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A new solid-state NMR method is presented for estimating homonuclear dipole–dipole couplings for selected groups of nuclear spins in a multiple-spin coupled network. The methodology combines off-magic-angle spinning, frequency selective spin echoes, and multiple quantum filtering. The new method is insensitive to incoherent relaxation effects and may be used to estimate weak couplings. Internuclear 13C–13C couplings are estimated in uniformly 13C-labelled l-Histidine·HCl·H2O. Weak intermolecular couplings between 13C nuclei separated by distances exceeding 6 are estimated. [ABSTRACT FROM AUTHOR]

Calcium ion complexation in aqueous solutions is of paramount importance in biology as it is related to cell signaling, muscle contraction, or biomineralization. However, Ca 2+ -complexes are dynamic soluble entities challenging to describe at the molecular level. Nuclear magnetic resonance appears as a method of choice to probe Ca 2+ -complexes. However, 43 Ca NMR exhibits severe limitations arising from the low natural abundance coupled to the low gyromagnetic ratio and the quadrupolar nature of 43 Ca, which overall make it a very unreceptive nucleus. Here, we show that 43 Ca dynamic nuclear polarization (DNP) NMR of 43 Ca-labeled frozen solutions is an efficient approach to enhance the NMR receptivity of 43 Ca and to obtain structural insights about calcium ions complexed with representative ligands including water molecules, ethylenediaminetetraacetic acid (EDTA), and l-aspartic acid (l-Asp). In these conditions and in combination with numerical simulations and calculations, we show that 43 Ca nuclei belonging to Ca 2+ complexed to the investigated ligands exhibit rather low quadrupolar couplings (with C Q typically ranging from 0.6 to 1 MHz) due to high symmetrical environments and potential residual dynamics in vitrified solutions at a temperature of 100 K. As a consequence, when 1 H→ 43 Ca cross-polarization (CP) is used to observe 43 Ca central transition, "high-power" ν RF ( 43 Ca) conditions, typically used to detect spin 1/2 nuclei, provide ∼120 times larger sensitivity than "low-power" conditions usually employed for detection of quadrupolar nuclei. These "high-power" CPMAS conditions allow two-dimensional (2D) 1 H- 43 Ca HetCor spectra to be readily recorded, highlighting various Ca 2+ -ligand interactions in solution. This significant increase in 43 Ca NMR sensitivity results from the combination of distinct advantages: (i) an efficient 1 H-mediated polarization transfer from DNP, resembling the case of low-natural-abundance spin 1/2 nuclei, (ii) a reduced dynamics, allowing the use of CP as a sensitivity enhancement technique, and (iii) the presence of a relatively highly symmetrical Ca environment, which, combined to residual dynamics, leads to the averaging of the quadrupolar interaction and hence to efficient high-power CP conditions. Interestingly, these results indicate that the use of high-power CP conditions is an effective way of selecting symmetrical and/or dynamic 43 Ca environments of calcium-containing frozen solution, capable of filtering out more rigid and/or anisotropic 43 Ca sites characterized by larger quadrupolar constants. This approach could open the way to the atomic-level investigation of calcium environments in more complex, heterogeneous frozen solutions, such as those encountered at the early stages of calcium phosphate or calcium carbonate biomineralization events., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Three anthraquinone-based chromophores (9,10-anthraquinone, alizarin, purpurin) are compared from the point of view of their experimental and computed NMR and UV-visible light absorption spectra. Using a hybrid (explicit/implicit) solvent model, each proton chemical shift can be reproduced with an error of less than 7%, even when such protons are engaged in inter-molecular hydrogen bonds with the solvent or when the analyzed sample contains a significant amount of impurities, for instance, 9,10-anthraquinone in purpurin. All the steady-state UV-visible absorption spectra feature a significant vibrational progression in the first absorption band. The shape of the corresponding computed spectra, including vibronic couplings obtained with the adiabatic Hessian approach and the Franck-Condon and Herzberg-Teller approximation of the transition dipole, are in excellent agreement with the experimental ones. The importance and the nature of the vibronic couplings are different for the three molecules, even if they only differ by the number of hydroxyl groups.