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W Knoll

Publications and source records attributed to W Knoll.

At least 73 records · Page 4Linked to original sources
PubMed1988

Structural properties of a phosphatidylcholine-cholesterol system as studied by small-angle neutron scattering: ripple structure and phase diagram.

Small-angle neutron scattering has been used to study structural features of lamellar bilayer membranes of dimyristoylphosphatidylcholine (DMPC) and DMPC mixed with various amount of cholesterol. The studies were recorded at a fixed hydration level of 17% 2H2O, i.e. just below saturation. Bragg reflections gives information on the ripple structure and on the bilayer periodicity. The crystalline Lc phase, which was stabilized after long time storage at low temperature, exhibits major small angle scattering when cholesterol is mixed into the membrane. The intermediate P beta' gel-phase, which is characteristic by the rippled structure, is dramatically stabilized by the introduction of cholesterol. The ripple structure depends significantly both on the cholesterol content and on the temperature. At high temperatures, T greater than 15 degrees C, the inverse ripple periodicity varies basically linearly with cholesterol content, and approach zero (i.e. periodicity goes to infinite) at 20 mol% cholesterol, approximately. At lower temperatures the correlation is more complex. The data indicate additional phase boundaries below 2 mol% and at approx. 8 mol%. Secondary rippled structures are observed in the low temperature L beta'-phase for cholesterol content below approx. 8 mol%. The data gives detailed insight into the phosphatidylcholine cholesterol phase diagram, which is discussed on the basis of a simple model in which the cholesterol complexes are fixed to the defect stripes of the rippled structure.

Cholesterol↗
PubMed1988

[Diagnostic chorionic biopsies in the first trimester. Report of experiences over a 2 year period].

We report about 58 ultrasonographically guided transcervical chorionic villus biopsies from January 1985 to November 1987. Maternal age greater than 35 years (n = 28), followed by trisomy 21 or 18 (n = 10) were the mean indications. Biochemically evaluation of storage diseases (n = 6) and genomically DNA-analysis because of phenylketonuria (n = 2) were combined in each case with cytogenetic diagnosis. In the other cases certain indications were the reasons for biopsy. In 52 of 58 cases we were successful in biopsies and diagnoses. In the other 6 biopsy specimen we didn't found chorionic villi. 3 abortions we observed up to day 3 after operation (n = 3) and after 6 weeks (n = 1). Pathological findings were 1 trisomy 16, 1,47,XYY-karyotype and 1 embryo with phenylketonuria. Another reason for termination of pregnancy was male karyotype in a Morbus Duchenne-risk and 1 risk for Rett-syndrome. Meanwhile 30 healthy babies were born.

Adult↗
PubMed1986

Direct evidence for Ca++-induced lateral phase separation in black membranes of lipid mixtures by the analysis of gramicidin A single-channels.

Single-channel conductance fluctuations are analysed for gramicidin A incorporated into binary-mixed black lipid membranes of charged phosphatidic acid and neutral lecithin in different molar ratios. At very low Ca++ concentrations in the electrolyte (i.e. in the presence of EDTA) homogeneous lipid mixtures are identified through their conductance and life time probability distributions for integral gramicidin pores. As for the pure lipid components, the conductance histograms each show a single maximum with regular width and for all channels a single mean lifetime is found. For Ca++-levels (10(-6) - 10(-5) M) that are close to the critical demixing concentration (approximately 10(-4) M) unusually broad conductance distributions and reduced lifetimes are found provided the PC content, chi, of the membrane is close to the critical mixture (chi crit approximately equal to 0.5). We interpret this as a first example of the coupling of a membrane function (the transport of ions) to a lipid matrix with locally fluctuating composition close to a critical demixing point. For cCA++ = 10(-2) M the conductance histogram of gramicidin A in an equimolar mixture of PA and PC shows two well-separated maxima. A correlation analysis between conductance and lifetime of the single pores shows that the two channel populations also differ significantly in their mean channel lifetime, tau *. This finding is interpreted as being direct evidence for Ca++-induced lateral phase separation in black lipid membranes, as has been postulated recently.

Calcium↗
PubMed1985

Small-angle neutron scattering study of lateral phase separation in dimyristoylphosphatidylcholine-cholesterol mixed membranes.

The small-angle neutron scattering (SANS) technique developed previously is used to study the lateral phase separation in dimyristoylphosphatidylcholine (DMPC)-cholesterol mixed vesicles in the L alpha (35 degrees C) and L beta' (7 degrees C) phase of DMPC. To increase the sensitivity of the previous method, we apply the so-called inverse contrast variation technique where contrast matching is performed at a constant H2O/D2O ratio by varying the ratio of DMPC with deuterated and protonated hydrocarbon chains. Phase boundaries can be determined to an accuracy of +/- 0.5 mol %. In parallel experiments phase separation in the L beta' phase was also studied by freeze-fracture electron microscopy. For DMPC in the L alpha phase complete miscibility is clearly established up to cholesterol molar fractions of xc = 0.14. Strong evidence is provided that this is also the case up to xc approximately equal to 0.45. Cholesterol is no longer soluble above this limit and precipitates as small crystallites. For the L beta' phase (7 degrees C) phase boundaries are clearly established at xc1 = 0.08 and xc2 = 0.24, and very strong evidence is provided for two additional boundaries at xc3 = 0.435 and xc4 approximately equal to 1.0. At 0 less than or equal to xc less than or equal to xc1 the mixture forms a tilted solid solution in both the L beta' and P beta' phase while at xc1 less than or equal to xc less than or equal to xc2 this phase coexists with a nontilted mixture containing 24 mol % cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol↗
PubMed1985

