dehyde forms upon the oxidation of PUFA hitherto has been unknown. Now, Pryor and Stanley suggest that the malonaldehyde arises from the ac id-catalyzed or thermal decomposition of endoperoxides formed by the oxida tion of PUFA. Pryor observes, "Some of the physio logical results that have been thought to be. the result of autoxidation of PUFA actually may be caused by the very specific effect of a prostaglan din or endoperoxide. A host of unnatu ral isomers are formed and we must ask: Of the physiological symptoms that are observed in some experiments, how many of them may be caused by prostaglandin endoperoxides, or by en doperoxides and prostaglandins with unnatural structures? 55 Pryor and Stanley calculate that more than 100 different compounds can be produced in the autoxidation of the fatty acid ester, methyl linolenate. The endoperoxides, which represent about 20% of their isolated products, show a half-life of four hours at 80° C, accord ing to Pryor. Enzymic preparations of endoperoxide give material with a shorter half-life, Pryor observes. These preparations contain metal ion impuri ties that may catalyze decomposition, he suggests. Pryor and Stanley first suspected that the malonaldehyde precursor of the thiobarbituric acid test is an endo peroxide when they observed that autoxidized solutions of methyl linolenate also give a positive response to a test developed for prostaglandins. In this test, alcoholic base is believed to dehy drate PGE compounds (keto-alcohol forms of prostaglandins) into conjugat ed dienones that absorb at 278 nm. The explanation of this response, they reason, is that endoperoxides form upon autoxidation, and these endo peroxides decompose, at least partially, to the PGE-type compounds. Moreover, kinetic studies of the ap pearance of the thiobarbituric acid color test reactive material and the PGE test material suggest that both are from the same precursor, namely endoperoxides, Pryor and Stanley be lieve. The chemists confirm the pres ence of endoperoxides by preparing trimethylsilyl derivatives of the interme diates, and comparing gas chromato graphic and mass spectrometric analy ses of these derivatives with those for genuine prostaglandins. Karen Joy Skinner, C&EN Washington
its specific task—compared, that is, to a laboratory prototype model or the adaptation of an existing machine through various attachments. Among its advantages is a point-resolving power of about 2 A and the possibility of directly computerizing and process ing the data it provides. It has a mag nification capability ranging from 10 million X down to 50 X. The Elmiskop ST100F—which stands for Scanning Transmission 100-kV Accelerating Voltage Field Emitter—operates on an unusual prin ciple developed from the pioneering work of Dr. Albert V. Crewe and his as sociates at the University of Chicago. Instead of a static beam of electrons, a moving electron probe scans the speci men line by line. The extremely fine electron beam is formed by an electro magnetic lens for high magnifications or by an integrated miniature lens for low magnifications. Line-by-line scanning is achieved with the aid of electromag netic deflector systems. Between 400 and 3200 lines can be selected as the scan frequency. Alternatively, a raster consisting of up to 1000 by 1000 image points can be formed. Detectors record the penetrating electrons according to their scattering angle. The image is formed by syn chronization of the line deflection in the picture tube of an observation monitor with the deflections of the
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Microscope features high resolving power Molecular biologists likely will be in terested in a new scanning transmis sion electron microscope developed by engineers at Siemens A.G. in West Berlin. The microscope, the company claims, is the first commercial unit of its kind designed and built to perform
probe beam. Electrical signals coming from the detectors are used for intensi ty control. The magnified scan image may be observed and photographed on the monitor in an undarkened room. In addition, the image signals can be fed to a computer. The special field emission system in the ST100F operates at a maximum electron accelerating voltage of 100 kV. This device allows the machine to be used for almost all the conventional image-forming and diffraction tech niques, such as bright- and dark-field microscopy. Moreover, electrons may be selected according to their individu al energies. This enables signals indi cating elastic and inelastic electron dispersion to be separated or linked up with each other, a particular advantage in microscopic studies of viruses or bio logical membranes. Siemens engineers point to the image storage capability of the ST100F, which will be of considerable advantage to scientists working with labile samples that are sensitive to damage by the electron beam. It now will be possible to make a single beam scan of the sample and record the re sulting picture electronically. After studying this picture, a segment of the specimen that warrants more detailed examination may be singled out and exposed to a second scan at a higher magnification. ' Π
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