But there is a more fundamental difference. Crystallography determines the
positions (more or less accurate) whereas NMR measures distances between
certain atoms. NMR is local, 1-dimensinal information that -with enough data-
can be used to generate a three dimensional model compatible with it,
crystallography delivers a thre dimensional representation (e-density) that
you can (but don't have to!) interprete in terms of a model.
Case in point (with regard to the original question): I belive to remeber some
discussion about the 'bent structure' of DNA determined with NMR that turned
out to be an artifact due to the lack of long range infortmation to
restrain/determine the 'straightness' of a helix.
btw. 'NMR at atomic resolution' should better be called 'NMR at interatomic
resolution' as it never resolves an atom
thatthat On Friday 14 November 2008 02:36:13 Mark J. van Raaij wrote:
> although I agree with the statement below, I would say that X-ray
> crystallography also only generates a molecular model (not "a
> structure"). Rather than a structure, X-ray crystallography generates
> an electron density map. The quality of the model based in it depends
> on the quality of this electron density map (esp. resolution) and the
> quality of the researchers interpretation of it.
> Mark
>
> >>> As a physical principle, spectroscopic
> >>> methods do not deliver atomic resolution structures, but a set of
> >>> inferences that may or may no be compatible with a molecular model.
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