Daphna Scharf

Distribution of Halothane in a Dipalmitoylphosphatidylcholine Bilayer from Molecular Dynamics Calculations

We report a 2-ns constant pressure molecular dynamics simulation of halothane, at a mol fraction of 50%, in the hydrated liquid crystal bilayer phase of dipalmitoylphosphatidylcholine. Halothane molecules are found to preferentially segregate to the upper part of the lipid acyl chains, with a maximum probability near the C(5) methylene groups. However, a finite probability is also observed along the tail region and across the methyl trough. Over 95% of the halothane molecules are located below the lipid carbonyl carbons, in agreement with photolabeling experiments. Halothane induces lateral expansion and a concomitant contraction in the bilayer thickness. A decrease in the acyl chain segment order parameters, S(CD), for the tail portion, and a slight increase for the upper portion compared to neat bilayers, are in agreement with several NMR studies on related systems. The decrease in S(CD) is attributed to a larger accessible volume per lipid in the tail region. Significant changes in the electric properties of the lipid bilayer result from the structural changes, which include a shift and broadening of the choline headgroup dipole (P-N) orientation distribution. Our findings reconcile apparent controversial conclusions from experiments on diverse lipid systems.

Effects of Anesthetics on the Structure of a Phospholipid Bilayer: Molecular Dynamics Investigation of Halothane in the Hydrated Liquid Crystal Phase of Dipalmitoylphosphatidylcholine

We report the results of constant temperature and pressure molecular dynamics calculations carried out on the liquid crystal (Lalpha) phase of dipalmitoylphosphatidylcholine with a mole fraction of 6.5% halothane (2-3 MAC). The present results are compared with previous simulations for pure dipalmitoylphosphatidylcholine under the same conditions (Tu et al., 1995. Biophys. J. 69:2558-2562) and with various experimental data. We have found subtle structural changes in the lipid bilayer in the presence of the anesthetic compared with the pure lipid bilayer: a small lateral expansion is accompanied by a modest contraction in the bilayer thickness. However, the overall increase in the system volume is found to be comparable to the molecular volume of the added anesthetic molecules. No significant change in the hydrocarbon chain conformations is apparent. The observed structural changes are in fair agreement with NMR data corresponding to low anesthetic concentrations. We have found that halothane exhibits no specific binding to the lipid headgroup or to the acyl chains. No evidence is obtained for preferential orientation of halothane molecules with respect to the lipid/water interface. The overall dynamics of the lipid-bound halothane molecules appears to be reminiscent of that of other small solutes (Bassolino-Klimas et al., 1995. J. Am. Chem. Soc. 117:4118-4129).

Path‐integral Monte Carlo studies of para‐hydrogen clusters

Path‐integral Monte Carlo calculations have been used to study para‐hydrogen clusters, (p‐H2)N, with N=13, 19, 33, and 34. Particular attention has been given to the low temperature structures and their evolution with increasing temperature. The structures of these quantum clusters have a clear relationship to their classical counterparts. In particular, the core of the N=19, 33, and 34 clusters is bipyramidal, in constrast to that of the N=13 cluster, which is icosahedral. At T∼2 K, these clusters exhibit a large propensity for exchanging particles, even without explicitly including exchange permutations (Bose statistics). This behavior has its origin in the large zero‐point motion of the very weakly bound small p‐H2 clusters.

A Designed Four-α-Helix Bundle That Binds the Volatile General Anesthetic Halothane with High Affinity

The structural features of volatile anesthetic binding sites on proteins are being examined with the use of a defined model system consisting of a four-alpha-helix bundle scaffold with a hydrophobic core. Previous work has suggested that introducing a cavity into the hydrophobic core improves anesthetic binding affinity. The more polarizable methionine side chain was substituted for a leucine, in an attempt to enhance the dispersion forces between the ligand and the protein. The resulting bundle variant has an improved affinity (K(d) = 0.20 +/- 0.01 mM) for halothane binding, compared with the leucine-containing bundle (K(d) = 0.69 +/- 0.06 mM). Photoaffinity labeling with (14)C-halothane reveals preferential labeling of the W15 residue in both peptides, supporting the view that fluorescence quenching by bound anesthetic reports both the binding energetics and the location of the ligand in the hydrophobic core. The rates of amide hydrogen exchange were similar for the two bundles, suggesting that differences in binding affinity were not due to changes in protein stability. Binding of halothane to both four-alpha-helix bundle proteins stabilized the native folded conformations. Molecular dynamics simulations of the bundles illustrate the existence of the hydrophobic core, containing both W15 residues. These results suggest that in addition to packing defects, enhanced dispersion forces may be important in providing higher affinity anesthetic binding sites. Alternatively, the effect of the methionine substitution on halothane binding energetics may reflect either improved access to the binding site or allosteric optimization of the dimensions of the binding pocket. Finally, preferential stabilization of folded protein conformations may represent a fundamental mechanism of inhaled anesthetic action.

Path‐integral Monte Carlo study of a lithium impurity in para‐hydrogen: Clusters and the bulk liquid

Simulation studies using the path‐integral formulation of quantum statistical mechanics are reported for single atomic lithium impurities in bulk liquid para‐hydrogen and in clusters, Li(p‐H2)n, with n=12, 13, 32, 33, and 34. Over the range of temperatures studied in the clusters (T=2.5–6.0 K), the lithium impurity is found to reside outside or at the surface of the clusters. Nevertheless, perturbations of the structure are observed in comparison to neat para‐hydrogen clusters. The solvation energy of the lithium in the bulk liquid and subcritical gas (T=14–25 K) is found to be slightly positive. In both the clusters and the liquid, the inhomogeneously broadened dipole spectrum of the lithium atom was calculated using the radial fast Fourier transform Lanczos method. In the clusters, the spectra exhibit a main absorption band near the unperturbed atomic Li value and a second, asymmetric band shifted to the blue. The latter can be identified as the p orbital oriented radially towards the cluster, while the...

