The crystal and molecular structure of VH amylose by electron diffraction analysis

The crystal and molecular structures of VH amylose were determined by a constrained linked-atom least-squares refinement, utilizing intensities measured from electron diffraction patterns and stereochemical restraints. Hexagonal platelet single crystals were grown from dilute aqueous ethanol solution and their electron diffraction diagrams analysed. These data indicated that the amylose chains were crystallized in a hexagonal lattice with a = b = 13.65 A, c (chain axis) = 8.05 A and space group P6(5)22. The best model obtained using the base plane data coupled with a stereochemical refinement yielded R = 0.24 (R' = 0.25). It corresponded to a system of left-handed 6-fold helices packed on an hexagonal net but with statistically random up/down chain disorder. A column of six water molecules was present within each helical repeat. Additionally, the gap between each pair of adjacent helices was bridged by two water molecules positioned so as to allow hydrogen bonding with chains of either sense. This proposed crystal structure differs somewhat from previous reports which invoked orthorhombic lattices and requires a regularly alternating arrangement of up and down chains to account for the intensity. Suggestions are made to account for these differences.     

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.     

Water-silica gel interactions,X-ray diffraction study at room and low temperature

X-ray diffraction experiments on water confined in silica gel powder hydrated at about 20% are presented and analyzed at room temperature and down to 77 K. The structural modification of confined water observed at second neighbors is due to the competition between the confinement effect and the water-silica interaction.     

Characterization of the sub-transition of hydrated dipalmitoylphosphatidylcholine bilayers: X-ray diffraction study

The structural changes accompanying the recently described sub-transition of hydrated dipalmitoylphosphatidylcholine (Chen, S.C., Sturtevant, J.M. and Gaffney, B.J. (1980) Proc. Natl. Acad. Sci. USA 77, 5060–5063) have been defined using X-ray diffraction methods. Following prolonged storage at ?4°C the usual L_β′ gel form of hydrated dipalmitoylphosphatidylcholine (DPPC) is converted into a more ordered stable ‘crystal’ form. The bilayer periodicity is 59.1 Å and the most striking feature is the presence of a number of X-ray reflections in the wide angle region. The most prominent of these are a sharp reflection at $\text{1}\text{4.4}\text{A}\text{?}{}^{\mathrm{?1}}$ and a broader reflection at $\text{1}\text{3.9}\text{A}\text{?}{}^{\mathrm{?1}}$. This diffraction pattern is indicative of more ordered molecular and hydrocarbon chain packing modes in this low temperature ‘crystal’ bilayer form. At the sub-transition (T_rmsub = 15–20°C) an increase in the bilayer periodicity occurs ($\text{d=63.6}\text{A}\text{?}$) and a strong reflection at approx. $\text{1}\text{4.2}\text{A}\text{?}{}^{\mathrm{?1}}$ with a shoulder at approx. $\text{1}\text{4.1}\text{A}\text{?}{}^{\mathrm{?1}}$ is observed. This diffraction pattern is identical to that of the bilayer gel (L_β′) form of hydrated DPPC. Thus, the sub-transition corresponds to a bilayer ‘crystal’ → bilayer L_β′ gel structural rearrangement accompanied by a decrease in the lateral hydrocarbon chain interactions. Differential scanning calorimetry and X-ray diffraction show that on further heating the usual structural changes L_β′ → P_β′ and P_β′ → L_α occur at the pre- and main transitions, at approx. 35°C and 41°C, respectively.     

Surface structure of baboon (Papio anubis) hydrated articular cartilage: study of low temperature replicas by transmission electron microscopy

Platinum/carbon replicas of the natural articular surfaces of the femoral condyles of normal adult baboons (Papio anubis) were made at approximately -130 degrees C. Washed surfaces displayed arrays of approximately 50-200 nm diameter collagen fibers and low parallel ridges approximately 5 micron apart. Untouched surfaces were often obscured by a featureless lamina. The ultrastructure of the bearing surface of baboon articular cartilage resembles that of quadrupeds such as the dog.     

