Chemistry Reference
In-Depth Information
2.3.1 Polarizing Agents for the SE
The SE polarizing mechanism requires that the zero and double quantum forbidden
transitions are not simultaneously excited and that the enhancement effect is
without positive and negative transitions cancellation. To date, two radicals,
BDPA and trityl, can satisfy these requirements at high magnetic fields [
35
,
36
].
BDPA is the first polarizing agent applied to investigate the SE mechanism in
polystyrene matrix, which reveals an inhomogeneous linewidth of
~ 20 MHz at a
211-MHz machine [
37
-
39
]. However, BDPA is insoluble in aqueous solution,
making it very difficult to use in biological science. In contrast, the trityl has
good solubility in water and a narrow EPR spectrum width; it has therefore
been successfully used in aqueous solution DNP applications with an Oxford
HyperSense Oxford DNP polarizer [
40
].
O
2.3.2 Polarizing Agents for TM and CE
As mention above, under the situation that
O > o
0
N
, the TM and CE mechanisms
are the main polarizing mechanisms in DNP experiments. The homogeneously
broadened EPR leads to TM while the inhomogeneously broadened EPR results
in CE. Nitroxide based radicals and biradicals are suitable polarizing agents for TM
and CE because their EPR inhomogeneous line widths are about 600MHz at a
1
H
Larmor frequency of 211-MHz. Furthermore, these chemical compounds, which
have an EPR spectrum consisting of two narrow sharp lines separated by
o
0
N
, are
ideal polarizing agents for CE polarization.
3
Instrumentation
The instrumentation for DNP experiments has enjoyed great developments in
recent years. Generally speaking, on top of an existing NMR system, a microwave
source, a waveguide, and a new probe are extra instrumentation required for
performing DNP experiments. The microwave source is used to create the micro-
wave and the waveguide will transmit the microwaves from the source to the NMR
probe. The new probe is designed with the additional function that it can irradiate
the sample with microwaves. In addition, there are some other requirements for
special cases, such as performing MAS below 90 K, irradiating the EPR spectrum
with maximum enhancement, and so on.
3.1 Microwave Sources
The power of microwave sources for DNP depends strongly on the nature of
the experiment to be performed. The field strength of the microwave
o
1
E
has
