Sensitivity of MAS NMR has been dramatically improved by the advent of the high-field DNP techniques. Especially, we have shown that implementing DNP at ultra-low temperatures (ULT, T «100 K) using helium greatly improves the DNP efficiency at high field, high-resolution conditions (B0 »10 T). A key technological breakthrough therein consists of the closed-cycle helium gas circulation (CHC) MAS system and dedicated DNP probe [1,2], that maintains the sample spinning at 20 K (±0.1 K) and 8 kHz (±0.003 kHz) for weeks without consuming any helium. The long-term stability and its seamless temperature controlling ability between 20 and 200 K has enabled a number of applications in biological [3] and material sciences [4], and contributed to understanding of the mechanistic origin of the Overhauser DNP [5]. The system is also combined with a cryogenic signal detection system for the thermal noise suppression, cooled "for free" using the cold (~40 K) return helium gas on its way back to the CHC compressor [6]. Based on these instruments, the net sensitivity gain of over 4000 was recorded at T = 30 K, B0 =16.4 T.
Beyond the sensitivity improvement, we have been exploring new DNP methods e.g., for the nano space-selective observation [7] and background signal suppression, both of which being the key for looking at target molecules within unpurified mixture samples. We also demonstrate nanodiamond is a promising polarizing agent for MAS DNP on proteins at 16.4 T in strongly reducing environments such as within biological cells. These basic technical advances combined should open up a new avenue to highly sensitive and target molecule-selective protein structural analysis in cellular setups. In the presentation, we will discuss the latest results on such methods, instrument and applications.
[1] Y. Matsuki et al., JMR 259, 76- (2015)
[2] Y. Matsuki et al., eMagRes 7, 9- (2018)
[3] M. Takamuku et al., Neurochem. Int. 157, 105345 (2022)
[4] Y. Matsuki et al., PhysChemChemPhys 23, 4919- (2021)
[5] F. A. Perras et al., JCP accepted. (2023)
[6] Y. Matsuki et al., JMR 335, 107139 (2021)
[7] Y. Matsuki et al., JPC C 124, 18609-18514 (2020)