In vivo NMR spectroscopy (here, in the context of vertical bore NMR magnets vs MRI systems) is a powerful, non-destructive technique, but is challenged by low biomass in aquatic organisms (~90% water) and biological heterogeneity. Consequently, many in vivo NMR experiments utilize isotopic enrichment (e.g., 13C) to obtain enough signal, making studies costly and largely restricted to lab-raised organisms. Steady State Free Precession (SSFP) holds potential for in vivo NMR, which collects hundreds of scans within the timespan of a single traditional NMR scan by maintaining the signal in a steady state, offering improved sensitivity. Coupling SSFP with large-diameter NMR probes (i.e., 10 mm or 20 mm) which accommodate greater biomass versus standard 5 mm probes, further improves concentration-sensitivity and reduces dependence on expensive sample enrichment. To illustrate this potential, this work discusses in vivo process monitoring in relation to general concepts of traditional reaction monitoring. First, 31P is monitored in vivo in a continuous fashion, arguably analogous to in situ reaction monitoring. Next, salt impacts are studied by systematic media changes, reflecting batch reaction monitoring. Finally, multivariable processes (temperature responses vs changes over time) can be differentiated through careful variable manipulations, similar to modern operando monitoring of complex processes. Finally, SSFP is integrated with Complete Reduction to Amplitude Frequency Tables (CRAFT), which overcomes limitations of SSFP (i.e., data truncation), while sensitivity advantages are retained. Using a 20 mm probe, this SSFP-CRAFT approach is applied to a freshwater plant, sea anemone and earthworms, highlighting applications across various biological systems.
Electron nuclear double resonance (ENDOR) spectroscopy is a method to detect the interactions between electron and nuclear spins to obtain structural information on molecular systems. Usually, this information is derived by extracting magnetic parameters from experimental spectra using spectral simulations. Thus, the derived information is limited to the representation of the experiment by the simulation. Advances in experimental setup constantly increase the resolution of spectra, and more sophisticated and detailed simulations are required, especially when the effect of pulse sequences can no longer be approximated by simple line shape functions. A suitable method to quantify these effects are spin dynamics simulations. Here, we describe the spin dynamics simulation package SimDyn, which is optimised to efficiently simulate ENDOR spectra of S = 1/2 paramagnetic systems, particularly Mims ENDOR for nitroxide-based 19F-ENDOR distance measurements. Besides the effects of orientation selection, the software is able to treat small terms in the spin Hamiltonian that are usually neglected in EPR and ENDOR but can still be directly observed, such as nuclear chemical shielding and nuclear-nuclear dipolar couplings. Moreover, relaxation effects are phenomenologically implemented as decay rates of density matrix elements in the Liouville space. Applications are presented for the analysis of 19F-ENDOR spectra, demonstrating how spin dynamics simulations can be used to gain insight on the experiment and subtle features of the detected spectra.
The experimental NMR/MRI studies demonstrate the anisotropies of the second moment M2 and dipolar order relaxation rate R1D in myelin-like bilayers. In this paper, to analyze the anisotropy, we consider the dipole-dipole Hamiltonian averaged accounting for the restricted motion of hydrogen protons. The averaging of the Hamiltonian's spatial part enables the derivation of analytical expressions for the angular dependence of the second moment M2 and the relaxation rate R1D. Comparison of theoretical results and experimental data allowed us to determine the average orientation and standard deviation of the hydrogen pair of a CH₂ group within the phospholipid bilayer. Some differences in these parameters, estimated from angular dependences of the second moment and the relaxation rate, can be explained by using different methods to measure the moment and rate, as well as by the fact that the environment can influence transverse and spin–lattice relaxations differently. The correlation times of the fluctuating CH2 groups were estimated as 7.9×10−7s.
Double-rotation (DOR) solid-state NMR spectroscopy (ssNMR) enables higher-resolution spectra of quadrupolar nuclei to be acquired as compared to the ubiquitous magic-angle spinning (MAS). However, due to low filling factors, slow spinning speeds and the cost and complexity of the specialised probes required, multiple quantum MAS (MQMAS) has almost completely superseded DOR for acquiring high-resolution spectra of quadrupolar nuclei. Herein, we investigate the use of spherical rotors to achieve DOR. Such rotors overcome the filling-factor issue, and the stator can be 3D-printed. Further, since there is only a single spinning surface and no nested rotors, the spinning speeds can, in principle, approach that of conventional MAS rotors. We present a proof-of-concept stator and sphere undergoing double rotation driven by pulsed gas streams, precessing at low tens of Hz, from which NMR data is acquired. We discuss the weight distribution in spheres for DOR and the implications for the ratio of the spinning and precession frequency, and conclude by outlining a potential avenue for achieving faster DOR using piezoelectric valves.
