Heteronuclear Cross-Relaxation under Solid-State Dynamic Nuclear Polarization

Abstract

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.

Bis‐Gadolinium Complexes for Solid Effect and Cross Effect Dynamic Nuclear Polarization

“…[12] However, these features seem to be centered at the 13 C and 15 N Larmor frequency offsets which suggest cross-talk between the polarizations of different nuclear species, [8,25] or might be a sign of multiple nuclei undergoing concerted DNP transitions. Earlier studies have shown such features to occur at field offsets corresponding to nuclear combination frequencies, [26] a situation not given in this case.…”

Efficient Building Blocks for Solid‐Phase Peptide Synthesis of Spin Labeled Peptides for Electron Paramagnetic Resonance and Dynamic Nuclear Polarization Applications

“…To quantify the DNP efficiency in 13 C direct polarization experiments, 13 C MAS experiments were recorded employing the saturation recovery pulse sequence. Since next to the direct polarization transfer to 13 C (direct channel), an alternative polarization pathway may occur, where polarization is transferred via the proton spin reservoir (indirect channel), [54][55][56][57][58] additional experiments with a pulse sequence containing rotor synchronized 180°pulses on the 1 H channel [57] were recorded. The spectra for the different 13 C MAS experiments are shown in Figure 6.…”

Complex Formation of the Tetracycline‐Binding Aptamer Investigated by Specific Cross‐Relaxation under DNP

“…[12] Furthermore,t his method is generally applicable to any system where isotope-labeled "reporter" methyl groups can be specifically introduced. We have shown that this effect can be used to specifically edit NMR spectra for high selectivity towards bound complexes in the presence of unbound species.…”

“…[9] These metal ions may specifically occupy endogenous binding sites, [10] or may be introduced into diamagnetic biomolecules by ac helate tag. [12] Besides the highly efficient, three-fold symmetric methyl reorientation, other molecular motions also seem to support this effect. [10] However,inthis case the close vicinity to the metal ion often causes NMR line broadening and thus al oss in resolution.…”

“…[10] However,inthis case the close vicinity to the metal ion often causes NMR line broadening and thus al oss in resolution. [12] Mathematical subtraction of a 1 H-saturated and an on-saturated spectrum (DDP sat )c an then be used as ahighly specific filter, exclusively selecting 13 Cmagnetization generated by cross-relaxation. We have recently described this effect on proteins and amino acids and have shown that it can efficiently transfer DNP-enhanced polarization from 1 Ht o 13 Cw ithin methyl groups under typically employed DNP conditions (i.e., utilizing bis-nitroxides in ag lassy matrix at cryogenic temperatures of approximately 100 K).…”