The magnetic properties of MAl4(OH)12SO4·3H2O with M = Co2+, Ni2+, and Cu2+ determined by a combined experimental and computational approach.

The magnetic properties of the nickelalumite-type layered double hydroxides (LDH), MAl4(OH)12(SO4)·3H2O (MAl4-LDH) with M = Co2+ (S = 3/2), Ni2+ (S = 1), or Cu2+ (S = 1/2) were determined by a combined experimental and computational approach. They represent three new inorganic, low-dimensional magnetic systems with a defect-free, structurally ordered magnetic lattice. They exhibit no sign of magnetic ordering down to 2 K in contrast to conventional hydrotalcite LDH. Detailed insight into the complex interplay between the choice of magnetic ion (M2+) and magnetic properties was obtained by a combination of magnetic susceptibility, heat capacity, neutron scattering, solid-state NMR spectroscopy, and first-principles calculations. The NiAl4- and especially CoAl4-LDH have pronounced zero-field splitting (ZFS, easy-axis and easy-plane, respectively) and weak ferromagnetic nearest-neighbour interactions. Thus, they are rare examples of predominantly zero-dimensional spin systems in dense, inorganic matrices. In contrast, CuAl4-LDH (S = 1/2) consists of weakly ferromagnetic S = 1/2 spin chains. For all three MAl4-LDH, good agreement is found between the experimental magnetic parameters (J, D, g) and first-principles quantum chemical calculations, which also predict that the interchain couplings are extremely weak (< 0.1 cm-1). Thus, our approach will be valuable for evaluation and prediction of magnetic properties in other inorganic materials.

Picosecond dynamics in haemoglobin from different species: a quasielastic neutron scattering study.

BACKGROUND: Dynamics in haemoglobin from platypus (Ornithorhynchus anatinus), chicken (Gallus gallus domesticus) and saltwater crocodile (Crocodylus porosus) were measured to investigate response of conformational motions on the picosecond time scale to naturally occurring variations in the amino acid sequence of structurally identical proteins. METHODS: Protein dynamics was measured using incoherent quasielastic neutron scattering. The quasielastic broadening was interpreted first with a simple single Lorentzian approach and then by using the Kneller-Volino Brownian dynamics model. RESULTS: Mean square displacements of conformational motions, diffusion coefficients of internal dynamics and residence times for jump-diffusion between sites and corresponding effective force constants (resilience) and activation energies were determined from the data. CONCLUSIONS: Modifications of the physicochemical properties caused by mutations of the amino acids were found to have a significant impact on protein dynamics. Activation energies of local side chain dynamics were found to be similar between the different proteins being close to the energy, which is required for the rupture of single hydrogen bond in a protein. GENERAL SIGNIFICANCE: The measured dynamic quantities showed significant and systematic variations between the investigated species, suggesting that they are the signature of an evolutionary adaptation process stimulated by the different physiological environments of the respective protein.

Rotational motion in LiBH4/LiI solid solutions.

We investigated the localized rotational diffusion of the (BH(4))(-) anions in LiBH(4)/LiI solid solutions by means of quasielastic and inelastic neutron scattering. The (BH(4))(-) motions are thermally activated and characterized by activation energies in the order of 40 meV. Typical dwell times between jumps are in the picosecond range at temperatures of about 200 K. The motion is dominated by 90° reorientations around the 4-fold symmetry axis of the tetrahedraly shaped (BH(4))(-) ions. As compared to the pure system, the presence of iodide markedly reduces activation energies and increases the rotational frequencies by more than a factor of 100. The addition of iodide lowers the transition temperature, stabilizing the disordered high temperature phase well below room temperature.

Low-Frequency Dynamics of BSA Complementarily Studied by Raman and Inelastic Neutron Spectroscopy.

The present study focuses on protein motions on the picosecond time scale, generally characterized by the overlapping of vibrational and relaxational dynamics in disordered molecular systems. Recently, it has been demonstrated that a dry protein, bovine serum albumin (BSA), shows a glass-like transition in the temperature range between 240 and 260 K. Here, we present the results of combined low-frequency Raman and inelastic neutron scattering studies of dry BSA under conditions similar to those of this glass-like transition. The use of both techniques allows us to perform a detailed comparison of the dynamic susceptibility and vibrational density of states of BSA obtained at different temperatures and to calculate the light-vibration coupling coefficient, C(ω). Moreover, we analyzed the temperature evolution of the boson peak and a peak located at ∼80 cm-1, which has previously been identified to originate from protein dynamics, and observed that both modes show anomalous temperature behavior in the vicinity of Tg.

Confinement Effects for Lithium Borohydride: Comparing Silica and Carbon Scaffolds.

LiBH4 is a promising material for hydrogen storage and as a solid-state electrolyte for Li ion batteries. Confining LiBH4 in porous scaffolds improves its hydrogen desorption kinetics, reversibility, and Li+ conductivity, but little is known about the influence of the chemical nature of the scaffold. Here, quasielastic neutron scattering and calorimetric measurements were used to study support effects for LiBH4 confined in nanoporous silica and carbon scaffolds. Pore radii were varied from 8 Å to 20 nm, with increasing confinement effects observed with decreasing pore size. For similar pore sizes, the confinement effects were more pronounced for silica than for carbon scaffolds. The shift in the solid-solid phase transition temperature is much larger in silica than in carbon scaffolds with similar pore sizes. A LiBH4 layer near the pore walls shows profoundly different phase behavior than crystalline LiBH4. This layer thickness was 1.94 ± 0.13 nm for the silica and 1.41 ± 0.16 nm for the carbon scaffolds. Quasi-elastic neutron scattering confirmed that the fraction of LiBH4 with high hydrogen mobility is larger for the silica than for the carbon nanoscaffold. These results clearly show that in addition to the pore size the chemical nature of the scaffold also plays a significant role in determining the hydrogen mobility and interfacial layer thickness in nanoconfined metal hydrides.

Does a dry protein undergo a glass transition?

Bovine serum albumin (BSA) with extremely low hydration level 0.04, which is usually defined as dry, has been investigated in the temperature range between 200 and 340 K by incoherent inelastic neutron scattering using the neutron time-of-flight spectrometer FOCUS (PSI, Switzerland). Anomalous temperature behavior has been revealed for relaxational and low-frequency vibrational dynamics of BSA in the vicinity of 250 K. The mean-square atomic displacement has been shown to exhibit a change in the slope of temperature dependence near the same temperature. The presented results point out that the glass-like transition occurs in the dry protein.