Judith Schlagnitweit

Non-linearity and frequency shifts of nuclear magnetic spin-noise

We have systematically investigated the line shapes observed in proton nuclear spin-noise spectra which depend in a complex way on the properties of the resonance circuit, the number of spins present, transverse relaxation, inhomogeneous broadening, and radiation damping. Using highly sensitive high resolution probes these dependencies are investigated by way of high resolution (1)H noise power NMR spectra of liquid samples. Significant deviations from intuitive expectations are observed. Simulations based on an adapted Nyquist noise equation are used to understand and interpret the experimental findings.

The first observation of Carbon-13 spin noise spectra

Graphical abstract Highlights ► We report the first observation of Carbon-13 spin noise spectra. ► Non-linear amplitude response at high concentration indicates the influence of radiation damping. ► The proton decoupled noise spectra show absence of heteronuclear NOE enhancement.

Non-Linear Signal Detection Improvement by Radiation Damping in Single-Pulse NMR Spectra

When NMR lines overlap and at least one of them is affected by radiation damping, the resonance line shapes of all lines are no longer Lorentzian. We report the appearance of narrow signal distortions, which resemble hole-burnt spectra. This new experimental phenomenon facilitates the detection of tiny signals hidden below the main resonance. Theoretical analysis based on modified Maxwell–Bloch equations shows that the presence of strong transverse magnetization creates a feedback through the coil, which influences the magnetization of all spins with overlapping resonance lines. In the time domain this leads to cross-precession terms between magnetization densities, which ultimately cause non-linear behavior. Numerical simulations corroborate this interpretation.

Spin-Noise-Detected Two-Dimensional Fourier-Transform NMR Spectroscopy

We introduce two-dimensional NMR spectroscopy detected by recording and processing the noise originating from nuclei that have not been subjected to any radio frequency excitation. The method relies on cross-correlation of two noise blocks that bracket the evolution and mixing periods. While the sensitivity of the experiment is low in conventional NMR setups, spin-noise-detected NMR spectroscopy has great potential for use with extremely small numbers of spins, thereby opening a way to nanoscale multidimensional NMR spectroscopy.

Observation of NMR noise from solid samples.

We demonstrate that proton NMR noise signals, i.e. NMR spectra without excitation by radio frequency, can be obtained from solid samples. Experimental results are shown for static and magic-angle spinning conditions. In addition, a tuning procedure based on the probes’ NMR noise characteristics and similar to the one described previously for liquids probes can also be used to optimize signal-to-noise ratios in 1H-MAS experiments.

On the tuning of high-resolution NMR probes.

Three optimum conditions for the tuning of NMR probes are compared: the conventional tuning optimum, which is based on radio-frequency pulse efficiency, the spin noise tuning optimum based on the line shape of the spin noise signal, and the newly introduced frequency shift tuning optimum, which minimizes the frequency pushing effect on strong signals. The latter results if the radiation damping feedback field is not in perfect quadrature to the precessing magnetization. According to the conventional RLC (resistor–inductor–capacitor) resonant circuit model, the optima should be identical, but significant deviations are found experimentally at low temperatures, in particular on cryogenically cooled probes. The existence of different optima with respect to frequency pushing and spin noise line shape has important consequences on the nonlinearity of spin dynamics at high polarization levels and the implementation of experiments on cold probes.

Determination of the xanthate group distribution on viscose by liquid-state 1H NMR spectroscopy

An analytical method for determination of the xanthate group distribution on viscoses based on liquid-state NMR spectroscopy was developed. Sample preparation involves stabilization of the xanthate group by allylation followed by derivatization of the remaining free hydroxyl groups at the glucose unit. The method was applied for studying (1) the γ-value (number of xanthate groups per 100 glucose units) of viscose, (2) the distribution of the xanthate groups on the anhydroglucose unit (AGU), and (3) changes of the xanthate group distribution during ripening. Results of the γ-value determination are well comparable with reference methods. Elucidation of the xanthate group distribution on the AGU gives the percentage at the C-6 position and a cumulative share of the positions C-2 and C-3. During ripening, xanthate groups at C-2 and C-3 degrade first, while xanthates at C-6 decompose at a slower rate.

Toward multipurpose NMR experiments

Standard phase cycled NMR pulse sequences were generalized such that for each individual step of the pulse phase cycle the free induction decay is stored separately without phase correction. This is in contrast to the usual practice, where pulse responses are phase shifted immediately (by applying a ‘receiver phase’) and co‐added as they are stored. The approach used here allows one to extract different types of NMR information, which are usually referred to as different ‘experiments’, from the same raw data set a posteriori by using complex linear combinations. Storing the free induction decays of individual phase cycle steps separately and using specific linear combinations of these data to obtain a particular type of information increase the overall efficiency of a given set of NMR experiments substantially, because all information can be derived from a single multiplexed data set. This ‘super‐experiment’ requires only as much time as the most complex of the derived specific experiments alone.

Backbone assignment and secondary structure of the PsbQ protein from Photosystem II

PsbQ is one of the extrinsic proteins situated on the lumenal surface of photosystem II (PSII) in the higher plants and green algae. Its three-dimensional structure was determined by X-ray crystallography with exception of the residues 14–33. To obtain further details about its structure and potentially its dynamics, we approached the problem by NMR. In this paper we report 1H, 15N, and 13C NMR assignments for the PsbQ protein. The very challenging oligo-proline stretches could be assigned using 13C-detected NMR experiments that enabled the assignments of twelve out of the thirteen proline residues of PsbQ. The identification of PsbQ secondary structure elements on the basis of our NMR data was accomplished with the programs TALOS+, web server CS23D and CS-Rosetta. To obtain additional secondary structure information, three-bond HN-Hα J-coupling constants and deviation of experimental 13Cα and 13Cβ chemical shifts from random coil values were determined. The resulting “consensus” secondary structure of PsbQ compares very well with the resolved regions of the published X-ray crystallographic structure and gives a first estimate of the structure of the “missing link” (i.e. residues 14–33), which will serve as the basis for the further investigation of the structure, dynamics and interactions.

Nonlinear detection of secondary isotopic chemical shifts in NMR through spin noise

The detection of minor species in the presence of large amounts of similar main components remains a key challenge in analytical chemistry, for instance, to obtain isotopic fingerprints. As an alternative to the classical NMR scheme based on coherent excitation and detection, here we introduce an approach based on spin-noise detection. Chemical shifts and transverse relaxation rates are determined using only the detection circuit. Thanks to a nonlinear effect in mixtures with small chemical shift dispersion, small signals on top of a larger one can be observed with increased sensitivity as bumps on a dip; the latter being the signature of the main magnetization. Experimental observations are underpinned by an analytical theory: the coupling between the magnetization and the coil provides an amplified detection capability of both small static magnetic field inhomogeneities and small NMR signals. This is illustrated by two-bond 12C/13C isotopic measurements.