No abstract

Lipid vesicle encapsulation is an efficient approach to transfer quantum dots (QDs) into aqueous solutions, which is important for renewable energy applications and biological imaging. However, little is known about the molecular organization at the interface between a QD and lipid membrane. To address this issue, we investigated the properties of 3.0 nm CdSe QDs encapsulated within phospholipid membranes displaying a range of phase transition temperatures (Tm). Theoretical and experimental results indicate that the QD locally alters membrane structure, and in turn, the physical state (phase) of the membrane controls the optical and chemical properties of the QDs. Using photoluminescence, ICP-MS, optical microscopy, and ligand exchange studies, we found that the Tm of the membrane controls optical and chemical properties of lipid vesicle-embedded QDs. Importantly, QDs encapsulated within gel-phase membranes were ultrastable, providing the most photostable non-core/shell QDs in aqueous solution reported to date. Atomistic molecular dynamics simulations support these observations and indicate that membranes are locally disordered displaying greater disordered organization near the particle–solution interface. Using this asymmetry in membrane organization near the particle, we identify a new approach for site-selective modification of QDs by specifically functionalizing the QD surface facing the outer lipid leaflet to generate gold nanoparticle–QD assemblies programmed by Watson–Crick base-pairing.

Living cells contain a range of densely phosphorylated surfaces, including phospholipid membranes, ribonucleoproteins, and nucleic acid polymers. Hyperphosphorylated surfaces also accumulate in neurodegenerative diseases as neurofibrillar tangles. We have synthesized and structurally characterized a precisely patterned phosphotyrosine surface and establish this assembly as a surrogate of the neuronal tangles by demonstrating its high-affinity binding to histone H1. This association with nucleic acid binding proteins underscores the role such hyperphosphorylated surfaces may play in disease and opens functional exploration into protein-phosphorylated surface interactions in a wide range of other complex assemblies.

The dynamics of the gel to fluid phase transformation in 100 nm large unilamellar vesicles (LUV) of 1,2-dipalmitoyl(d62)-sn-glycero-3-phosphocholine (d62-DPPC), has been studied by laser-induced temperature-jump initiation coupled with time-resolved infrared spectroscopy and by MD simulations. The infrared transients that characterize the temperature dependent phase transformation are complex, extending from the nanosecond to the millisecond time scales. An initial fast (submicrosecond) component can be modeled by partial melting of the gel domains, initiated at pre-existing defects at the edges of the faceted structure of the gel phase. Molecular dynamics simulations support the model of fast melting from edge defects. The extent of melting during the fast phase is limited by the area expansion on melting, which leads to a surface pressure that raises the effective melting temperature. Subsequent melting is observed to follow highly stretched exponential kinetics, consistent with collective relaxation of the surface pressure through a hierarchy of surface undulations with different relaxation times. The slowest step is water diffusion through the bilayer to allow the vesicle volume to grow along with its expanded surface area. The results demonstrate that the dominant relaxation in the gel to fluid phase transformation in response to a large T-jump perturbation (compared to the transition width) is fast (submicrosecond), which has important practical and fundamental consequences.