Reflectometry and molecular dynamics study of the impact of cholesterol and melatonin on model lipid membranes.

The effect of melatonin and/or cholesterol on the structural properties of a model lipid bilayer prepared from 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) has been investigated both experimentally and via molecular dynamics (MD) simulations. Neutron reflectometry experiments performed with single supported membranes revealed changes in lipid bilayer thickness upon the introduction of additional components. While the presence of cholesterol led to an increase in membrane thickness, the opposite effect was observed in the case of melatonin. The results obtained are in a good agreement with MD simulations which provided further information on the organization of components within the systems examined, indicating a mechanism underlying the membranes' thickness changes due to cholesterol and melatonin that had been observed experimentally. Cholesterol and melatonin preferentially accumulate in different membrane regions, presumably affecting the conformation of lipid hydrophobic moieties differently, and in turn having distinct impacts on the structure of the entire membrane. Our findings may be relevant for understanding the effects of age-related changes in cholesterol and melatonin concentrations, including those in the brains of individuals with Alzheimer's disease.

The spatial diamond-graphite transition in detonation nanodiamond as revealed by small-angle neutron scattering.

A spatial transition of the carbon state in detonation nanodiamond (DND) from crystalline diamond inside the particle to a graphite-like state at the DND surface is proposed on the basis of small-angle neutron scattering (SANS) analysis. The SANS contrast variation from concentrated (5 wt%) dispersions of DND in liquids (water, dimethylsulfoxide) reveals a shift in the mean scattering length density of DND as compared to pure diamond, which is related to the presence of a non-diamond component in the DND structure. At the same time, the diffusive character of the particle surface is deduced based on the deviation from the Porod law. The two observations are combined to conclude that the continuous radial density profile over the whole particle volume conforms to a simple power law. The profile naturally suggests that non-diamond states are concentrated mainly close to the particle surface; still there is no sharp boundary between the radial distributions of the two states of carbon in DND.