PubMed Health⌕ Search

Biomedical subjects

W A Pangborn

Publications and source records attributed to W A Pangborn.

7 recordsLinked to original sources
PubMed1992

Molecular interactions in binary solids: crystal structure of a cholesteryl ester solid solution.

The x-ray crystal structure analysis of a cholesteryl ester solid solution, cholesteryl undecanoate/cholesteryl laurate in a molar ratio 0.52/0.48, is described. The unit cell is monoclinic with a = 13.005(2) A, b = 9.005(1) A, c = 31.421(3) A, and beta = 90.82(1) degrees and the space group P2(1) with Z = 4 (two molecules per asymmetric unit). Thus, the d001 spacing is almost the value predicted by Vegard's law from the values for the pure compounds. The room-temperature crystal structure is very much like that of cholesteryl laurate monolayer I packing, in the form where the esterified fatty acid chains are fully extended, with no salient sign of conformational disorder seen in this binary solid. The final R factor for 6571 observed reflections is 0.097.

Cholesterol Esters↗
PubMed1989

Molecular conformation and ion transport of cyclic and linear ionophores.

X-ray crystal structure determinations and energy-minimization techniques provide conformational data on the complexed and uncomplexed forms of ion transport antibiotics of the shuttle and channel types. In the solid state, hexadecaisoleucinomycin (HEXIL), an analogue of valinomycin, is observed as an asymmetric macrocycle stabilized by eight intramolecular (4----1) hydrogen bonds. The structure obtained from energy-minimization procedures exhibits a greater variation in phi and psi angles of chemically equivalent residues than does the crystallographically observed structure. The structure has eight carbonyl groups directed toward its interior and is capable of providing flexible coordination to a positively charged ion or molecule. These structural findings are consistent with the observed capacity of HEXIL to complex cesium ions, tetramethyl ammonium ions and acetylcholine. Gramicidin A is a pentadecapeptide that functions as a transmembrane channel for transporting monovalent cations. Uncomplexed gramicidin A crystallizes as a left-handed, antiparallel, double-stranded, helical dimer with 5.6 amino acid residues per turn. The helix has an overall length of 31 A and an average inner channel diameter of 4.8 A. The channel of this crystalline form does not contain ions or solvent molecules. Transporting ions through this channel could be achieved only by some expansion of the channel opening that would involve breaking and reforming hydrogen bonds that stabilize the double-stranded helix.

Gramicidin↗
PubMed1988

Polymorphic forms of 1,2-dipalmitoyl-sn-glycerol: a combined X-ray and electron diffraction study.

Quantitative crystallographic structure analyses are carried out for two polymorphic forms of 1,2-dipalmitoyl-sn-glycerol. A single crystal X-ray determination on the higher melting beta'L-form reveals that the hairpin conformer structure is essentially identical to that of the dilauroyl homolog reported earlier (I. Pascher, S. Sundell and H. Hauser (1981) J. Mol. Biol. 153, 791-806) with inclined acyl chain packing in the O perpendicular methylene subcell. Lamellar electron diffraction intensity data from epitaxially crystallized samples were used to determine the structure of the lower melting alpha L-form. The chains pack in the hexagonal subcell and are perpendicular to the lamellar surface. An appropriately oriented molecular model based on the beta'L-polymorph does not lead to a satisfactory structure solution but models based on the conformationally different 1,2-diglyceride moiety of several phospholipid structures does lead to a closer match to the observed diffraction data. In this proposed packing model for the alpha L-form, the hydroxyl oxygens are somewhat farther away from the unit cell origin than in the beta'L-form crystal structure, and, in combination with the different molecular conformation, this might explain the observed stability of this crystal polymorph against acyl shifts.

Crystallography↗
PubMed1983

Epitaxial crystallization of alkane chain lipids for electron diffraction analysis.

Thin microcrystals of a wide variety of polymethylene chain materials, including n-alkanes, linear waxes, glycerides, a detergent, phospholipids and phospholipid analogs based on cyclopentane-1,2,3-triol, are epitaxially grown on naphthalene to give an orientation with long chain axes parallel to the best developed crystal face. These crystals, which represent a different orientation than those grown from solution, facilitate ab initio quantitative crystal structure analysis from electron diffraction intensity data from the projection yielding the most crystallographic information.

Alkanes↗
PubMed1976

Studies on membrane fusion. II. Induction of fusion in pure phospholipid membranes by calcium ions and other divalent metals.

The effect of divalent metals on the interaction and mixing of membrane components in vesicles prepared from acidic phospholipids has been examined using freeze-fracture electron microscopy and differential scanning calorimetry. Ca2+, and to a certain extent Mg2+, induce extensive mixing of vesicle membrane components and drastic structural rearrangements to form new membranous structures. In contrast to the mixing of vesicle membrane components in the absence of Ca2+ described in the accompanying paper which occurs via diffusion of lipid molecules between vesicles, mixing of membrane components induced by Ca2+ or Mg2+ results from true fusion of entire vesicles. There appears to be a "threshold" concentration at which Ca2+ and Mg2+ become effective in inducing vesicle fusion and the threshold concentration varies for different acidic phospholipid species. Different phospholipids also vary markedly in their relative responsiveness to Ca2+ and Mg2+, with certain phospholipids being much more susceptible to fusion by Ca2+ than Mg2+. Vesicle fusion induced by divalent cations also requires that the lipids of the interacting membranes be in a "fluid" state (T greater than Tc). Fusion of vesicle membranes by Ca2+ and Mg2+ does not appear to be due to simple electrostatic charge neutralization. Rather the action of these cations in inducing fusion is related to their ability to induce isothermal phase transitions and phase separations in phospholipid membranes. It is suggested that under these conditions membranes become transiently susceptible to fusion as a result of changes in molecular packing and creation of new phase boundaries induced by Ca2+ (or Mg2+).

Calcium↗
PubMed1975

Lysolecithin and cholesterol interact stoichiometrically forming bimolecular lamellar structures in the presence of excess water, of lysolecithin or cholesterol.

The structural interaction of egg lysolecithin, derived from egg lecithin, and cholesterol in aqueous solution has been investigated using X-ray diffraction. When mixed in any proportions, either suspended in excess buffer or up to 85% lipid by dry weight, a separate lamellar phase containing equimolar proportions of lysolecithin and cholesterol forms, separate from excess water, or lysolecithin or cholesterol. The cholesterol disorders the crystalline chains of the lysolecithin. The equimolar phase is stable up to 50 degrees C unlike lysolecithin alone, which forms micelles, Thes results show that lysolecithin and cholesterol combine stoichiometrically in a stable complex. We propose as a structural model, that cholesterol fills the space of the missing fatty acyl chain making the lysolecithin more cylindrical rather than wedge shaped. This interaction could reduce both the lytic action of lysolecithin on membranes and its induction of cell fusion. It suggest another role of cholesterol in cell membranes: namely, to act as a stabilizer of bilayer structure by being a mobile component that can fill free volume in the hydrocarbon interior. Lysolecithin-cholesterol interaction may also be important in the early events of atherosclerosis where lysolecithin levels in vessel walls increase fivefold.

Binding Sites↗