Structure, effective pair potential and properties of halothane

Abstract Halothane, 2-bromo-2-chloro,1,1,1-trifluoroethane, is an important inhalation general anesthetic. We performed energy minimization within the Car-Parrinello scheme of density functional theory to obtain the detailed structure of a halothane molecule, in the gas phase. Then, effective model pair potentials for a flexible halothane molecule have been developed that describe halothane in solution around room temperature. The potential parameter were fitted to reproduce the known density at 298 K. Constant pressure and temperature molecularddynamics simulations were carried out at room temperature as well as at 310 K. The results are in excellent agreement with the available experimental data.

Nature of lithium trapping sites in the quantum solids para‐hydrogen and ortho‐deuterium

Quantum mechanical studies of a lithium impurity in solid para‐hydrogen and ortho‐deuterium have been performed using the path integral formulation of statistical mechanics. Since an isolated lithium atom is much larger than the host molecules, trapping sites consisting of from one to six vacancies have been investigated. Interestingly, all of the sites are comparable in energy. This is due to the large compressibility of para‐hydrogen and ortho‐deuterium solids, which permits the lattice to relax to comfortably accommodate the impurity. The inhomogeneously broadened dipole spectrum of the lithium impurity in the various sites was calculated using the radial fast Fourier transform Lanczos method and compared to experiments by Fajardo [J. Chem. Phys. 98, 110 (1993)]. Based on the present calculations, lithium atoms appear to occupy preferentially a three‐vacancy trapping site in para‐hydrogen while in ortho‐deuterium a four‐vacancy trapping site seems to be favored. Complementary variational Einstein model...

Isotope effect on the melting of para-hydrogen and ortho-deuterium clusters

Abstract Path integral Monte Carlo calculations have been used to investigate the isotope effect on the “melting” of small clusters of para-hydrogen ( p -H 2 ) and ortho-deuterium ( o -D 2 ). While clusters of ( o -D 2 ) N with N =13 and 55 exhibit a quantum-lipid→quantum-solid “freezing” transition in a temperature regime where the effects of Bose statistics are unimportant, the corresponding ( p -H 2 ) N clusters do not.

Effects of the nonimmobilizer hexafluroethane on the model membrane dimyristoylphosphatidylcholine.

Background Nonimmobilizers are agents that lack anesthetic properties, although their chemical structure is very similar to known anesthetics. The primary action site of both agents, whether at the membrane or target protein level, is still a matter of debate. However, increasing evidence points to the distinct modifications of the membrane physical properties that such agents induce. Such modification may play a role in the mechanism of anesthesia, and may therefore be related to the differences in their clinical behavior. Methods Molecular dynamics (MD) computer simulations have been used to investigate the distribution of a nonimmobilizer, hexafluroethane (HFE, C2F6), in a lipid membrane. The biologically relevant liquid-crystal phase of a hydrated dimyristoyl phosphatidyl choline (DMPC) bilayer was used as a membrane model. Two MD simulations corresponding to HFE mole fractions of 6% and 25% have been performed at room temperature and constant ambient pressure, for a duration of 2 nanoseconds each. Results The equilibrium configurations of HFE in the bilayer show that the nonimmobilizer molecules are evenly distributed along the lipid hydrocarbon chains with a slight preference for the bilayer center. This partitioning induces an expansion of the bilayer thickness and a lateral contraction of the membrane (decrease of the area per lipid). The presence of HFE has essentially no effect on the lipid acyl chain conformations in agreement with nuclear magnetic resonance (NMR) measurements of the chain order parameters. The partitioning of the nonimmobilizer does not influence the orientation of the lipid head-group dipole moment. Conclusions The modifications induced by the presence of the nonimmobilizer HFE on a model membrane are distinct from those previously found for halothane (CF3CHBrCl), its anesthetic analogue, and appear to result from different distributions in the lipid bilayer. The results of the MD simulations show that (1) the changes in the average area per lipid and in the membrane thickness are opposite for the two agents and (2) HFE induces no change in the lipid head-group orientation, in contrast to halothane. These different effects (1) on the physical properties of the lipid bilayer and (2) on the electrostatic properties of the membrane–water interface may be linked to different clinical effects, and thus might contribute to the mechanism of general anesthesia.

Molecular dynamics simulation of four-α-helix bundles that bind the anesthetic halothane

The mutation of a single leucine residue (L38) to methionine (M) is known experimentally to significantly increase the affinity of the synthetic four‐α‐helix bundle (Aα2)2 for the anesthetic halothane. We present a molecular dynamics study of the mutant (Aα2–L38M)2 peptide, which consists of a dimer of 62‐residue U‐shaped di‐α‐helical monomers assembled in an anti topology. A comparison between the simulation results and those obtained for the native (Aα2)2 peptide indicates that the overall secondary structure of the bundle is not affected by the mutation, but that the side chains within the monomers are better packed in the mutant structure. Unlike the native peptide, binding of a single halothane molecule to the hydrophobic core of (Aα2–L38M)2 deforms the helical nature of one monomer in a region close to the mutation site. Increased exposure of the cysteine side chain to the hydrophobic core in the mutant structure leads to the enhancement of the attractive interaction between halothane and this specific residue. Since the mutated residues are located outside the hydrophobic core the observed increased affinity for halothane appears to be an indirect effect of the mutation.