In this proposal we plan to examine pulse polarization schemes to enhance the sensitivity of EPR and specifically electron nuclear double resonance (ENDOR) experiments. This latter technique is fundamental in mechanistic studies of enzyme catalysis, such as in ribonucleotide reductase (RNR), to elucidate the structure of formed radical intermediates obtainable only in very small volumes and concentrations (<< 100 µL and << 100 µM). Currently, these investigations are considerably aggravated by the well-known low sensitivity of pulsed ENDOR experiments, which is about 1 to 2 orders of magnitude smaller than in the respective electron spin echo (ESE) experiment. To address this issue, pulsed polarization schemes derived from nuclear magnetic resonance and dynamic nuclear polarization, such as the cross-polarization (CP) technique, will be extensively tested to optimize coherent polarization transfer between electron and nuclei. In a second step, these schemes will be extended to detect the generated nuclear polarization in an ENDOR-type of experiment that measures the hyperfine spectrum of nuclei interacting with the electron spin. The overall performance (sensitivity and resolution) of a so-called CP-ENDOR experiment will be compared with traditional Davies and Mims ENDOR in applications on real samples, such as in the spectra of the radical intermediates in RNR catalytic cycle.