天然保鲜物料在食品保鲜上的应用研究

食品变质主要是由氧化和微生物作用引起的,目前使用的化学合成添加剂,其毒性和对人体的潜在危险,已引起严重关注,研究和开发天然的保鲜防腐物料已成为必然趋势,从辛香物料中提取保鲜组分,从茶叶中提取茶多酚,是目前最有希望的开发天然保鲜添加剂的途径,本文初步分析了食品的变质过程和天然保鲜组分的特性,并为这方面的深入研究提出了看法。     

Specificity of peptides; new aspects of the specificity of peptides with vitamin and hormone action are described

The data now at hand show that specificity among biologically active peptides is probably not as exquisite as might have been deduced from experiences with some of the water-soluble vitamins. It is plain that a given biological effect can be evoked by peptides which differ considerably. It is likewise plain that the effect cannot be evoked by a number of other peptides. There is apparently some specificity, but more is involved than that incorporated in the classical view.     

Permethylpolysilanes: Silicon Analogs of Hydrocarbons: Cyclic polysilanes resemble aromatic hydrocarbons in electronic and chemical properties

Studies of the spectra of cyclopolysilanes have led to several significant conclusions. These molecules possess both easily ionized Si-Si bonding electrons and low energy delocalized antibonding orbitals, and can therefore serve either as electron donors or acceptors. This model explains the similarity in properties between cyclopolysilanes and aromatic hydrocarbons. Further investigations of cyclopolysilanes seem likely to provide the key to understanding of controversial questions of bonding in metalloid compounds even as studies of their carbon analogs, the cyclic and cage hydrocarbons, have been crucial to present knowledge of organic stereochemistry and reaction mechanisms. The reactions of cyclopolysilanes are not only interesting in themselves, but have opened the way to the synthesis of complex polysilanes and thus to whole new areas of study. Improved methods of synthesis and isolation are needed, but the number and kinds of compounds that can be prepared seem almost limitless. Perhaps a polymetal chemistry comparable in breadth and variety to carbon chemistry is now developing.     

Hydride Complexes: The chemistry and current significance of covalent hydrogen compounds of transition metals are discussed

Hydride complexes of transition metals were virtually unknown 12 years ago, but they have now been found to lie at the root of many interesting catalytic chemical reactions, some of which were used in industry even before the unique properties of hydride complexes, which formed their basis, had been realized. This article surveys the history, preparations, important properties, and reactions of hydride complexes, surveys their part in important catalytic industrial processes, and speculates about their possible role in the biological fixation of nitrogen.     

Electronic Properties of Amorphous Materials: Changes are considered which occur when the long-range order typical for crystals disappears

A possible approach to the understanding of electronic properties of amorphous materials is to compare them with the corresponding crystalline materials, whose properties have been well explained. This approach has been exploited in the simple case of amorphous germanium, and I have indicated how the observed optical properties can be used to obtain information on the changes of electronic states, and what complications arise when we try to understand the transport properties.     

High-luminosity blue and blue-green gallium nitride light-emitting diodes

Compact and efficient sources of blue light for full color display applications and lighting eluded and tantalized researchers for many years. Semiconductor light sources are attractive owing to their reliability and amenability to mass manufacture. However, large band gaps are required to achieve blue color. A class of compound semiconductors formed by metal nitrides, GaN and its allied compounds AIGaN and InGaN, exhibits properties well suited for not only blue and blue-green emitters, but also for ultraviolet emitters and detectors. What thwarted engineers and scientists from fabricating useful devices from these materials in the past was the poor quality of material and lack of p-type doping. Both of these obstacles have recently been overcome to the point where highluminosity blue and blue-green light-emitting diodes are now available in the marketplace.     

Porphyrin and phthalocyanine antiscrapie compounds

The transmissible spongiform encephalopathies (TSEs) are fatal, neurodegenerative diseases for which no effective treatments are available. The likelihood that a bovine form of TSE has crossed species barriers and infected humans underscores the urgent need to identify anti-TSE drugs. Certain cyclic tetrapyrroles (porphyrins and phthalocyanines) have recently been shown to inhibit the in vitro formation of PrP-res, a protease-resistant protein critical for TSE pathogenesis. We now report that treatment of TSE-infected animals with three such compounds increased survival time from 50 to 300%. The significant inhibition of TSE disease by structurally dissimilar tetrapyrroles identifies these compounds as anti-TSE drugs.     

