Monday, January 22, 2018

Chromophore Distortions in Photointermediates of Proteorhodopsin Visualized by Dynamic Nuclear Polarization-Enhanced Solid-State NMR #DNPNMR #NMR #SSNMR


Mehler, M., et al., Chromophore Distortions in Photointermediates of Proteorhodopsin Visualized by Dynamic Nuclear Polarization-Enhanced Solid-State NMR. J. Am. Chem. Soc., 2017. 139(45): p. 16143-16153.


Proteorhodopsin (PR) is the most abundant retinal protein on earth and functions as a light-driven proton pump. Despite extensive efforts, structural data for PR photointermediate states have not been obtained. On the basis of dynamic nuclear polarization (DNP)-enhanced solid-state NMR, we were able to analyze the retinal polyene chain between positions C10 and C15 as well as the Schiff base nitrogen in the ground state in comparison to light-induced, cryotrapped K- and M-states. A high M-state population could be achieved by preventing reprotonation of the Schiff base through a mutation of the primary proton donor (E108Q). Our data reveal unexpected large and alternating (13)C chemical shift changes in the K-state propagating away from the Schiff base along the polyene chain. Furthermore, two different M-states have been observed reflecting the Schiff base reorientation after the deprotonation step. Our study provides novel insight into the photocycle of PR and also demonstrates the power of DNP-enhanced solid-state NMR to bridge the gap between functional and structural data and models.

Friday, January 19, 2018

Improved strategies for DNP-enhanced 2D 1H-X heteronuclear correlation spectroscopy of surfaces #DNPNMR


Kobayashi, T., et al., Improved strategies for DNP-enhanced 2D 1H-X heteronuclear correlation spectroscopy of surfaces. Solid State Nuclear Magnetic Resonance, 2017. 87: p. 38-44.


We demonstrate that dynamic nuclear polarization (DNP)-enhanced 1H-X heteronuclear correlation (HETCOR) measurements of hydrogen-rich surface species are better accomplished by using proton-free solvents. This approach notably prevents HETCOR spectra from being obfuscated by the solvent-derived signals otherwise present in DNP measurements. Additionally, in the hydrogen-rich materials studied here, which included functionalized mesoporous silica nanoparticles and metal organic frameworks, the use of proton-free solvents afforded higher sensitivity gains than the commonly used solvents containing protons. We also explored the possibility of using a solvent-free sample formulation and the feasibility of indirect detection in DNP-enhanced HETCOR experiments.

Wednesday, January 17, 2018

Magic angle spinning NMR below 6 K with a computational fluid dynamics analysis of fluid flow and temperature gradients

This article is not specifically about DNP spectroscopy. However, magic angle spinning at 6K is definitely of interest to DNP, especially when using low-power, solid-state microwave sources.


Sesti, E.L., et al., Magic angle spinning NMR below 6 K with a computational fluid dynamics analysis of fluid flow and temperature gradients. J. Magn. Reson., 2018. 286(Supplement C): p. 1-9.


We report magic angle spinning (MAS) up to 8.5 kHz with a sample temperature below 6 K using liquid helium as a variable temperature fluid. Cross polarization 13C NMR spectra exhibit exquisite sensitivity with a single transient. Remarkably, 1H saturation recovery experiments show a 1H T1 of 21 s with MAS below 6 K in the presence of trityl radicals in a glassy matrix. Leveraging the thermal spin polarization available at 4.2 K versus 298 K should result in 71 times higher signal intensity. Taking the 1H longitudinal relaxation into account, signal averaging times are therefore predicted to be expedited by a factor of >500. Computer assisted design (CAD) and finite element analysis were employed in both the design and diagnostic stages of this cryogenic MAS technology development. Computational fluid dynamics (CFD) models describing temperature gradients and fluid flow are presented. The CFD models bearing and drive gas maintained at 100 K, while a colder helium variable temperature fluid stream cools the center of a zirconia rotor. Results from the CFD were used to optimize the helium exhaust path and determine the sample temperature. This novel cryogenic experimental platform will be integrated with pulsed dynamic nuclear polarization and electron decoupling to interrogate biomolecular structure within intact human cells.

Monday, January 15, 2018

Spectral diffusion and dynamic nuclear polarization: Beyond the high temperature approximation #DNPNMR


Wenckebach, W.T., Spectral diffusion and dynamic nuclear polarization: Beyond the high temperature approximation. J. Magn. Reson., 2017. 284(Supplement C): p. 104-114.


