Scheme 2:
Schematics of the proton CE MAS-DNP mechanism in the ideal case of a biradical with two non-overlapping EPR spectra separated by the proton Larmor frequency. (A) Conventional MAS-DNP where the spins of electron b are saturated, while the spins of the electron a are at thermal equilibrium. The proposed optically pumped hyperpolarization method in which the polarization of electron spin a is increased while the electron b is at thermal equilibrium (B) or saturated by microwave irradiation (C).