Constant pressure and temperature molecular dynamics simulation of a fully hydrated liquid crystal phase dipalmitoylphosphatidylcholine bilayer.

We report a constant pressure and temperature molecular dynamics simulation of a fully hydrated liquid crystal (L alpha) phase bilayer of dipalmitoylphosphatidylcholine at 50 degrees C and 28 water molecules/lipid. We have shown that the bilayer is stable throughout the 1550-ps simulation and have demonstrated convergence of the system dimensions. Several important aspects of the bilayer structure have been investigated and compared favorably with experimental results. For example, the average positions of specific carbon atoms along the bilayer normal agree well with neutron diffraction data, and the electron density profile is in accord with x-ray diffraction results. The hydrocarbon chain deuterium order parameters agree reasonably well with NMR results for the middles of the chains, but the simulation predicts too much order at the chain ends. In spite of the deviations in the order parameters, the hydrocarbon chain packing density appears to be essentially correct, inasmuch as the area/lipid and bilayer thickness are in agreement with the most refined experimental estimates. The deuterium order parameters for the glycerol and choline groups, as well as the phosphorus chemical shift anisotropy, are in qualitative agreement with those extracted from NMR measurements.     

Raman microscopy and X-ray diffraction,a combined study of fibrillin-rich microfibrillar elasticity

Fibrillin-rich microfibrils are essential elastic structures contained within the extracellular matrix of a wide variety of connective tissues. Microfibrils are characterized as beaded filamentous structures with a variable axial periodicity (average 56 nm in the untensioned state); however, the basis of their elasticity remains unknown. This study used a combination of small angle x-ray scattering and Raman microscopy to investigate further the packing of microfibrils within the intact tissue and to determine the role of molecular reorganization in the elasticity of these microfibrils. The application of relatively small strains produced no overall change in either molecular or macromolecular microfibrillar structure. In contrast, the application of larger tissue extensions (up to 150%) resulted in a markedly different structure, as observed by both Raman microscopy and small angle x-ray scattering. These changes occurred at different levels of architecture and are interpreted as ranging from alterations in peptide bond conformation to domain rearrangement. This study demonstrates the importance of molecular elasticity in the mechanical properties of fibrillin-rich microfibrils in the intact tissue.     

The projection structure of the low temperature K intermediate of the bacteriorhodopsin photocycle determined by electron diffraction

Bacteriorhodopsin (bR) is an integral membrane protein which absorbs visible light and pumps protons across the cell membrane of Halobacterium salinarium. bR is one of the few membrane-bound pumps whose structure is known at atomic resolution. Changes in the protein structure of bR are a crucial element in the mechanism of proton pumping and can be followed by a variety of spectroscopic, and diffraction methods. A number of intermediates in the photocycle have been identified spectroscopically and a number of laboratories have been successful in reporting the structural changes taking place in the later stages of the photocycle over the millisecond time-scale using diffraction techniques. These studies have revealed significant changes in the protein structure, possibly involving changes in flexibility and/or movement of helices. Earlier intermediates which arise and decay on the picosecond to microsecond time-scale have proven more difficult to trap. Here, we report for the first time the successful trapping and diffraction analysis of bR in a low temperature state resembling the very early intermediate, K. We have calculated a projection difference map to 3.5 A resolution. The map reveals no significant structural changes in the molecule, despite having a very low background noise level. This does not rule out the possibility of movements in a direction perpendicular to the plane of the membrane. However, the data are consistent with other evidence that significant structural changes do not occur in the protein itself.     

Time-resolved x-ray diffraction and calorimetric studies at low scan rates: II. On the fine structure of the phase transitions in hydrated dipalmitoylphosphatidylethanolamine

