
> Hi everyone
>
> I got a question concerning the treatment of Rex in reduced spectral
> density mapping as implemented in relax...
>
> My data include R1, R2 and {1H}15N-NOE at three magnetic fields (500,
> 600 and 800 MHz). When proceeding to the reduced spectral density
> mapping, I obtain J at three values (0, wN and wH). However, a slight
> trend is observable between those extracted values, i.e. the values from
> data at 500 MHz are higher than those at 600, which are higher than
> those at 800...
>
> Is it normal ?
>
> Is there a way to extract the contribution from Rex (conformational
> exchange, slow us-ms timescale motions) from my data using reduced
> spectral density mapping at those three fields ?
>
> Thanks !!
>
>
> Sébastien :)
>
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