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Biomedical subjects

S I Chan

Publications and source records attributed to S I Chan.

At least 145 records · Page 8Linked to original sources
PubMed1977

More on the motional state of lipid bilayer membranes: interpretation of order parameters obtained from nuclear magnetic resonance experiments.

Proton and deuterium order parameters measured for the liquid crystalline phase of unsonicated lipid bilayer membranes are interpreted in terms of two motions: (i) chain reorientation and (ii) chain isomerization via kink diffusion. The observed order parameters are found to be compatible with angular deflections of the chain of about 50 degrees with respect to the bilayer normal, coupled with a probability of trans orientation of a methylene segment in the upper part of the chain of about 0.8-0.9. The motional model can be shown to account for the dynamic properties of the membrane system as measured by nuclear magnetic relaxation measurements, assuming that the chain isomerization occurs at a rate of approximately 10(10) s-1 and chain reorientation at a rate of approximately 10(7) s-1. Analysis of proton and deuterium line-width data in terms of this model shows that sonication has the effect of increasing the rate and amplitude of chain reorientation without substantially changing the isomerization motion along the acyl chain. These conclusions are briefly compared with similar observations recently reported in Raman spectroscopic studies.

Lipids↗
PubMed1977

Effect of lysolecithin on the structure and permeability of lecithin bilayer vesicles.

In order to elucidate the role of lysolecithin in membranes, we have examined the effect of lysolecithin on the structure and permeability of lecithin bilayer membranes. Small L-alpha-dimyristoyllecithin (DML) vesicles with myristoyllysolecithin (MLL) incorporated as well as small L-alpha-dipalmitoyllecithin (DPL) vesicles with palmitoyllysolecithin (PLL) were studied by nuclear magnetic resonance (NMR) methods at temperatures both above and below the alpha-gel in equilibrium liquid crystalline phase transition temperature (Tc) and as a function of the concentration of the incorporated lysolecithin. Europium (III) ion was used as a probe to measure the permeability of the vesicular bilayer membrane. At temperatures below Tc, these vesicles were found to be extremely permeable to europium (III) ions. The ion translocation was found to be too fast to be measured by the NMR method under these conditions. However, above the phase transition temperature the ionic permeability decreases to a rate which could be conveniently monitored, and the permeability was shown to increase with temperature and lysolecithin concentration. Analysis of the lysolecithin concentration dependence suggests the formation of ion channels within the lipid bilayer involving four lysolecithin molecules. The data below Tc suggest a phase separation below the phase transition temperature of the host lipid, leading to the formation of patches of lysolecithin molecules within the lecithin matrix. These lysolecithin clusters are presumably long-lived under these conditions and are sufficiently structurally perturbed or disordered to serve as channels for rapid ion permeation.

Kinetics↗
PubMed1977

X-ray absorption edge studies on oxidized and reduced cytochrome c oxidase.

The x-ray absorption edge spectra of the Cu and Fe-centers in oxidized and reduced cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase: EC 1.9.3.1) have been obtained using synchrotron radiation from the SPEAR storage ring at the Stanford Linear Accelerator Center. In addition, oxidized and reduced plastocyanin as well as a number of model copper compounds in various oxidation states were also examined. A comparison of the absorption edge fine structure of cytochrome oxidase with those of the models indicates that one of the two coopers in the oxidized protein is in the +1 oxidation state. Upon reduction of the protein with dithionite, the second copper becomes Cu(I). The shift in the Fe K-edge of cytochrome oxidase upon reduction is small (about 2 e V or 3 times 10(-19 J) and is comparable to that previously observed for the reduction of the heme iron of cytochrome c.

Animals↗
PubMed1977

The secondary structure of histone IV and its stability.

The secondary structure within histone IV and its fragments obtained by cyanogen bromide (CNBr) and cleavage at Met 84 has been examined by circular dichroism and spectophotometric pH titration measurements. These studies have confirmed the existence of stable secondary structure within the C-terminal fragment of histone IV (C-peptide which can be perturbed only by 6M urea at pH greater than 8 or 8 M guanidine-HCL. In contrast, the N-terminal fragment (N-peptide) appears to lack significant secondary structure at low ionic strengths but acquires approximately 15% betasheet conformation and 5% alpha-helix upon aggregation at ionic strengths larger than or equal to 0.4. The rates of nitration of the N- and C-peptides by tetranitromethane (TNM) have also been measured as a function of ionic strengths. Under comparable conditions, the rate constant for nitration of the N-peptide was found to be about six times greater than that for the C-peptide, further evidence in support of the presence of stable secondary structure within the C-terminal region of histone IV. After binding these histone IV fragments to DNA, however, the nitration reaction rate constants for the N- and C-peptide in the bound form are found to be 2% and 27% of the corresponding free peptides. Reconstituted nucleohistone IV is about 10% as reactive to TNM as histone IV at comparable ionic strength.

Amino Acids↗
PubMed1976

The formation and annealing of structural defects in lipid bilayer vesicles.

