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titania, where the PO(OH) 2 moiety is known to tightly bind to
ZrO 2 surfaces. While in the bulk diacids, only hydrogen-bonded
homodimers (head-to-head, tail-to-tail) are present, hydrogenbonding
among carboxylic and phosphonic acid groups (headto-tail)
is found in multilayers of the diacids on ZrO 2 nanopowder.
Notably, the monolayers appeared less ordered compared
to respective multilayers, possibly due to a superposition of
intramonolayer and interparticle hydrogen-bonding. 80
4. CONCLUSION
Highly resolved solid-state MAS NMR spectra of supramolecular
organic systems can nowadays be recorded on rather
small amounts (10 - 15 mg) of as-synthesized compounds.
The methodology, however, can be applied in general. For instance,
double-quantum fi ltered heteronuclear correlation experiments
have been used to selectively study interactions of
water molecules and solid microcrystalline proteins, which play
a major role in protein folding, structure, and function. Notably,
the experimental data revealed no evidence for solid-like water
molecules, thus indicating that residence times of solvent molecules
in the proximity of the protein are shorter than required
for DQ creation. 82 Similarly, heteronuclear dipolar couplings can
be exploited for the structure determination of transmembrane
proteins. 83 In the fi eld of metabonomics, proton double-quantum
fi lter have been applied for the quantifi cation of highly relevant
γ-aminobutyric acid (GABA) levels in the human brain. 84
Furthermore, in earlier studies, high resolution DQ MAS NMR
spectroscopy of isotopically labelled systems yielded torsional
angles in peptides 85,86 and thus important information about
the chain conformation. In addition, the secondary structure
and topology of an entire seven-helix receptor protein in a native
membrane environment could be determined via both
homonuclear dipolar correlation and heteronuclear scalar correlation
MAS NMR spectra. 87 High-resolution solid-state NMR
also facilitated an assessment of structural features that defi
ne interfaces among proline-rich motifs that are frequently
involved in signaling events. 88 The sensitivity of 1 H chemical
shifts to ring currents in aromatic systems has been exploited
in determining the packing of porphyrines. 89 The full information
about distances and the relative orientation of specifi c
moieties is available from DQ-NMR spectroscopy on static
samples of specifi cally labeled materials 90 and has provided
important information about the chain conformation and the
packing of synthetic macromolecules. 91
The development of the technique is far from being complete. 55
Major advances are expected in the years to come so that solid
state NMR is envisaged to become a rather indispensable tool
in macromolecular, supramolecular and biological science as
is solution state NMR today.
This work highly benefi ted from fi nancial support of the Deutsche
Forschungsgemeinschaft (DFG) (SFB 625).
72
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