The phase transitions of dipalmitoylphosphatidylethanolamine (DPPE) in excess water have been examined by low-angle time-resolved x-ray diffraction and calorimetry at low scan rates. The lamellar subgel/lamellar liquid-crystalline (L_c → L_α), lamellar gel/lamellar liquid-crystalline (L_β → L_α), and lamellar liquid-crystalline/lamellar gel (L_α → L_β) phase transitions proceed via coexistence of the initial and final phases with no detectable intermediates at scan rates 0.1 and 0.5°C/min. At constant temperature within the region of the L_β → L_α transition the ratio of the two coexisting phases was found to be stable for over 30 min. The state of stable phase coexistence was preceded by a 150-s relaxation taking place at constant temperature after termination of the heating scan in the transition region. While no intermediate structures were present in the coexistence region, a well reproducible multipeak pattern, with at least four prominent heat capacity peaks separated in temperature by 0.4-0.5°C, has been observed in the cooling transition (L_α → L_β) by calorimetry. The multipeak pattern became distinct with an increase of incubation time in the liquid-crystalline phase. It was also clearly resolved in the x-ray diffraction intensity versus temperature plots recorded at slow cooling rates. These data suggest that the equilibrium state of the L_α phase of hydrated DPPE is represented by a mixture of domains that differ in thermal behavior, but cannot be distinguished structurally by x-ray scattering.     

X-ray diffraction and electron microscope studies on the structure of bacterial F pili.

F pili are hollow cylinders with 80 Å outer diameter and 20 Å inner diameter. Both X-ray fibre diffraction and optical diffraction of electron micrographs show a strong layer-line corresponding to a spacing of 32 Å, to which a J₄ Bessel function is assigned on the basis of the optical diffraction. X-ray diffraction patterns show near-meridional intensity on a layer-line corresponding to a spacing of 12.8 Å, to which a J₁ Bessel function is assigned. Mass per length measurements on unstained specimens in the scanning transmission electron microscope give 3000 daltons/Å, indicating that the 11,200 dalton pilin subunits are 3.7 Å apart along the axial direction of the pili. These observations show that the pilus structure can be represented as four coaxial helices of pitch 128 Å with the pilin subunits elongated and overlapping along the line of these helices. Each of these helices of subunits is translated axially with respect to its neighbour, to give a basic helix of 3.6 units per turn of 12.8 Å pitch. Radial electron density calculations indicate a 50 Å diameter girdle of hydrophobic amino acids between the inner and outer diameters of the protein shell. A molecular model of the structure at low resolution is presented.     

A new crystal form of tropomyosin. Preliminary X-ray diffraction analysis

A new crystalline form of tropomyosin has been produced that diffracts to about 4 A resolution. The crystals are grown at room temperature by slowly lowering the concentration of spermine. This polyamine apparently neutralizes the acidic amino acid side-chains of tropomyosin and allows close side-by-side packing of molecules. The space group is C2, with unit cell dimensions a = 259.7 A, b = 55.3 A, c = 135.6 A, and beta = 97.2 degrees. The tropomyosin molecules appear to be bonded head-to-tail to form straight filaments that run along the crystallographic (332) direction in an arrangement closely related to thin crystalline sheets previously described.     

Unique molecular conformation of aureobasidin A, a highly amide N-methylated cyclic depsipeptide with potent antifungal activity: X-ray crystal structure and molecular modeling studies.

A structural feature of aureobasidins, cyclic depsipeptide antibiotics produced by Aureobasidium pullulans R106, is the N-methylation of four out of seven amide bonds. In order to investigate possible relationship between the molecular conformation and the amide N-methylation, aureobasidin A (AbA), which exhibits the potent antifungal activity, was subjected to X-ray crystal analysis. The crystal, recrystallized from ether (orthorhombic, space group P2(1)2(1)2(1), a = 21.643 (3) A, b = 49.865(10) A, c = 12.427 (1) A, z= 8), contained two independent conformers per asymmetric unit and they took on a similar arrowhead-like conformation. The conformation consisted of three secondary structures of antiparallel beta-sheet, and beta- and gamma-turns, and was stabilized by three intramolecular and transannular N-H O=C hydrogen bonds. The beta-hydroxy-N-methyl-l-valine residue, which is indispensable for its bioactivity, was located at the tip of the corner. Since a nearly identical conformation has been observed for aureobasidin E, a related cyclic depsipeptide, this arrowhead-like conformation may be energetically stable and important for biological activity. The contribution of the amide N-methylation to the conformation was investigated by model building and energy calculations. The energy-minimizations of AbA analogs, in which some (one to four) of four N-methylated amide bonds were replaced with usual amide bond, led to some conformers which are fairly different from the arrowhead form of AbA, although they are stabilized by three intramolecular N-H...O=C hydrogen bonds. This result explains the reason why four out of the seven amide bonds have to be methylated to manifest biological activity, i.e. the high N-methylation of aureobasidin is necessary to form only one well-defined conformation.     

