Dynamic nuclear polarization (DNP) is presented as an applicaformed with insoluble (16) or soluble free radicals (5,6), tion for EPR spectroscopy in low magnetic field (below 40 G). but the most widely used free radicals are nitroxides dis-An updated theory of DNP allows the calculation of the maximum solved in aqueous solutions. They are stable free radicals DNP factor for all EPR transitions of 14 N or 15 N nitroxides. The with one unpaired electron (S Γ
1
2 ) coupled by a scalar DNP factor is considered as a function of the magnetic field and interaction to a nitrogen nucleus (spin K Γ
1 for 14 N and as a function of the direction of the applied RF field. The equations spin K Γ
1 2 for 15 N). Similar to in vivo EPR experiments, make it possible to calculate the theoretical DNP spectra of ni-DNP biological applications could be considered only if the troxide which is essential for the interpretation of the DNP spectra RF field used for the EPR saturation is weak enough to of nitroxides in a very low magnetic field. The EPR spectra obtained by field-cycling dynamic nuclear polarization with an RF allow good penetration of the electromagnetic waves in vivo field for the EPR irradiation at 64, 72, and 74 MHz for the without overheating the sample. Therefore, the RF frequency TXO( 14 N), and 62 and 66 MHz for the PCD( 15 N) nitroxides are must be below 200 MHz, making the use of a weak magnetic presented.
The RF field for EPR saturation was applied perpendicfield necessary. Classical EPR spectroscopy at X Band (9.5 ular to the static magnetic field in order to induce p transitions, GHz) shows nitroxide spectra containing three lines split by except for the 64 and 74 MHz spectra of the TXO( 14 N) nitroxide, hyperfine coupling. L Band EPR spectroscopy (17) allows in which the RF field was applied parallel to the static magnetic the detection of nitroxide spectrum in a magnetic field of field in order to induce s transitions.