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Biomedical subjects

A W Robards

Publications and source records attributed to A W Robards.

16 recordsLinked to original sources
PubMed2000

Direct visualization of changes in deacylated Na(+) gellan polymer morphology during the sol-gel transition.

Changes in gellan polymer morphology during the sol-gel transition were directly visualized by transmission electron microscopy and a model incorporating these changes and existing physical data is proposed. Our observations suggest that the most thermodynamically stable conformations of gellan polymers in solution, in the absence of added cations, are the double helix and double-helical duplexes. We have demonstrated two forms of lateral aggregation of gellan helices in the presence of Ca(2+) and K(+) ions. One type forms junction zones that lead to network formation and gelation, while the second type leads to the formation of isolated fibers of aggregated helices and inhibition of gelation. The proposed model of gellan gelation is based on these observations where thermoreversibility, gel strength, and endothermic transitions of gellan gels can be explained.

Acetylation↗
PubMed1993

Ultrastructural evidence for intramolecular double stranding in iota-carrageenan.

Kinetic studies of primary processes of conformational ordering in gel-forming biopolymers have suggested that a change in mechanism from intermolecular to intramolecular multistrand formation occurs on lowering the concentration of biopolymer. We report here ultrastructural observations consistent with intramolecular double stranding in a carbohydrate polymer, iota-carrageenan, by arresting this process of primary conformational ordering by an ultra-rapid freeze fixation technique. High-resolution transmission electron microscopy (TEM) revealed isolated iota-carrageenan chains showing a range of morphologies (linear, circular, and hairpin) consistent with intramolecular stranding. Control experiments in which iota-carrageenan was frozen in the disordered form revealed longer and thinner strands.

Carbohydrate Conformation↗
PubMed1993

Improved visualization of folded collagen alpha-chains by ultra-rapid freezing.

Transmission electron microscopy techniques are commonly employed to examine the folded polymer structure of collagen polypeptides. These techniques include deposition of a sample by spraying, slow freeze-fixation, air drying and vacuum drying the specimen at room temperature, and using additives such as glycerol in the collagen preparation. Here we report preliminary observations of type I collagen alpha-chains, folded in water, at a concentration of 35 micrograms ml-1 and 10 micrograms ml-1, visualized by an ultra-rapid, freeze-fixation technique designed to minimize structural deformation caused by spraying, additives and poor freeze-fixation. The technique also allows the use of submicrolitre sample volumes of known concentrations with negligible loss and shearing, while at the same time providing excellent contrast to the collagen polymer for electron microscopy. This technique can be employed to study the structure of a wide range of macromolecules (proteins and carbohydrates).

Animals↗
PubMed1982

Understanding the artefact problem in freeze-fracture replication: a review.

Freeze-fracture and freeze-etching techniques do not provide artefact-free images of native in vivo or in vitro cells and tissues. Each preparation stage can produce specific artefacts which must be recognized and understood if these methods are to contribute meaningful information to cell biology, This paper reviews the latest information available on artefacts in freeze-fracture replication (and etching) methods and points to possibilities for avoiding some of them. Different specimens show different sensitivity to artefactual changes and the final images must be interpreted carefully with regard to the multi-event process that has led to their production.

Cryoprotective Agents↗
PubMed1981

Specimen heating during sputter-coating.

It is possible to sputter thin films of gold on to surfaces of frozen biological specimens at very low temperatures (less than 120 K) without untoward effects from heating. This is achieved by using permanent magnets to confine the plasma and thus to minimize the energy required to give a reasonable sputtering yield. The system described uses only 250 V at 12-15 mA to give 15 nm films within 2-3 min. It is shown, from theory portraying 'worst-case' conditions, that the specimen temperature could not increase by more than 6.0 K at equilibrium. Practical results support the theoretical assumptions. Similar considerations have been applied to sputtering at normal ambient temperature where it is shown that appropriate design of simple apparatus and selection of operational conditions can give adequate films in a reasonable time with negligible (less than 3 K) temperature rise above the starting temperature.

Cold Temperature↗
PubMed1978

A device for the rapid freezing of biological specimens under precisely controlled and reproducible conditions.

The construction and preliminary testing of a device is described which can be used to freeze biological specimens in any cryogenic liquid at temperatures down to the nitrogen freezing point (63 K) and which can operate in the pressure range 1.3 kNm-2 to 1 MNm-2. Ultra-rapid freezing can be carried out in a subcooled cryogenic liquid either hyperbarically or at atmospheric pressure. Slow freezing rates can be achieved by cooling the specimens in a controlled manner in the vapour phase above the liquid bath.

Atmospheric Pressure↗
PubMed1973

Ultrastructural study of poly- -hydroxybutyrate granules from Bacillus cereus.

The freeze-etching technique was used to examine the effects of fracturing and etching on the appearance of poly-beta-hydroxybutyrate granules from Bacillus cereus. These granules were examined in extracts isolated by hypochlorite or by sonic treatment, and also in fixed and unfixed intact cells; in the latter case they were compared with granules in thin sections of intact cells. After freeze-fracturing, the diameter of the granules in intact cells was between 240 and 720 nm. The granules consisted of a central core, of diameter between 140 and 370 nm, which occupied less than 50% of the volume of the granule and which was either stretched or removed on fracturing; the core was surrounded by an outer coat which may be bounded by a membrane.

Bacillus cereus↗
PubMed1972

Freeze-etching.

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Freeze Etching↗