Unusual Local Molecular Motions in the Solid State Detected by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy

Abstract

Polyethylene glycol (PEG) and three related surfactants were studied by dynamic nuclear polarization (DNP) enhanced solid state NMR spectroscopy and differential scanning calorimetry (DSC). DNP enhanced solid state NMR surprisingly reveals the presence of local molecular motions that are normally understood to be inactive at temperatures ∼100 K. This surprising phenomenon could be explained by the experimentally necessary rapid freezing of the studied samples. Specifically, DSC shows that PEG 200 forms a glass upon freezing and that the three PEG-related surfactants are at least partially in a glass state or some other thermodynamic nonequilibrium state when rapidly frozen to the temperatures of the DNP enhanced solid state NMR experiments. This effect of preserving local molar motions by rapid freezing also holds true for solutions of organic solutes in the PEG 200 solvent matrix.

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

“…The spectra for the different 13 C MAS experiments are shown in Figure . As also found in other systems, the vicinity of the radicals broadens the line‐width of the 13 C‐signals in the direct channel, compared to the line‐width in the indirect channel …”

“…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.…”

“…As also found in other systems, the vicinity of the radicals broadens the line-width of the 13 C-signals in the direct channel, compared to the line-width in the indirect channel. [54][55][56][57] Hyp (21) Ser (22) Tyr (23 The enhancement for 13 C MAS direct polarization (direct channel) was extracted by comparing the mw on spectra in Figure 6 II and the mw off spectra in Figure 6 IV. Enhancement factors of 12 � 1, 9 � 1 and 7 � 1 ( Table 1) were obtained for the hydroxyproline, tyrosine and serine building blocks, respectively.…”

Recent developments in MAS DNP-NMR of materials

“…a DNP-enhanced 13 C polarization pointing in opposite direction to the polarization at thermal equilibrium [146,217]. Such incoherent indirect DNP transfer was first reported for biomolecules [146,217] and more recently for nonionic surfactants, such as polyethylene glycol (PEG) and solutions of organic molecules, such as cyclohexane, in PEG (see Figure 14a) [209,218,219]. Local molecular motions, such as the rotation of the methyl groups or cyclohexane ring inversion, promote the 1 H- 13 C dipolar cross-relaxation.…”

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

“…Next, we checked for background SCREAM‐DNP, which could potentially be induced by the dynamics of other functional groups . In Figure A, we show the ΔDP sat spectra of the bound TC–aptamer complex recorded at different polarization periods between 2 and 16 s, showing a clear signature of SCREAM‐DNP of RNA resonances between approximately 50 and 180 ppm.…”