4 research outputs found
Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy
Dynamic
nuclear polarization (DNP) is shown to greatly improve
the solid-state nuclear magnetic resonance (SSNMR) analysis of synthetic
polymers by allowing structural assignment of intrinsically diluted
NMR signals, which are typically not detected in conventional SSNMR.
Specifically, SSNMR and DNP SSNMR were comparatively used to study
functional polymers for which precise structural elucidation of chain
ends is essential to control their reactivity and to eventually obtain
advanced polymeric materials of complex architecture. Results show
that the polymer chain-end signals, while hardly observable in conventional
SSNMR, could be clearly identified in the DNP SSNMR spectrum owing
to the increase in sensitivity afforded by the DNP setup (a factor
ā¼10 was achieved here), hence providing access to detailed
structural characterization within realistic experimental times. This
sizable
gain in sensitivity opens new avenues for the characterization of
āsmartā functional polymeric materials and new analytical
perspectives in polymer science
Optimizing Sample Preparation Methods for Dynamic Nuclear Polarization Solid-state NMR of Synthetic Polymers
This work compares the overall sensitivity
enhancements provided
by dynamic nuclear polarization (DNP) for the solid-state NMR characterization
of polymer samples doped with biradicals and prepared either by <i>film casting</i> (FC), or by <i>glass forming</i> (GF)
using 1,1,2,2-tetrachloroethane as the solvent. Analysis of amorphous
and semicrystalline polymers (polystyrene, polyĀ(ethylene oxide), polylactide,
polyĀ(methyl methacrylate)) of varying molecular weights showed that
GF provided larger sensitivity enhancements than FC but yielded DNP-enhanced <sup>13</sup>C CPMAS spectra of lower resolution for semicrystalline polymers,
owing to line-broadening due to conformational distribution of the
polymer chains in frozen solution. Moreover, use of deuterated solvents
significantly reduced the intensity of the solvent signals in the
DNP-enhanced <sup>13</sup>C CPMAS spectra of polymers prepared by
GF, while preserving the sensitivity enhancement observed for the
polymer signals. For the polymers investigated here, both FC and GF
performed better than <i>incipient wetness impregnation</i>, yielding overall sensitivity enhancements between 5 and 40
Spin Exchange Monitoring of the Strong Positive Homotropic Allosteric Binding of a Tetraradical by a Synthetic Receptor in Water
The
flexible tetranitroxide <b>4T</b> has been prepared and
was shown to exhibit a nine line EPR spectrum in water, characteristic
of significant through space spin exchange (<i>J</i><sub><i>ij</i></sub>) between four electron spins interacting
with four nitrogen nuclei (<i>J</i><sub><i>ij</i></sub> ā« <i>a</i><sub>N</sub>). Addition of CB[8]
to <b>4T</b> decreases dramatically all the <i>J</i><sub><i>ij</i></sub> couplings, and the nine line spectrum
is replaced by the characteristic three line spectrum of a mononitroxide.
The supramolecular association between <b>4T</b> and CB[8] involves
a highly cooperative asymmetric complexation by two CB[8] (<i>K</i><sub>1</sub> = 4027 M<sup>ā1</sup>; <i>K</i><sub>2</sub> = 202āÆ800 M<sup>ā1</sup>; Ī± = 201)
leading to a rigid complex with remote nitroxide moieties. The remarkable
enhancement for the affinity of the second CB[8] corresponds to an
allosteric interaction energy of ā13 kJ mol<sup>ā1</sup>, which is comparable to that of the binding of oxygen by hemoglobin.
These results are confirmed by competition and reduction experiments,
DFT and molecular dynamics calculations, mass spectrometry, and liquid
state NMR of the corresponding reduced complex bearing hydroxylamine
moieties. This study shows that suitably designed molecules can generate
allosteric complexation with CB[8]. The molecule must (i) carry several
recognizable groups for CB[8] and (ii) be folded so that the first
binding event <i>reorganizes</i> the molecule (unfold) for
a better subsequent recognition. The presence of accessible protonable
amines and H-bond donors to fit with the second point are also further
stabilizing groups of CB[8] complexation. In these conditions, the
spin exchange coupling between four radicals has been efficiently
and finely tuned and the resulting allosteric complexation induced
a dramatic stabilization enhancement of the included paramagnetic
moieties in highly reducing conditions through the formation of the
supramolecular <b>4T</b>@CBĀ[8]<sub>2</sub> complex
Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K
A series of seven functionalized
nitroxide biradicals (the bTbK
biradical and six derivatives) are investigated as exogenous polarization
sources for dynamic nuclear polarization (DNP) solid-state NMR at
9.4 T and with ca. 100 K sample temperatures. The impact of electron
relaxation times on the DNP enhancement (Īµ) is examined, and
we observe that longer inversion recovery and phase memory relaxation
times provide larger Īµ. All radicals are tested in both bulk
1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and
the difference in Īµ between the two cases is discussed. The
impact of the sample temperature and magic angle spinning frequency
on Īµ is investigated for several radicals each characterized
by a range of electron relaxation times. In particular, TEKPol, a
bulky derivative of bTbK with a molecular weight of 905 gĀ·mol<sup>ā1</sup>, is presented. Its high-saturation factor makes it
a very efficient polarizing agent for DNP, yielding unprecedented
proton enhancements of over 200 in both bulk and materials samples
at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of
33 at 180 K and 12 at 200 K, suggesting that with the continued improvement
of radicals large Īµ may be obtained at higher temperatures