It is shown that sonication of phospholipid-water dispersions below the crystalline leads to liquid crystalline phase transition temperature (Tc) produces bilayer vesicles with structural defects within the bilayer membrane, which permit rapid permeation of ions and catalyze vesicle-vesicle fusion. These structural defects are annihilated simply by annealing the vesicle suspension above Tc. The rate of annealing was found to be slow, of the order of an hour for T = 3 degrees C above Tc, but annealing is complete within 10 min for T = 10 degrees C above Tc. It is proposed that these structural defects are fault-dislocations in the bilayer structure, which arise from a population defect in the distribution of the lipid molecules between the outer and inner monolayers, when small bilayer fragments reassemble to form the small bilayer vesicles during the sonication procedure. Such a population defect can only be remedied by lipid transport via the inside in equilibrium outside flip-flop mechanism, which would account for the slow kinetics of annealing observed even at 3 degrees C above the phase transition.

Binding Sites↗
PubMed1976

Voltage-induced formation of alamethicin pores in lecithin bilayer vesicles.

The interaction of alamethicin with lecithin bilayer vesicles in the presence of a transmembrane potential difference has been studied by proton magnetic resonance spectroscopy. Asymmetrical vesicles with alamethicin trapped within the intravesicular compartment were prepared and the formation of ion channels, were then triggered by imposing a suitable potassium ion gradient across the bilayer membrane in the presence of valinomycin. These alamethicin channels were found to permit the outward passage of europium ions into the extravesicular medium when the transmembrane potential is more positive for the inner compartment.

Alamethicin↗
PubMed1975

Nuclear magnetic resonance studies of histone IV solution conformation.

The 220-MHz high-resolution proton magnetic resonance (PMR) spectrum of histone IV has been examined as a function of histone concentration, salt concentration, and pD. The hydrophobic C-terminal portion of the histone IV monomer appears to be largely PMR "invisible" indicating that this region of the polypeptide contains rigid secondary structure. Further loss of PMR resonance areas with increased histone IV concentration in neat D2O has been attributed to self-aggregation involving a monomer-dimer equilibrium. An equilibrium between the monomer and large aggregates, on the other hand, appears to dominate at NaCl concentrations above 0.01 M. pD studies reveal an abrupt increase in histone IV aggregation at pD smaller than 0.8 and precipitation of histone IV at pD values in the neighborhood of its isoelectric point, pD similar to 11.

Animals↗
PubMed1975

Nuclear magnetic resonance studies on the solution conformation of histone IV fragments obtained by cyanogen bromide cleavage.

Two histone IV fragments obtained by cleavage at Met-84 by cyanogen bromide have been examined by proton magnetic resonance (PMR) spectroscopy as a function of temperature, peptide concentration, ionic strength, and pD. Sedimentation and gel electrophoresis studies on these peptides have also been carried out. The 220-MHz PMR spectrum of the N-peptide in both the high- and low-field regions was shown to be almost identical with that calculated for an extended coil N-peptide monomer. The calculated random coil and experimental C-peptide spectra, on the other hand, differ in many respects. Evidence was obtained for the presence of rigid secondary structure in the C-peptide. In addition, the Val, Leu, Ile CH3 resonance displays a prominent high-field satellite band which shifts downfield with increasing temperature. Sedimentation studies on the N-peptide reveal the formation of extremely large, remarkably homogeneous aggregates at ionic strengths larger than or equal to 0.01. The C-peptide, on the other hand, does not appear to form aggregates of sufficient size to be detectable in velocity sedimentation studies of a few hours duration. The relative area changes which have previously been noted in the PMR spectrum of histone IV with increasing ionic strength were also observed for the N-peptide but not the C-peptide. Interpretation of these relative area changes has been made in terms of the amino acid sequence of histone IV, and an effort was made to identify that segment of the polypeptide which undergoes secondary structural change with increasing ionic strength.

Amino Acids↗
PubMed1975

The interpretation of proton magnetic resonance linewidths for lecithin dispersions. Effect of particle size and chain packing.

Two previously reported theoretical treatments of the effect of sonication on the PMR spectrum of phospholipid bilayer membranes have led to divergent conclusions regarding the effects of sonication on the structure of the bilayer membrane. In this report these two theoretical treatments will be critically reviewed, and it will be shown that only the theory of Seiter and Chan (Seiter, C.H.A. and Chan, S.I. (1973) J. Am. Chem. Soc. 95, 7541-7553) yields predictions which are in agreement with experiment. Analysis of available and newly acquired NMR results for sonicated bilayer vesicles of different sizes, both above and below the thermal phase transition, indicates that sonication does disrupt the regular molecular packing of the phospholipid molecules in these systems.

Binding Sites↗
PubMed1975

Alamethicin-mediated fusion of lecithin vesicles.

It was recently shown that alamethicin greatly facilitates the fusion of small, sonicated, lecithin bilayer vesicles. In the present work the details of this fusion process have been followed by monitoring the inner and outer choline methyl signals separately by proton magnetic resonance spectroscopy. It is shown that during the alamethicine-induced fusion some of the antibiotic molecules become translocated from the extravesicular aqueous medium into the enclosed intravesicular space, and these alamethicine molecules were found to affect the choline methyl signals from the inner half of the bilayer only. No evidence was obtained for transmembrane coupling of the two halves of the bilayer in the presence of alamethicin or for any effects that might be construed as due to incorporation of alamethicin molecules into the hydrophobic core of the bilayer.

Alamethicin↗