Structure and dynamics of lipids.
Hybrid lipid membranes and comparison of molecular dynamics simulations to neutron reflectivity
Collaborations:
D. J. Tobias, University of California Irvine, CA.
N. F. Berk, C. F. Majkrzak and S. Krueger, NCNR, NIST
A. L. Plant, Biotechnology division, NIST
Neutron reflectivity is a powerful technique for studying the composition and structure of layered films on a nanometer scale. On the other hand, molecular dynamics simulation is nowadays a standard research tool for studying complex systems. With the advance of computers and the development of new methodologies, simulations have become widely used to assist in the interpretation of experiments. The aim of the present studies is to show that molecular dynamics simulation can complement neutron reflectometry experiments, both by providing data against which models for reflectivity data reduction are scrutinized, and ultimately predict optimal experimental conditions before the experiments are carried out.
- Investigation of Hybrid Membranes with Neutron Reflectometry: Probing the Interactions of Melittin
- First-Principle Determination of Hybrid Bilayer First-Principle Determination of Hybrid Bilayer Membrane Structure by Phase Sensitive Neutron Reflectometry
- "Molecular Dynamics Study of Supported Phospholipid/Alkanethiol Bilayers on a Gold(111) surface"
Krueger S.; Meuse C.W.; Majkrzak C.F.; Dura J.A.; Berk N.F.; Tarek M. and
Plant A.L. Langmuir 17, 511-521 (2001)
Recent improvements in neutron reflectometry
methodology have afforded enhanced sensitivity for the study of biomimetic
membranes. The technique has been used to probe the interactions of the
peptide toxin, melittin, with supported bilayers of phospholipid and octadecanethiol
or thiahexa(ethylene oxide) alkane on gold. Improvements in instrumentation
and experimental design permit neutron reflectivity measurements out to
a wavevector transfer of 0.7 Å-1 and down to reflectivities approaching
10-8, allowing unprecedented resolution of structural details in the bilayer.
The data indicate that melittin strongly perturbs the phospholipid headgroup
region and also affects the alkane chain region of the bilayer. There is
no evidence for hydration of the ethylene oxide spacer region between the
gold and alkane regions of the thiahexa(ethylene oxide) alkane/phospholipid
bilayer, but a distinct shift of up to 3 Å in the apparent location
of the interface between the alkane and phospholipid regions is observed.
This work shows that the neutron reflectometry technique is now sensitive
to small changes in the reflected intensities, and these small changes
can result in significant contributions to the resultant scattering length
density profiles. ( includes comparison to MD results)
Majkrzak C.F., Berk N.F., Krueger S., Dura J.A., Tarek M., Tobias D.J., Silin V., Meuse C.W., Woodward J. and Plant A.L. Bioph. J. 79, 3330-3340 (2000)
The application of a new, phase sensitive
neutron reflectometry method to reveal the compositional depth profiles
of biologically relevant membranes is reported. The scattering length density
profiles so obtained is ensured to be unique and can therfore be directly
compared with that corresponding to the chemical compositional profile
of the film predicted by Molecular dynamics simulations. Measurements which
demonstrate the practical realization of this phase-sensitive technique
were performed on a hybrid bilayer membrane, consisting of a single functional
thickness, in intimate contact with a water reservoir. Results are compared
to atomistic molecular dynamics simulaitions performed on a similar hybrid
bilayer at 300K.
Tarek M.; Tu K.; Klein M.L. and Tobias D.J. Bioph. J. 77, 964-972 (1999)
Molecular dynamics simulations have been
used to investigate the structure of hybrid bilayers (HB) formed by dipalmitoylphosphatidylcholine
(DPPC) lipid monolayers adsorbed on a hydrophobic alkanethiol self-assembled
monolayer (SAM). The HB system was studied at 20°C and 60°C, and
the results were compared with recent neutron reflectivity measurements
(Meuse, C. W., S. Krueger, C. F. Majkrzak, J. A. Dura, J. Fu, J. T. Connor,
and A. L. Plant. 1998. Biophys. J. 74:1388) and previous simulations of
hydrated multilamellar bilayers (MLB) of DPPC (Tu, K., D. J. Tobias, and
M. L. Klein. 1995. Biophys. J. 69:2558; and 1996. 70:595). The overall
structures of the HBs are in very good agreement with experiment. The structure
of the SAM monolayer is hardly perturbed by the presence of the DPPC overlayer.
The DPPC layer presents characteristics very similar to the MLB gel phase
at low temperature and to the liquid crystal phase at high temperature.
Subtle changes have been found for the lipid/water interface of the HBs
compared to the MLBs. The average phosphatidylcholine headgroup orientation
is less disordered, and this produces changes in the electric properties
of the HB lipid/water interface. These changes are attributed to the fact
that the aqueous environment of the lipids in these unilamellar films is
different from that of MLB stacks. Finally, examination of the intramolecular
and whole-molecule dynamics of the DPPC molecules in the fluid phase HB
and MLB membranes revealed that the reorientations of the upper part of
the acyl chains (near the acyl ester linkage) are slower, the single molecule
protrusions are slightly damped, and the lateral rattling motions are significantly
reduced in the HB compared with the MLB.