Reactions of organ crystals with gases

Although reactions of molecular crystals are much more widespread than has been generally appreciated (53), the great sensitivity of their rates to seemingly trivial changes in molecular structure clearly provides a major stumbling block to the general utilization of such processes. A key to understanding the effects of structural changes has been provided by the demonstration of anisotropy of such reactions discussed in this article. It is recognized, however, that many factors in addition to those aspects of the crystal structure utilized in the discussion above may play a part in determining reactivity of crystalline solids with gases. The recent ready availability of crystal structure data for molecular crystals should lead to a more rapid development of the principles governing the packing of such crystals and their reactivity. The resulting control of such reactions should not only provide the means of causing them when they are desired, as, for example, for synthesis, but also the means of preventing them when they are to be avoided, as in the stabilization of pharmaceuticals. Finally, it appears that we have only just begun to exploit the potential reaction specificity and the exciting possibilities resulting from the unique symmetry properties of the crystalline state.     

Laser techniques in high-pressure geophysics

Laser techniques in conjunction with the diamond-anvil cell can be used to study high-pressure properties of materials important to a wide range of problems in earth and planetary science. Spontaneous Raman scattering of crystalline and amorphous solids at high pressure demonstrates that dramatic changes in structure and bonding occur on compression. High-pressure Brillouin scattering is sensitive to the pressure variations of single-crystal elastic moduli and acoustic velocities. Laser heating techniques with the diamond-anvil cell can be used to study phase transitions, including melting, under deep-earth conditions. Finally, laser-induced ruby fluorescence has been essential for the development of techniques for generating the maximum pressures now possible with the diamond-anvil cell, and currently provides a calibrated in situ measure of pressure well above 100 gigapascals.     

热带亚热带地区山羊营养需要研究进展

山羊适应性强、易于饲养,是我国主要的小型反刍动物,而且羊肉营养价值丰富、市场需求量大,山羊养殖业已经成为我国重要的农业产业之一。因此,加强对山羊营养需要量的研究,为科学饲养、提高经济效益提高理论依据。文章对国内外热带亚热带地区山羊的采食量、能量、蛋白质、矿物质、维生素及水等营养需要量的最新研究进展进行了综述分析,以期为我国热带亚热带地区山羊饲养标准的制定提供参考。     

Structure and Properties of Crystalline Polymers: Supramolecular structure is complex, can be controlled, and influences mechanical properties

Through control of the crystallization process many polymers can be made to exhibit a broad spectrum of structural possibilities, ranging from a state of high disorder to one of high order. Emphasis has been placed on bulk-crystallized polymers which normally fall near the center of this spectrum. This was done in part because polymers are most commonly fabricated in this manner, and also because the amount of attention given in recent years to crystals grown from dilute solution has tended to obscure important aspects of the structure of bulk-crystallized polymers. Differences in the structure of the interlamellar regions account most reasonably for many of the variations in mechanical properties that crystalline polymers can be made to display. The interlamellar amorphous model, proposed here as a structural model for bulk-crystallized polymers, largely neglects the lamellae themselves, because so little is known about the structure of the lamellae of bulk-crystallized polymers and how variations in lamellar structure can affect properties. This model is intended, therefore, as both a working hypothesis to aid further investigation and as an outline to be filled in as more is learned about the supramolecular structure of crystalline polymers (31).     

Some marine ecological phenomena: chemical basis and biomedical potential

Analysis of secondary metabolites derived from marine organisms has revealed a broad spectrum of novel molecular architecture. The function of these compounds in their natural habitat is linked to various aspects of species survival, and the compounds have also served as characteristic chemical markers through successive trophic levels. Fundamental questions concerning the locus of synthesis in complex and intricate assemblies of plants and animals and the pathways of biosynthesis are beginning to be answered. It is now apparent that the marine environment gives rise to some distinctive chemistry, which is generated along characteristic pathways. Some of the newly described compounds have already become valuable tools in biomedicine.     

