Testing High Concentrations of Membrane Active Antibiotic Chlorhexidine Via Computational Titration and Calorimetry

Coarse-grained strategies for membrane simulations are designed to increase efficiency for larger and more complex molecular dynamics simulations. For membrane active antibiotics, the concentration dependence of their action presents a tremendous challenge in simulation scale. In this study, we examine the effects of concentration for the popular membrane active antibacterial drug chlorhexidine. It presents an interesting biophysical modeling test, where from experimental experience we know that model membranes of 1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphocholine (DMPC) can absorb very high quantities of the drug without disruption. We construct a coarse-grained model of chlorhexidine in three different charged states and compare to previous all-atom simulations and new experiments. Using large, long-time, and unbiased simulations of chlorhexidine inserting into the lipid bilayer, we find little changes to the functional structure of a DMPC membrane up through concentrations of 15:100 drug:lipid, where the slowing rate of continued insertion tests the capabilities of even this coarse-grained approach. We validate our simulations with computational calorimetry measurements, and show that they agree with new experimental data from differential scanning calorimetry

Macrocyclic Oligoesters Incorporating a Cyclotetrasiloxane Ring

Macrocyclic oligoester structures based on a cyclotetrasiloxane core consisting of tricyclic (60+ atoms) and pentacycylic (130+ atoms) species were identified as the major components of a lipase-mediated transesterification reaction. Moderately hydrophobic solvents with log <i>P</i> values in the range of 2–3 were more suitable than those at lower or higher log <i>P</i> values. Temperature had little effect on total conversion and yield of the oligoester macrocycles, except when a reaction temperature of 100 °C was employed. At this temperature, the amount of the smaller macrocycle was greatly increased, but at the expense of the larger oligoester. For immobilized lipase B from Candida antarctica (N435), longer chain length esters and diols were more conducive to the synthesis of the macrocycles. Langmuir isotherms indicated that monolayers subjected to multiple compression/expansion cycles exhibited a reversible collapse mechanism different from that expected for linear polysiloxanes

Molecular Structures of Fluid Phosphatidylethanolamine Bilayers Obtained from Simulation-to-Experiment Comparisons and Experimental Scattering Density Profiles

Following our previous efforts in determining the structures of commonly used PC, PG, and PS bilayers, we continue our studies of fully hydrated, fluid phase PE bilayers. The newly designed parsing scheme for PE bilayers was based on extensive MD simulations, and is utilized in the SDP analysis of both X-ray and neutron (contrast varied) scattering measurements. Obtained experimental scattering form factors are directly compared to our simulation results, and can serve as a benchmark for future developed force fields. Among the evaluated structural parameters, namely, area per lipid <i>A</i>, overall bilayer thickness <i>D</i>_B, and hydrocarbon region thickness 2<i>D</i>_C, the PE bilayer response to changing temperature is similar to previously studied bilayers with different headgroups. On the other hand, the reduced hydration of PE headgroups, as well as the strong hydrogen bonding between PE headgroups, dramatically affects lateral packing within the bilayer. Despite sharing the same glycerol backbone, a markedly smaller area per lipid distinguishes PE from other bilayers (i.e., PC, PG, and PS) studied to date. Overall, our data are consistent with the notion that lipid headgroups govern bilayer packing, while hydrocarbon chains dominate the bilayer’s response to temperature changes

Tocopherol Activity Correlates with Its Location in a Membrane: A New Perspective on the Antioxidant Vitamin E

We show evidence of an antioxidant mechanism for vitamin E which correlates strongly with its physical location in a model lipid bilayer. These data address the overlooked problem of the physical distance between the vitamin’s reducing hydrogen and lipid acyl chain radicals. Our combined data from neutron diffraction, NMR, and UV spectroscopy experiments all suggest that reduction of reactive oxygen species and lipid radicals occurs specifically at the membrane’s hydrophobic–hydrophilic interface. The latter is possible when the acyl chain “snorkels” to the interface from the hydrocarbon matrix. Moreover, not all model lipids are equal in this regard, as indicated by the small differences in vitamin’s location. The present result is a clear example of the importance of lipid diversity in controlling the dynamic structural properties of biological membranes. Importantly, our results suggest that measurements of aToc oxidation kinetics, and its products, should be revisited by taking into consideration the physical properties of the membrane in which the vitamin resides

Comparison between the in-plane scans of DPPC-d62 bilayers with 32.5 mol% cholesterol using a) the conventional high energy resolution (small Δ<i>E</i>) setup and b) the low energy resolution (large Δ<i>E</i>) setup.

<p>The data are denoted by circles with the fit shown as a solid line. A disordered structure was observed in a), while the sharp features in b) are indicative of the presence of highly ordered lipid domains. A top view of the corresponding molecular structures are shown in the insets to the Figure using the same symbols as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066162#pone-0066162-g001" target="_blank">Figure 1</a> b); the quasi-Bragg reflections are indicated by vertical dashed lines and their associated Miller indices, [<i>hkl</i>]. Peaks resulting from the silicon substrates and the aluminum sample chamber (as described in the Materials and Methods Section) are highlighted in grey, but not accounted for in the fit.</p

Phase diagram of phospholipid/cholesterol complexes, such as DMPC/cholesterol and DPPC/cholesterol, as reported by, for example,

<p><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066162#pone.0066162-Vist1" target="_blank">[35]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066162#pone.0066162-Marsh1" target="_blank">[40]</a>. Besides the well known gel and fluid phases, the so-called liquid-order phase is observed at high cholesterol concentrations. The 32.5 mol% sample, as depicted by the ⊛, was determined to be in the <i>l_o</i> phase.</p

Geometry of the triple axis spectrometer.

<p>Orientation of the sample for in-plane scans, such that the scattering vector, <i>Q</i>, lies in the plane of the membrane (<i>q_||</i>). <i>k_i</i> and <i>k_f</i> are the incident and final neutron wave vectors (<i>k = </i>2<i>π = λ</i>) and the c’s denote the location of collimators along the beam line.</p

Area per molecule and partial areas for DPPC and cholesterol molecules as function of cholesterol concentration.

<p>Curves were calculated using Eq. (1) from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066162#pone.0066162-Edholm1" target="_blank">[60]</a>.</p

Schematics of the studied systems.

<p>a) Schematic of a lipid bilayer containing a lipid domain as studied by neutron scattering techniques using a low (top) and high (bottom) spatial resolution setup. b) In-plane representation of saturated hydrocarbon-chain lipid-cholesterol interactions in accordance with the umbrella model, whereby each lipid is associated with 2 cholesterol molecules. This structural arrangement results when the cholesterol content is 66 mol%. The blue squares represent lipid head groups, the yellow circles correspond to lipid tails, and the red circle are cholesterol molecules. c) Schematic molecular structures of DPPC and cholesterol molecules.</p

In-plane diffraction data and peak assignments.

<p>Data were taken with and without a PG filter, which was used to suppress higher order reflections.</p