Investigations of Nonionic Surfactants and a Novel Polarization Transfer Mechanism for Dynamic Nuclear Polarization Enhanced NMR
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Beschreibung
In this work, the signal enhancement in solid-state DNP NMR experiments was investigated on PEG and surfactants on PEG basis. In the past, these substances were only little noticed as solids in their bulk phase. In their liquid phase, however, they are ideal as a replacement for common organic solvents, as they have high biocompatibility. The PEG related nonionic surfactants, namely C10E6, C11E6P1 and Triton X-100 and polyethylene glycol (PEG 200) itself were studied by dynamic nuclear polarization (DNP) enhanced solid state NMR spectroscopy and differential scanning calorimetry (DSC). All samples were successfully hyperpolarized with DNP and it was shown that DNP NMR is a suitable tool to analyze interactions of radicals as solutes with surfactants as solvents.
During these studies it was discovered for DNP enhanced 13C magic angle spinning (MAS) spectra, that surprisingly two sets of resonances are present in each spectrum, one with broad and one with sharp spectral features that are 180° opposite in phase. Therefore, these resonances were analyzed in detail and the existence of two separate polarization transfer processes for 13C DNP NMR spectra was discovered.
The first one is the well-known mechanism that occurs in any direct polarization 13C DNP NMR experiment, where the polarization is transferred directly from the electrons to the 13C nuclei. For this direct channel mechanism mainly the nuclei in the vicinity of the radical are polarized. This mechanism becomes dominant with increasing radical concentration. The second mechanism is a newly observed indirect polarization transfer mechanism, in which the 1H nuclei are first polarized, and then the polarization is transferred to the 13C nuclei through the proton spin pool through a NOE-based cross-relaxation mechanism. The resulting resonances are 180 ° out of phase narrow signals in the 13C DNP spectrum. It was feasible to establish experiments to suppress the indirect channel pathway by application of a sequence of 180° pulses on the 1H nuclei during the 13C DNP build-up or the use of a T2 filter. The indirect channel polarized 13C spectra can be separated from the direct channel spectra by taking the difference of the 13C spectra with and without 1H 180° pulse train.
This novel polarization transfer mechanism was detected and studied in detail for the four analyzed surfactants as well as for organic solutes in PEG 200. Further studies revealed the same mechanism for several novel DNP radicals and three amino acid building-blocks in TCE. DSC measurements verified that the NOE type cross relaxation causing the indirect channel mechanism was affected by molecular motions in the samples, which were unknown thus far. Furthermore, it was investigated, if other nuclei, like 31P can also be polarized via the indirect channel mechanism. This was studied for spin-labeled model protein sulfoxidase, where no indirect channel signals were recorded.
Autor*in
Sarah Catherine Bothe
Themen in »Investigations of Nonionic Surfactants and a Novel Polarization Transfer Mechanism for Dynamic Nuclear Polarization Enhanced NMR«
Dynamic Nuclear Polarization Green chemistry NMR Nonionic Surfactants Nuclear Magnetic Resonance PEG PEG related surfactants Polarization Transfer Mechanism Polyethylenglykol green solvent signal enhancement surfactants