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Fig. 5 13 C CP MAS spectra
of a silica framework with
MW irradiation at 263 GHz to
induce DNP. The figures
compare the best
enhancements observed using
TEMPO and TOTAPOL
radicals. It should be noted
that significant DNP
enhancement of the alkyl
moiety of the surface ethoxy
groups was also observed.
Reproduced with permission
from [ 48 ]
yield a 50-fold signal enhancement on 13 C signals for surface species covalently
incorporated into a silica framework (Fig. 5 ). Lelli et al . [ 49 ] have applied the
technique to study the distribution of surface bonding modes and interaction of
functionalized silica materials by observing the 29 Si signals directly (Fig. 6 ). The
remarkable gain in sensitivity and time provided by surface-enhanced silicon-29
DNP NMR spectroscopy is on the order of a factor of 400. This has also allowed the
acquisition of the previously inconceivable two-dimensional correlation spectra,
enabling more detailed characterizations of these functionalized surfaces [ 48 , 49 ].
The unprecedented sensitivity observed in DNP-NMR means that sufficient
signals can be obtained from a single scan, allowing reactions even far from
equilibrium to be studied in real time. Of great importance, the DNP-NMR method
is compatible with quantitative rate determination experiments where a single spin
in the reactant is labeled in its spin state by a selective radio frequency pulse for
subsequent tracking through the reaction, allowing the unambiguous identification
of its position in the product molecule. Zeng et al . [ 50 ] have demonstrated the
application of DNP-NMR to the Diels-Alder reaction of 1,4-diphenylbutadiene
with 4-phenyl-1,2,4-triazole-3,5-dione, where reaction rates could be obtained
accurately and reproducibly. In particular, the high chemical shift specificity
afforded by high-resolution NMR permitted the simultaneous determination of
reaction rates and mechanistic information in one experiment. Similarly, time-
resolved DNP-enhanced NMR has been used to study the enzymatic reaction of
the conversion of N -benzoyl- L -arginine-ethyl ester into the product N -benzoyl-
L -arginine by trypsin enzyme (Fig. 7 )[ 51 ].
The control of circulating drug concentrations by therapeutic drug monitoring
(TDM) is vital for drugs when under- or overdosing may lead to loss of therapeutic
efficacy or to adverse effects. Effective TDM depends on effective analytical
platforms for the fast detection, identification, and quantification of circulating
drugs with a narrow therapeutic range. As a result of the low concentrations of
drugs and their metabolites in blood plasma, analytical tools are needed to provide
high sensitivity and specificity. NMR spectroscopy is quantitative if care is taken
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