Abstract
Several parameters in NMR depend on the magnetic field strength. Field-cycling NMR is an elegant way to explore the field dependence of these properties. The technique is well developed for solution state and in relaxometry. Here, a shuttle system with magic-angle spinning (MAS) detection is presented to allow for field-dependent studies on solids. The function of this system is demonstrated by exploring the magnetic field dependence of the solid-state photochemically induced nuclear polarization (photo-CIDNP) effect. The effect allows for strong nuclear spin-hyperpolarization in light-induced spin-correlated radical pairs (SCRPs) under solid-state conditions. To this end, 13C MAS NMR is applied to a photosynthetic reaction center (RC) of the purple bacterium Rhodobacter (R.) sphaeroides wildtype (WT). For induction of the effect in the stray field of the magnet and its subsequent observation at 9.4 T under MAS NMR conditions, the sample is shuttled by the use of an aerodynamically driven sample transfer technique. In the RC, we observe the effect down to 0.25 T allowing to determine the window for the occurrence of the effect to be between about 0.2 and 20 T.
Highlights
Several parameters in NMR depend on the magnetic field strength
Solids are shuttled in relaxation experiments which do not require chemical shift resolution[2,4,5,13] or the sample is not moved between fields but the magnetic field is provided by an electro-magnet with rapidly switched electric current[2]
In systems showing the solid-state photo-CIDNP effect, the spincorrelated radical pairs (SCRPs) is created upon photo-excitation of the primary electron donor which leads to a charge-separated state via an electron transport to an acceptor molecule (Fig. S1)
Summary
Magnetic field dependence of the photo-CIDNP enhancement factor for selected carbon positions in the selectively 4-ALA labeled bacterial RCs of R. sphaeroides WT in comparison with the natural abundance (n.a.) case observed in the unlabelled sample[6]. The experimental observation of the low-field wing of the enhancement window of the solid-state photo-CIDNP effect demonstrates the applicability of the shuttle MAS NMR system. This device might help to demonstrate the effect in more and non-photosynthetic electron-transfer systems opening a new avenue for the induction of hyperpolarization by sample illumination. Due to its easy implementation, it delivers a base for field-cycled solid-state dynamic nuclear polarization[44] (DNP) and optical nuclear polarization[45] (ONP) experiments under MAS, which so far were just done under static conditions[5,9,13,46]
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