The structure of DNA-DLPC-cationic gemini surfactant aggregates: a small angle synchrotron X-ray diffraction study

The structure of aggregates formed by interaction of DNA with unilamellar dilauroylphosphatidylcholine (DLPC) vesicles (DNA:DLPC=1:1 base/mol) in the presence of gemini surfactant butane-1,4-diyl-bis(dodecyldimethylammonium bromide) (C12GS) was investigated using synchrotron small angle X-ray diffraction. In the concentration range C12GS+:DLPC< or =1 mol/mol, a condensed lamellar Lalphac phase was found with a repeat period of lipid bilayer stacking in the range d approximately 5.70-6.53 nm and the DNA interhelical distance d(DNA) approximately 3.52-3.99 nm, depending on the concentration of C12GS. At molar ratio C12GS+:DLPC> or =0.35:1, the diffractograms have shown the presence of a second lamellar phase with the repeat period d approximately 5.31 nm which slightly decreases with increasing concentration of C12GS+. The increasing fraction of this phase in the aggregates with increasing concentration of C12GS supports the association of this phase with microscopic domains enriched by surfactant molecules. The temperature behaviour of aggregates was investigated in the range 25-60 degrees C and the transversal thermal expansivities of the observed phases were determined.     

Structure and phase behavior of O-stearoylethanolamine: A combined calorimetric, spectroscopic and X-ray diffraction study

Recent studies show that O-acylethanolamines (OAEs), structural isomers of the putative stress-fighting lipids, namely N-acylethanolamines (NAEs), can be derived from NAEs and are present in biological membranes under physiological conditions. In view of this, we have synthesized O-stearoylethanolamine (OSEA) as a representative OAE and investigated its phase behavior and crystal structure. The thermotropic phase transitions of OSEA dispersed in water and in 150 mM NaCl were characterized using calorimetric, spectroscopic, turbidimetric and X-ray diffraction studies. These studies have revealed that when dispersed in water OSEA undergoes a cooperative phase transition centered at 53.8 °C from an ordered gel phase to a micellar structure whereas in presence of 150 mM NaCl the transition temperature increases to 55.8 °C and most likely the bilayer structure is retained above the phase transition. O-Stearoylethanolamine crystallized in the orthorhombic space group P2₁2₁2₁ with four symmetry-related molecules in the unit cell. Single-crystal X-ray diffraction studies show that OSEA molecules adopt a linear structure with all-trans conformation in the acyl chain region. The molecules are organized in a tail-to-tail fashion, similar to the arrangement in a bilayer membrane. These studies are relevant to understanding the role of salt on the phase properties of this new class of lipids.     

Role of intestinal mucus in crystal biogenesis: an electron-microscopical,diffraction and X-ray microanalytical study

Summary In the posterior intestine of the sea-water eel, mucus plays an important role in biocrystallization of calcium ions. By means of transmission and scanning electron microscopy associated with X-ray microanalysis and X-ray diffraction it has been possible to determine the role of mucous fibers as nucleation sites. Biocrystallization occurs in 2 steps: (1) Calcification of mucus. As soon as mucus is excreted in the intestinal lumen, it is loaded with calcium, as shown by lanthanum affinity and X-ray microanalysis on freeze-dried tissues. (2) Genesis of crystals. Needleshaped crystallites build up in coalescent spherites in the intestinal lumen near the microvilli. Genesis occurs as follows: (a) crystallite mineralization by nucleation in an organic matrix composed of glycoproteinaceous mucous fibers, followed by the appearance of spherites; (b) coalescence in spherites and association of spherites in rhombohedra; (c) extrusion of organic material during the final step of crystallization.