Sensitivity enhancement by DNP

Solid-state NMR contributes very unique to structural biology among other available methods. One of the remarkable thing is its broad application in terms of the form of sample: It can analyze non-crystalline fibrils, insoluble membrane proteins, even proteins in a whole cell and organelles are conceivable as target because there is no molecular weight limit. However, the usefulness of solid-state NMR technique is offset mainly by its poor sensitivity, i.e. solid-state NMR requires a lot of protein (>1mg), or study of a huge molecular entities will be increadibly difficult since sensitivity per unit sample volume decrease when the molecular weight of the target grows.

Here, we bring up the DNP. Dynamic nuclear polarization (DNP) is a method to transfer the 660-times larger Boltzmann polarization of an electron to the nuclei of interest, thereby amplifying the signal intensity of NMR to the same extent.

On top of that, there is the temperature effect. According to the Boltzmann distribution, 10-fold decrease in the sample temperature polarizes it 10-times stronger. Overall, the enhancement would be huge (660x10=6600). However, a complete transfer of the electron polarization to nuclei would be very difficult in the real world because of non-zero electron spin relaxation. So, our realistic goal would be 1000-fold enhancement, at least 100-times from DNP.

We have developed the world's highest field DNP/NMR system, which is already up and running in our lab (see below). Our current challenge is to establish general ways to optimize the spectral resolution at cryogenic sample temperature, and ways to stably polarize the broad range of samples such as membrane proteins, amyloid fibris and micro-crystals.


Figure: Sensitivity enhancement via DNP at the external magnetic field condition of B₀=14T (600MHz for ¹H). Photos are the setup which is already up and running at IPR, Osaka University.