We report on the spontaneous polarization transfer from dynamically hyperpolarized H toC during magic-angle spinning dynamic nuclear polarization (DNP) at temperatures around 100 K. The transfer is mediated by H-C cross-relaxation within methyl groups due to reorientation dynamics, and results in an inverted C NMR signal of enhanced amplitude. Further spreading of transferred polarization can then occur viaC-C spin-diffusion. The resulting process is equal to the nuclear Overhauser effect (NOE) where typically continuous saturation of H by radio frequency irradiation is employed. Here, hyperpolarization by irradiation with microwaves in the presence of typical bis-nitroxide polarizing agents is utilized for steady-state displacement ofH polarization from thermal equilibrium and perpetual spin-lattice relaxation. An effective C enhancement factor of up to -15 has been measured. Presence of Gd(III) furthermore amplifies the effect likely by accelerated relaxation ofH. We provide experimental evidence for the proposed mechanism and show that DNP-induced cross-relaxation is a robust feature within proteins and single amino acids and discuss potential applications.

We recently engineered encodable lanthanide binding tags (LBTs) into proteins and demonstrated their applicability in Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray crystallography and luminescence studies. Here, we engineered two-loop-LBTs into the model protein interleukin-1b (IL1b) and measured 1 H, 15 Npseudocontact shifts (PCSs) by NMR spectroscopy. We determined the Dv-tensors associated with each Tm 3? -loaded loop-LBT and show that the experimental PCSs yield structural information at the interface between the two metal ion centers at atomic resolution. Such information is very valuable for the determination of the sites of interfaces in protein-protein-complexes. Combining the experimental PCSs of the two-loop-LBT construct IL1b-S2R2 and the respective single-loop-LBT constructs IL1b-S2, IL1b-R2 we additionally determined the distance between the metal ion centers. Further, we explore the use of two-loop LBTs loaded with Gd 3? as a novel tool for distance determination by Electron Paramagnetic Resonance spectroscopy and show the NMR-derived distances to be remarkably consistent with distances derived from Pulsed Electron-Electron Dipolar Resonance.

Paramagnetic relaxation enhancement (PRE) is a versatile tool for NMR spectroscopic structural and kinetic studies in biological macromolecules. Here, we compare the quality of PRE data derived from two spin labels with markedly different dynamic properties for large RNAs using the I-A riboswitch aptamer domain (78 nt) from Mesoplamsa florum as model system. We designed two I-A aptamer constructs that were spin-labeled by noncovalent hybridization of short spin-labeled oligomer fragments. As an example of a flexible spin label, U-TEMPO was incorporated into the 3' terminal end of helix P1 while, the recently developed rigid spin-label Çm was incorporated in the 5' terminal end of helix P1. We determined PRE rates obtained from aromaticC bound proton intensities and compared these rates to PREs derived from imino proton intensities in this sizeable RNA (~78 nt). PRE restraints derived from both imino and aromatic protons yielded similar data quality, and hence can both be reliably used for PRE determination. For NMR, the data quality derived from the rigid spin label Çm is slightly better than the data quality for the flexible TEMPO as judged by comparison of the structural agreement with the I-A aptamer crystal structure (3SKI).

Encodable lanthanide binding tags (LBTs) have become an attractive tool in modern structural biology as they can be expressed as fusion proteins of targets of choice. Previously, we have demonstrated the feasibility of inserting encodable LBTs into loop positions of interleukin-1β (Barthelmes et al. in J Am Chem Soc 133:808-819, 2011). Here, we investigate the differences in fast dynamics of selected loop-LBT interleukin-1β constructs by measuring N nuclear spin relaxation experiments. We show that the loop-LBT does not significantly alter the dynamic motions of the host protein in the sub-τ-timescale and that the loop-LBT adopts a rigid conformation with significantly reduced dynamics compared to the terminally attached encodable LBT leading to increased paramagnetic alignment strength. We further analyze residual dipolar couplings (RDCs) obtained by loop-LBTs and additional liquid crystalline media to assess the applicability of the loop-LBT approach for RDC-based methods to determine structure and dynamics of proteins, including supra-τc dynamics. Using orthogonalized linear combinations (OLCs) of RDCs and Saupe matrices, we show that the combined use of encodable LBTs and external alignment media yields up to five linear independent alignments.