Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Dynamic Nuclear Polarization (DNP)

Background:

• Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP) significantly enhances NMR sensitivity by increasing nuclear polarization.
• Exogenous nitroxide biradicals (e.g., TOTAPOL, AMUPOL) are crucial for proton polarization enhancement in MAS-DNP.
• Standard efficiency assessment (εon/off) assumes microwave irradiation is the only factor affecting signal intensity.

Purpose of the Study:

• To investigate nuclear depolarization effects caused by biradicals under MAS conditions at ~110 K.
• To analyze the radical-dependence of MAS-DNP-induced depolarization.
• To develop a theoretical model explaining MAS-DNP depolarization and propose a revised method for assessing radical performance.

Main Methods:

• Experimental MAS-DNP measurements at ~110 K with TOTAPOL, AMUPOL, and Trityl OX63 biradicals.
• Quantification of nuclear polarization loss compared to static samples at 10 kHz MAS frequency.
• Theoretical modeling to elucidate the depolarization mechanism under MAS conditions.

Main Results:

• TOTAPOL and AMUPOL induce significant nuclear depolarization at ~110 K, unlike Trityl OX63.
• Polarization losses reached up to 20% for TOTAPOL and 60% for AMUPOL at 10 kHz MAS.
• The observed depolarization is dependent on the specific biradical used.

Conclusions:

• The efficiency of MAS-DNP biradicals like AMUPOL may be overestimated due to unconsidered depolarization effects.
• A theoretical framework is established to understand MAS-DNP depolarization mechanisms.
• A new method for evaluating polarization gain is proposed for more accurate radical assessment.