Lipid Rearrangement in DSPC/DMPC Bilayers: A Neutron Reflectometry Study

Lipid translocation in membranes is still far from being understood and well characterized for natural cell membranes as well as for simpler bilayer model systems. Several discrepancies with respect to its occurrence and its characteristic time scale are present in the literature. In the current work, the structural changes induced by lipid rearrangement in a distearoyl-/dimyristoyl-phosphocholine binary lipid system have been addressed by means of neutron reflectivity. It has been shown that a fast, spontaneous compositional reorganization with lipid transfer between the two leaflets of the bilayer takes place only when the lipid species are both in the fluid phase. This process has been identified as the so-called lipid flip-flop. Moreover, the influence of the preparation protocol on the structural properties of the system has been investigated

Stacking and Branching in Self-Aggregation of Caffeine in Aqueous Solution: From the Supramolecular to Atomic Scale Clustering

The dynamical and structural properties of caffeine solutions at the solubility limit have been investigated as a function of temperature by means of MD simulations, static and dynamic light scattering, and small angle neutron scattering experiments. A clear picture unambiguously supported by both experiment and simulation emerges: caffeine self-aggregation promotes the formation of two distinct types of clusters: linear aggregates of stacked molecules, formed by 2–14 caffeine molecules depending on the thermodynamic conditions and disordered branched aggregates with a size in the range 1000–3000 Å. While the first type of association is well-known to occur under room temperature conditions for both caffeine and other purine systems, such as nucleotides, the presence of the supramolecular aggregates has not been reported previously. MD simulations indicate that branched structures are formed by caffeine molecules in a T-shaped arrangement. An increase of the solubility limit (higher temperature but also higher concentration) broadens the distribution of cluster sizes, promoting the formation of stacked aggregates composed by a larger number of caffeine molecules. Surprisingly, the effect on the branched aggregates is rather limited. Their internal structure and size do not change considerably in the range of solubility limits investigated