Dynamic Nuclear Polarization with a Rigid Biradical

摘要

Dynamic nuclear polarization (DNP) is an approach that can enhance NMR signal intensities of solids and liquids by two to three orders of magnitude. During a DNP experiment, the large Boltzmann polarization of an exogenous or endogenous paramagnetic species, such as a stable free radical, is transferred to the nuclei of interest by microwave (mw) irradiation of the sample at the electron paramagnetic resonance (EPR) frequency. The maximum theoretical enhancement possible is given by the ratio γ_s/γ_I, in which γ_s and γ_1 are the gyromagnetic ratios of the electron and the nucleus, respectively. Enhanced nuclear polarization is of considerable interest in a variety of applications ranging from particle physics to structural biology and clinical imaging. The mechanism that dominates the electron-nucleus polarization transfer depends on the relative sizes of the homogeneous line width, δ, and inhomogeneous breadth, A, of the EPR spectrum of the paramagnetic polarizing agent and the nuclear Larmor frequency, ω_(0I). When δ⊿ < ω_(01), DNP is dominated by the solid effect (SE), whereas the cross-effect (CE) is operative when ⊿ > ω_(0I) > δ.