Dynamic Nuclear Polarization (DNP) has proven itself most powerful for the orientation of nuclear spins in polarized targets and for hyperpolarization in magnetic resonance imaging (MRI). Unfortunately, the theoretical description of some of the processes involved in DNP invokes the high temperature approximation, in which Boltzmann factors are expanded up to first order, while the high electron and nuclear spin polarization required for many applications do not justify such an approximation. A previous article extended the description of one of the mechanisms of DNP—thermal mixing—beyond the high temperature approximation (Wenckebach, 2017). But that extension is still limited: it assumes that fast spectral diffusion creates a local equilibrium in the electron spin system. Provotorov’s theory of cross-relaxation enables a consistent further extension to slower spectral diffusion, but also invokes the high temperature approximation. The present article extends the theory of cross-relaxation to low temperature and applies it to spectral diffusion in glasses doped with paramagnetic centres with anisotropic g-tensors. The formalism is used to describe DNP via the mechanism of the cross effect. In the limit of fast spectral diffusion the results converge to those obtained in Wenckebach (2017) for thermal mixing. In the limit of slow spectral diffusion a hole is burnt in the electron spin resonance (ESR) signal, just as predicted by more simple models. The theory is applied to DNP of proton and 13C spins in samples doped with the radical TEMPO.

Friday, January 12, 2018

Overhauser effects in non-conducting solids at 1.2K #DNPNMR


Ji, X., et al., Overhauser effects in non-conducting solids at 1.2 K. J. Magn. Reson., 2018. 286: p. 138-142.


Recently, it was observed that protons in non-conducting solids doped with 1,3-bisdiphenylene-2-phenylallyl (BDPA) or its sulfonated derivative (SA-BDPA) can be polarized through Overhauser effects via resonant microwave irradiation. These effects were present under magic angle spinning conditions in magnetic fields between 5 and 18.8T and at temperatures near 100K. This communication reports similar effects in static samples at 6.7T and, more importantly, at temperatures as low as 1.2K, in a different dynamic regime than in the previous study. Our results provide new information towards understanding the mechanism of the Overhauser effect in non-conducting solids. We discuss possible origins of the fluctuations that can give rise to an Overhauser effect at such low temperatures.

Wednesday, January 10, 2018

Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy #DNPNMR


Suzuki, K., et al., Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy. Angew. Chem. Int. Ed., 2017. 56(47): p. 14842-14846.


Molecular orientation in amorphous organic semiconducting thin-film devices is an important issue affecting device performance. However, to date it has not been possible to analyze the "distribution" of the orientations. Although solid-state NMR (ssNMR) spectroscopy can provide information on the "distribution" of molecular orientations, the technique is limited because of the small amount of sample in the device and the low sensitivity of ssNMR. Here, we report the first application of dynamic nuclear polarization enhanced ssNMR (DNP-ssNMR) spectroscopy for the orientational analysis of amorphous phenyldi(pyren-1-yl)phosphine oxide (POPy2 ). The (31) P DNP-ssNMR spectra exhibited a sufficient signal-to-noise ratio to quantify the distribution of molecular orientations in amorphous films: the P=O axis of the vacuum-deposited and drop-cast POPy2 shows anisotropic and isotropic distribution, respectively. The different molecular orientations reflect the molecular origin of the different charge transport behaviors.

Monday, January 8, 2018

High-Field Solid-State NMR with Dynamic Nuclear Polarization #DNPNMR


Lee, D., S. Hediger, and G.D. Paƫpe, High-Field Solid-State NMR with Dynamic Nuclear Polarization, in Modern Magnetic Resonance, G.A. Webb, Editor. 2017, Springer International Publishing: Cham. p. 1-17.


Microwave-induced dynamic nuclear polarization (DNP) can produce hyperpolarization of nuclear spins, leading to substantial signal enhancement in NMR. This chapter discusses the contemporary application of DNP for solid-state NMR spectroscopy at high magnetic fields. The main mechanisms and polarizing agents that enable this hyperpolarization are presented, along with more practical aspects such as the effect of decreasing sample temperature and analyzing the absolute sensitivity gain from these experiments. Examples of the exploitation of DNP for studies of biomolecules, biominerals, pharmaceuticals, self-assembled organic nanostructures, and mesoporous materials are given as is an outlook as to the future of this powerful technique.