Biomedical subjects

Lai Ding

Publications and source records attributed to Lai Ding.

5 recordsLinked to original sources

PubMed2005

Distorted hexagonal phase studied by neutron diffraction: lipid components demixed in a bent monolayer.

The recent discovery of a distorted hexagonal phase in 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine/1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPE/DOPC) mixtures raised the intriguing question as to whether lipid mixtures demix in a bent monolayer. We performed neutron diffraction on a mixture of headgroup deuterated DOPC-d(13) and nondeuterated DOPE to study the lipid distribution in the distorted hexagonal phase. The 1:1 lipid mixture in full hydration and 25 degrees C was in a homogeneous lamellar phase. Upon dehydration the mixture transformed to a rhombohedral phase, then to a distorted inverted hexagonal phase, and finally to a regular inverted hexagonal phase. In the distorted hexagonal phase, the diffraction pattern showed a two-dimensional monoclinic lattice with two reciprocal vectors of equal length (1.5 nm(-)(1)) forming an angle 53 degrees between them. Diffraction intensities measured while varying the D(2)O/H(2)O ratio in the humidity was used to solve the phase problem. The neutron scattering length density distribution of the distorted hexagonal phase was constructed. The constant density contours are approximately elliptical. The difference in the eccentricities of the contours between the water and lipid distributions indicates that the DOPE/DOPC ratio is not uniform around the elliptical lipid tube in the unit cell. DOPE is preferentially distributed at the vertex regions where the curvature is the highest. Thus for the first time it is shown that when a monolayer of a homogeneous lipid mixture is bent, the lipid components are partially demixed in reaching the free energy minimum.

Lipids↗

PubMed2004

Diffraction techniques for nonlamellar phases of phospholipids.

A neutron diffraction method applicable to nonlamellar phases of substrate-supported lipid membranes is described and validated. When prepared on a flat substrate, the resulting nonlamellar phases have layered symmetry which provides some advantages over powder diffraction for detailed structure determination. This approach recently led to the detection of a rhombohedral phase and a distorted hexagonal phase of lipids. Here the determination of intensity and phase information for such phases is demonstrated by application to the hexagonal phase of diphytanoyl phosphatidylcholine (DPhPC). The hexagonal symmetry is used to verify the data reduction procedure for the intensities of the diffraction peaks. Diffraction intensities measured while varying the D2O/H2O ratio in the relative humidity was used to solve the phase problem. The neutron scattering length density distribution of the hexagonal phase was constructed and analyzed to elucidate the packing of the lipid molecules. The structure of DPhPC in the hexagonal phase is of interest in connection with its stalk structure in the rhombohedral phase. We also found that the incorporation of tetradecane into the DPhPC hexagonal phase is limited, similar to the case for dioleoyl phosphatidylethanolamine.

Neutron Diffraction↗

PubMed2003

Interaction of antimicrobial peptides with lipopolysaccharides.

We study the interaction of antimicrobial peptides with lipopolysaccharide (LPS) bilayers to understand how antimicrobial peptides interact with the LPS monolayer on the outer membrane of Gram-negative bacteria. LPS in water spontaneously forms a multilamellar structure composed of symmetric bilayers. We performed X-ray lamellar diffraction and wide-angle in-plane scattering to study the physical characteristics of LPS multilayers. The multilayer alignment of LPS is comparable to phospholipids. Thus, it is suitable for the application of oriented circular dichroism (OCD) to study the state of peptides in LPS bilayers. At high hydration levels, the chain melting temperature in multilamella detected by X-ray diffraction is the same as that of LPS aqueous dispersions, as measured by calorimetry. LPS has a strong CD, but with a careful subtraction of the lipid background, the OCD of peptides in LPS is measurable. The method was tested successfully with melittin. It was then applied to two representative antimicrobial peptides, magainin and protegrin. At peptide concentrations comparable to the physiological conditions, both peptides penetrate transmembrane in LPS bilayers. The results imply that antimicrobial peptides readily penetrate the LPS monolayer of the outer membrane.

Antimicrobial Cationic Peptides↗

PubMed2003

New phases of phospholipids and implications to the membrane fusion problem.

Membrane fusion is a ubiquitous process in eukaryotic cells. When two membranes fuse, lipid must undergo molecular rearrangements at the point of merging. To understand how lipid structure transitions occur, scientists studied the phase transition of lipid between the lamellar (L(alpha)) phase and the inverted hexagonal (H(II)) phase, based on the idea that lipid must undergo a similar rearrangement as in fusion. However, previous investigations on the system of dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE) did not reveal intermediate phases between the L(alpha) and H(II) phases. Recently, we found a rhombohedral phase of diphytanoylphosphatidylcholine between its L(alpha) and H(II) phases using substrate-supported samples. Here we report the observation of two new phases in the DOPC-DOPE system: a rhombohedral phase and a distorted hexagonal phase. The rhombohedral phase confirms the stalk hypothesis for the L(alpha)-H(II) transition, but the phase of stable stalks exists only for a certain range of spontaneous curvature. The distorted hexagonal phase exists only in a lipid mixture. It implies that lipids may demix to adjust its local spontaneous curvature in order to achieve energy minimum under stress.

Chemistry, Physical↗

PubMed2002

Two states of cyclic antimicrobial peptide RTD-1 in lipid bilayers.

RTD-1 is a recently discovered cyclic peptide that, like other well-studied antimicrobial peptides, appears to bind to the lipid matrix of cell membrane in the initial stage of activity. We studied the states of RTD-1 bound to lipid bilayers by two methods: oriented circular dichroism and X-ray diffraction. RTD-1 shows two physically distinct bound states in lipid bilayers like magainins, protegrins, alamethicin, and melittin that were previously studied. However, the nature of transition between the two states is different for RTD-1 as compared with the aforementioned peptides. In one of the two states, RTD-1 is oriented with its backbone ring parallel to the plane of the bilayer. Only in this state RTD-1 induces membrane thinning. But the effect of membrane thinning is much weaker than all other peptides, suggesting that the mechanism of RTD-1 may be different from the other peptides.

Anti-Bacterial Agents↗