Biomedical subjects

Steve O Nielsen

Publications and source records attributed to Steve O Nielsen.

5 recordsLinked to original sources

PubMed2005

Modeling surfactant adsorption on hydrophobic surfaces.

Surfactant adsorption on hydrophobic surfaces is of current interest in attempts to solubilize single-wall carbon nanotubes and to render quantum dots biocompatible. A coarse grained method is presented for incorporating a hydrophobic surface into existing liquid force fields by appealing to statistical mechanics and probability theory. The dimensionality problem which arises is overcome with an approximate treatment and the entire procedure is applied to aqueous n-alkyl poly(ethylene oxide) adsorbing onto a graphite surface. The simulations are in excellent agreement with atomic force microscopy data. The mechanism of micelle adsorption onto a partially coated surface is reported for the first time and has implications for the construction of nanotemplates.

Adsorption↗

PubMed2005

Lipid bilayer perturbations around a transmembrane nanotube: a coarse grain molecular dynamics study.

The perturbations induced in a lipid bilayer by the presence of a transmembrane nanotube are investigated using coarse grained molecular dynamics. Meniscus formation by the lipids and tilting of the nanotube occur in response to hydrophobic mismatch, although these two effects do not compensate completely for the total mismatch. The lipid head-to-tail vector field is examined and shows strong ordering in the membrane plane regardless of the nanotube length. Molecular layering at the lipid-nanotube interface is reported. This study extends previous theoretical approaches to a more realistic setting.

Biophysical Phenomena↗

PubMed2005

Structure and dynamics of model pore insertion into a membrane.

A cylindrical transmembrane molecule is constructed by linking hydrophobic sites selected from a coarse grain model. The resulting hollow tube assembly serves as a representation of a transmembrane channel, pore, or a carbon nanotube. The interactions of a coarse grain di-myristoyl-phosphatidyl-choline hydrated bilayer with both a purely hydrophobic tube and a tube with hydrophilic caps are studied. The hydrophobic tube rotates in the membrane and becomes blocked by lipid tails after a few tens of nanoseconds. The hydrophilic sites of the capped tube stabilize it by anchoring the tube in the lipid headgroup/water interfacial region of each membrane leaflet. The capped tube remains free of lipid tails. The capped tube spontaneously conducts coarse grain water sites; the free-energy profile of this process is calculated using three different methods and is compared to the barrier for water permeation through the lipid bilayer. Spontaneous tube insertion into an undisturbed lipid bilayer is also studied, which we reported briefly in a previous publication. The hydrophobic tube submerges into the membrane core in a carpetlike manner. The capped tube laterally fuses with the closest leaflet, and then, after plunging into the membrane interior, rotates to assume a transbilayer orientation. Two lipids become trapped at the end of the tube as it penetrates the membrane. The hydrophilic headgroups of these lipids associate with the lower tube cap and assist the tube in crossing the interior of the membrane. When the rotation is complete these lipids detach from the tube caps and fuse with the lower leaflet lipids.

Binding Sites↗

PubMed2004

Understanding nature's design for a nanosyringe.

Synthetic and natural peptide assemblies can possess transport or conductance activity across biomembranes through the formation of nanopores. The fundamental mechanisms of membrane insertion necessary for antimicrobial or synthetic pore formation are poorly understood. We observe a lipid-assisted mechanism for passive insertion into a model membrane from molecular dynamics simulations. The assembly used in the study, a generic nanotube functionalized with hydrophilic termini, is assisted in crossing the membrane core by transleaflet lipid flips. Lipid tails occlude a purely hydrophobic nanotube. The observed insertion mechanism requirements for hydrophobic-hydrophilic matching have implications for the design of synthetic channels and antibiotics.

Biosensing Techniques↗

PubMed2004

Transmembrane peptide-induced lipid sorting and mechanism of Lalpha-to-inverted phase transition using coarse-grain molecular dynamics.

Molecular dynamics results are presented for a coarse-grain model of 1,2-di-n-alkanoyl-sn-glycero-3-phosphocholine, water, and a capped cylindrical model of a transmembrane peptide. We first demonstrate that different alkanoyl-length lipids are miscible in the liquid-disordered lamellar (Lalpha) phase. The transmembrane peptide is constructed of hydrophobic sites with hydrophilic caps. The hydrophobic length of the peptide is smaller than the hydrophobic thickness of a bilayer consisting of an equal mixture of long and short alkanoyl tail lipids. When incorporated into the membrane, a meniscus forms in the vicinity of the peptide and the surrounding area is enriched in the short lipid. The meniscus region draws water into it. In the regions that are depleted of water, the bilayers can fuse. The lipid headgroups then rearrange to solvate the newly formed water pores, resulting in an inverted phase. This mechanism appears to be a viable pathway for the experimentally observed Lalpha-to-inverse hexagonal (HII) peptide-induced phase transition.

Computer Simulation↗