Collaborations:
M.L. Klein, LRSM, University of Pennsylvania, PA
L. Koubi, Department of Chemistry, University of Pennsylvania, PA
D. Scharf, Department of Anesthesia, University of Pennsylvania, PA
It is widely accepted that cellular membrane
lipids participate in mediating anesthesia. One of the most intriguing
questions in this context is how lipid properties are modulated in the
presence of clinically relevant concentrations of general anhestetics (GAs)
. Another key question pertains to the distribution of anesthetics
in the membrane lipid bilayer. The available experimental tools to study
GAs in lipid bilayers vary from the non-specific experiments on partition
coefficients to detailed observations using various spectroscopic techniques.
Computer simualtion studies of anesthetic and non anesthetics interactions
with a well characterized model lipid bilayer are crucial if subtle modifications
to the bulk behavior are to be understood. We are using Molecular Dynamics
simulations aimed at supplementing the available experimental data
with molecular details.
- Membrane Structural Perturbations Caused by Anesthetics and Non-immobilizers : A Molecular Dynamics Investigation.
Koubi L.; Tarek M.; S. Bandyopadhyay; M.L. Klein and Scharf D. Bioph. J. in press (2001)
- The structural perturbations of the fully
hydrated dimyristoyl-phosphatidylcholine (DMPC) bilayer induced by
the presence of hexafluoroethane C2F6, a "non-immobilizer",
have been examined by molecular dynamics simulations and compared to the effects produced by halothane CF3CHBrCl, an "anesthetic", on a similar bilayer (DPPC) (Koubi et al., Biophys. J. 2000 78:800). We find that the overall structure of the lipid bilayer and the zwiterionic head-group dipole orientation undergo only a slight modification compared to the pure lipid bilayer, with virtually no change in the potential across the interface. This is in contrast to the anesthetic case where the presence of the molecule led to a large perturbation of the electrostatic potential across to the membrane interface. Similarly, the analysis of the structural and dynamical properties of the lipid core are unchanged in presence of the non-immobilizer while there is a substantial increase in the microscopic viscosity for the system containing the anesthetic. These contrasting perturbations of the lipid membrane caused by those quite similar sized molecules may explain the difference in their physiological effects as respectively anesthetics and non-immobilizers.
- Distribution of Halothane in a DPPC Bilayer from MD Calculations
Koubi L.; Tarek M.; Klein M.L. and Scharf D. Bioph. J. 78, 800-811 (2000)
- We report a 2-ns constant pressure molecular
dynamics simulation of halothane, at a mol raction 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
- Effects of Anesthetics on the Structure of a Phospholipid Bilayer; MD Investigation of Halothane in the Hydrated Liquid Crystal Phase of Dipalmitoylphosphatidylcholine
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.
Tu, K..; Tarek M.; Klein M.L. and Scharf D. Bioph. J. 75, 2123-2134 (1998)
We report the results of constant temperature
and pressure molecular dynamics calculations carried out on the liquid
crystal (L-alpha) 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 ppears
to be remimiscent of that of other small solutes (Bassolino-Klimas et al.
1995. J. Amer. Chem. Soc. 117:4118-4129).
** Current work:
Effects of non-anesthetic solutes
on lipid bilayer and comparison to anesthetics.
Effects on unsaturated lipids.
Collaborations:
M.L. Klein, LRSM, University of Pennsylvania, PA
S. Bandyopadhyay, Department of Chemistry, University of Pennsylvania
- Nature of Lipid-DNA Interactions: A Molecular Dynamics Study of DNA Intercalated into a Lipid Bilayer
Bandyopadhyay S.; Tarek M.; and Klein M.L. J. Phys. Chem. B, 103, 10075-10080 (1999)
Lipid-DNA complexes are of topical interest
because of their potential for use as vectors in gene therapy. Herein,
molecular dynamics simulations have been carried out to probe the nature
of Lipid-DNA interactions and thereby provide a complement to recent experimental
and theoretical studies. Specifically, we have investigated the DNA duplex
d(CCAACGTTGG)(2), in its canonical B-form, intercalated into a lipid bilayer
consisting of a neutralizing binary mixture of cationic (dimyristoyl-trimethylammonium-propane-DMTAP)
and zwitterionic (dimyristoyl phosphatidyl choline DMPC) lipids. Surprisingly,
both lipids are involved in neutralizing the anionic DNA phosphate groups.
The electrostatic interactions between the cationic trimethylammonium (TAP)
and zwitterionic phosphocholine (PC) headgroups of the two lipids allow
the PC headgroups to orient out of the bilayer plane and thereby also become
available to screen the negative charges on the DNA.
NIST Center for Neutron Research