X-ray diffraction analysis of dehydrated myelin

Improved X-ray diffraction data from dry nerve myelin are presented. In addition to the spacings of approx. 150 Å, 60 Å, 44 Å and 34.6 Å, which have been previously reported, we identify a 14 Å series. The data suggests that the hydrocarbon chains in the single bilayer ($\text{≈ 60}\text{A}\text{?}$) is ordered, whereas in the double bilayer ($\text{≈ 150}\text{A}\text{?}$) and in the fluid phase ($\text{≈ 44}\text{A}\text{?}$) it is disordered. It is shown that cholesterol ($\text{≈34.6}\text{A}\text{?}$) exists as a bilayer, and the 14 Å series is probably another cholesterol phase.     

Parameters for crystal growth of ribosomal subunits

A systematic analysis of the parameters that control the crystal growth of the large subunit of ribosomes from B stearothermophilus has been carried out. Several parameters have been identified and classified according to their significance. It was found that only biologically active particles can crystallize and that the critical period for the crystallization process is the first few days, during which changes in the volume and content of the crystallization drop are of importance for both nucleation and crystal growth. Consequently, an experimental procedure for fine control of these variables has been developed. As a result of these studies, the reproducibility of crystal formation was increased, and larger and more stable crystals have been obtained.     

Structure and dynamics of the water around myoglobin.

The interplay between simulations at various levels of hydration and experimental observables has led to a picture of the role of solvent in thermodynamics and dynamics of protein systems. One of the most studied protein-solvent systems is myoglobin, which serves as a paradigm for the development of structure-function relationships in many biophysical studies. We review here some aspects of the solvation of myoglobin and the resulting implications. In particular, recent theoretical and simulation studies unify much of the diverse set of experimental results on water near proteins.     

Effects of chemical treatments and heating on the crystallinity of celluloses and their implications for evaluating the effect of crystallinity on cellulose biodegradation

Chemical treatments similar to those routinely used to extract cellulose from plant biomass caused significant increases in the relative crystallinity index (RCI) of Sig-macell 100 (a commercial cellulose of moderate crystallinity), as measured by x-ray powder diffraction in both the reflectance and transmittance modes. In general, the largest increases in RCI were observed following higher (rather than lower) temperature treatments. Substantial increases in crystalliity were also observed upon resuspension in water prior to drying, with higher temperatures again resulting in the greatest increases in RCI. Measurement of the RCIs of wetted Sigmacell 100 samples by acid hydrolysis kinetics revealed that most of the increased crystallinity occurred rapidly upon contact with water. In contrast to Sigmacell 100, a cellulose of higher initial crystallinity (the microcrystalline cellulose Sigmacell 50) showed little change in crystallinity following the above treatments. The results provide a partial explanation for the inconsistent relationships reported between cellulose crystallinity and cellulose biodegradation. (c) 1995 John Wiley & Sons, Inc.     

Morphological and structural studies of early mineral formation in enamel of rat incisors by electron spectroscopic imaging (ESI) and electron spectroscopic diffraction (ESD)

Morphological and structural analysis of the earliest stage of crystal formation in enamel of rat incisors, by use of energy filtering transmission electron microscopy (EFTEM), has shown needlelike crystallites with a dotlike substructure. We conclude that these dots (nanometer-sized particles) have developed at nucleating, active sites along the non-collagenous matrix proteins in enamel. Calcium and phosphate groups are bound at such active sites and develop to nuclei, which grow to these stable dots (nanometer-sized particles). The dots coalesce rapidly in longitudinal direction, along the matrix proteins, with neighbouring dots to form parallel arranged needlelike crystallites. These needles grow and coalesce in lateral directions to ribbon-platelike crystallites. In enamel most of the organic substance becomes decomposed and transported to the ameloblasts. Consequently, the ribbon-platelike crystallites can coalesce to form much thicker (hydroxy)-apatite crystals than in dentine. Already in the earliest stage of crystal formation the mineral chains of dots (nanometer-sized particles) and the needlelike crystallites show a parallel orientation in the direction of the c-axis of hydroxyapatite. This is supported by the texture of the 002 reflections in the corresponding electron spectroscopic diffraction patterns (ESD), which appear as the first Bragg reflections.     

The glassy state and accelerated aging of soybeans

Deteriorative changes in seeds may be expected to reflect changes in physical state as well as in chemical composition. In the present study, measurements of changes in glass transition, lipid phase transition and sugar content were made during accelerated aging of two cultivars of soybeans ( Glycine max Merrill cv. Chippewa 64 and cv. Hodgson 78). The glass transition in axes, as measured by the thermally stimulated depolarization current method, showed gradual decreases in both magnitude and transition temperature during accelerated aging, and eventually, axes of seeds lost their ability to enter the glassy state. Sucrose, raffinose and stachyose contents in seed axes showed little or no change during the aging treatment. Membrane lipids in aged axes retained the liquid crystalline phase during aging. These data suggest that the changes of glass transition during accelerated aging occurred without associated changes in soluble sugar contents or changes in the liquid crystalline state of membrane lipids. The loss of the glass transition during accelerated aging could be a consequence of the annealing effect due to elevated temperature and moisture content. We propose that a loss of the glassy state during accelerated aging leads to an enhanced rate of subsequent deteriorative reactions in seeds and accelerates the loss of viability.     

The effect of zeaxanthin on the thickness of dimyristoylphosphatidylcholine bilayer: X-ray diffraction study

Multilayers of dimyristoylphosphatidylcholine (DMPC) modified with a carotenoid pigment zeaxanthin were analyzed by means of small-angle X-ray diffractometry. The results are discussed in terms of the superlattice theory developed recently for metallic multilayers. Inclusion of zeaxanthin to a DMPC bilayer increases thickness of the hydrophobic core by about 3 Å in the whole lipid-pigment miscibility range. The thickness increases very sharply as pigment concentration approaches the threshold of miscibility. Zeaxanthin aggregation within a layer causes further changes in the thickness of the membrane: a local decrease followed by a gradual rise.     

The effect of ethidium bromide on the liquid crystalline phases of aqueous DNA.

The influence of the intercalation of ethidium bromide (EB) on the characteristics of the DNA cholesteric and hexagonal mesophases is studied by optical microscopy, circular dichroism, and X-ray diffraction. The distance between DNA rods in the hexagonal phase is not modified by the presence of EB whereas the pitch of the cholesteric mesophase is considerably shortened by the dye. This seems to be related to the stereochemical effect of the intercalation rather than to the presence of a random distribution of the positive charge of the dye.     

Ageing of the human corneal stroma: structural and biochemical changes

High and low angle X-ray diffraction patterns from the corneal stroma give information about the mean intermolecular spacing of the collagen molecules and the mean interfibrillar spacing of the collagen fibrils, respectively. X-ray data were collected, using a high intensity synchrotron source, from human corneas and sclera at approximately physiological hydration. The spacings were measured as a function of tissue age. Between birth and 90 years there is an increase in the cross-sectional area associated with each molecule in corneal collagen from approx. 3.04 nm² to 3.46 nm², and an increase in scleral collagen from approx. 2.65 nm² to 3.19 nm². These changes may be due to an increase in the extent of non-enzymatic cross-linking between collagen molecules over the age range. We have investigated this possibility by measuring collagen glycation using the thiobarbituric acid assay and the subsequent advanced glycation end-products (AGEs) using fluorescence emission. The results obtained have shown an age-related increase in glycation and AGEs in both tissues. We have also demonstrated a decrease in the interfibrillar spacing of corneal collagen with increasing age which may be related to changes in the proteoglycan composition of the interfibrillar matrix.