Ca2+-induced phase separation in black lipid membranes and its effect on the transport of a hydrophobic ion.

Voltage jump-current relaxation studies have been performed with dipicrylamine-doped black membranes of binary lipid mixtures. As in the case of the carrier-mediated ion transport (Schmidt, G., Eibl, H. and Knoll, W. (1982) J. Membrane Biol. 70, 147-155) no evidence was found that the neutral lipid phosphatidylcholine (DPMPC) and the charged phosphatidic acid (DPMPA) are heterogeneously distributed in the membrane over the whole range of composition. However, besides a continuous dilution of the surface charges of DPMPA by the addition of DPMPC molecules, different structural properties of mixed membranes influence the kinetics of the dipicrylamine transport. The addition of Ca2+ to the electrolyte induces a lipid phase separation within the membrane into two fluid phases of distinctly different characteristics of the translocation of hydrophobic ions. Thus, it is possible to determine a preliminary composition phase diagram for the DPMPA/DPMPC mixtures as a function of the Ca2+ concentration.

Adsorption↗
PubMed1981

Small-angle neutron scattering study of lipid phase diagrams by the contrast variation method.

A small-angle neutron scattering (SANS) study of slightly sonicated liposomes of binary lipid mixtures is presented. It is demonstrated that the neutron scattering of lipid lamellae may be analyzed in terms of the Kratky--Porod model of scattering by two-dimensional systems. The contrast variation technique may thus be applied in order to study the structure and phase diagrams of lipid layers not disturbed by heavy sonication. The thickness of isolated bilayers is measured, and molar volumes of pure lipid phases are determined. Mixtures of deuterated dimyristoylphosphatidylcholine with (1) protonated dipalmitoylphosphatidylcholine and (2) protonated distearoylphosphatidylcholine, respectively, are studied. Excess volumes of lipid mixtures are determined by the contrast variation. For the first mixture positive excess volumes of +86 A3 in the crystalline phase (5 degrees C) and of +49 A3 in the fluid phase (35 degrees C) are obtained. These large positive excess volumes are interpreted in terms of free volume creation at the interface between the monolayers, which indicates that the polar head groups are rather fixed with respect to the lipid--water interface. We show that the phase boundaries at a given temperature may be determined by performing contrast variation experiments for two mixtures with different initial composition. Good agreement with existing experimental data is observed for the first mixture. A miscibility gap is established in the crystalline state of the second mixture. A most interesting result is the finding of an immiscibility in the fluid state. This is interpreted in terms of critical concentration fluctuations caused by the critical demixing point of the solid-state miscibility gap hidden below the liquidus line.

Deuterium↗
PubMed1977

Temperature-jump experiments on thin lipid membranes in the presence of valinomycin.

Temperature jump relaxation experiments on planar lipid membranes in the presence of valinomycin were performed using the absorption of a strong light flash as an energy source for the generation of the T-jump. The relaxation of the current carried by valinomycin/Rb+ complexes was measured. The results were interpreted on the basis of a transport model which was also analyzed by voltage jump relaxation experiments. The study shows that the application of the T-jump technique provides valuable information about transport kinetics as well as the dynamics of the membrane structure. At the given experimental conditions the relaxation of the current is believed to reflect a temperature-dependent transition of the membrane to a new conformational state of low order. The relaxation could be resolved with the present technique only at low temperatures and for membranes of high microviscosity.

Biological Transport↗
PubMed1976

Optical and electrical studies on dansyllysine-valinomycin in thin lipid membranes.

Dansyllysine-valinomycin, a fluorescent analogue of the ionophore valinomycin was synthesized and incorporated into black lipid membranes. Its concentration inside the membrane was measured fluorometrically and was also determined from electrical relaxation experiments, which were analyzed on the basis of a previously proposed carrier model. The results of both methods agreed within less than one order of magnitude. This appears satisfactory in view of the sources of error inherent in both procedures. A conductance increment per carrier molecule of about 3 - 10(-17) omega-1 was obtained for dansyllysine-valinomycin in diphytanoyllecithin membranes at 25 degrees C and 1 M RbCl in the aqueous phases. This is about 400 times smaller compared to unmodified valinomycin in monoolein membranes. The difference is mainly caused by the change in the membrane properties and to a smaller extent by the structural modification of the carrier.

Electric Conductivity↗