Biomass recalcitrance: engineering plants and enzymes for biofuels production

Lignocellulosic biomass has long been recognized as a potential sustainable source of mixed sugars for fermentation to biofuels and other biomaterials. Several technologies have been developed during the past 80 years that allow this conversion process to occur, and the clear objective now is to make this process cost-competitive in today's markets. Here, we consider the natural resistance of plant cell walls to microbial and enzymatic deconstruction, collectively known as "biomass recalcitrance." It is this property of plants that is largely responsible for the high cost of lignocellulose conversion. To achieve sustainable energy production, it will be necessary to overcome the chemical and structural properties that have evolved in biomass to prevent its disassembly.     

Permanence in book papers; investigation of deterioration in modern papers suggests a practical basis for remedy

Deterioration of paper in the book stock-especially among books of recent decades-has become a serious problem for libraries, but exact measures of the extent or rate of this deterioration have not been available. By the same token, no good indications have been available of the expected useful life of paper in books currently coming off the press. Furthermore, although attention has been given to means of counteracting one of the recognized causes of deterioration-pollutants absorbed from the atmosphere-similar attention has not been given to the problems of identifying and counteracting the other recognized source of deterioration-agents left in or introduced into the paper at the time of manufacture. Our investigation was undertaken in an attempt to fill some of these gaps in our knowledge. It has been found that modern books-even those written with a serious or scholarly purpose ("nonfiction") and published ("to last!") in hard bindings-are deteriorating rapidly, and many of those issued 25 to 50 years ago are now almost unusable. The paper of an average American publication of the first decade of this century retains only 4 percent of the folding endurance of a typical new book paper of today; even the paper in the average publication of the 1940's has already declined in folding endurance to 36 percent of today's new book paper. Meanwhile this typical new book paper itself shows low initial strength (for example, folding endurance on the order of only 20 percent of that of book papers already 200 to 500 years old) as well as indications that it is subject to rapid deterioration. Acidity appears to be the principal cause of deterioration, both in the older papers and the new. A stabilizing process for neutralizing this acidity was developed and was brought during the investigation to an initial stage of economic feasibility. This process appears to inactivate the most injurious properties found in new book papers and precipitates compounds into the fibers which should, in addition, counteract the effect of pollutants absorbed from the air. A principal technique used for predicting the durability of paper is the accelerated aging procedure developed by the National Bureau of Standards; by extending the use of this technique, interesting decay curves have been obtained which facilitate comparison of modern papers with papers which have already demonstrated their stability for several centuries. These curves offer other values as well for the study of permanence in paper and suggest that a principle exists relating extended accelerated aging to even longer periods of natural aging than those with which it has hitherto been equated. During the past 300 years the papermaker has done an excellent job in meeting the demand for more and cheaper paper. A by-product of this accomplishment has been the production of many weak and unstable papers. This does not give rise to any problem where strength or stability are not critical, but where permanence is important, as in libraries of record, the problem becomes serious. While the present study describes a procedure for stabilizing initially unstable paper, much more remains to be done if relatively stable papers are to be made for books of lasting value. There is evidence that this is possible-that such papers can be made from certain types of chemical wood fibers at but little additional cost. But more research is needed, and the cooperation of the scientist, papermaker, printer, publisher, and librarian will be required to assure permanent books for the future.     

A new thiamine derivative,S-benzoylthiamine O-monophosphate

S-Benzoylthiamine O-monophosphate has been synthesized, and its physicochemical and biological properties have been investigated. It is a stable crystalline substance, it exerts thiamine activity approximately equivalent to that of thiamine hydrochloride in thiamine-requiring microorganisms, and it is easily absorbed in organisms, particularly by oral administration.     

Chemical anatomy of the brain

The development of sensitive histochemical-neuroanatomical techniques has made it possible to analyze the content of specific compounds in single nerve cells and their processes. In consequence, it has been possible to construct detailed maps of the distribution of various types of neurons on the basis of their transmitter substance. There are now many examples of neurons containing both a classical transmitter and a peptide. In some instances the peptides seem to support the action of the classical transmitters. This interaction may have applications in the prevention and treatment of nervous disease states.     

Targeting tyrosine kinases in cancer: the second wave

One of the most exciting developments in cancer research in recent years has been the clinical validation of molecularly targeted drugs that inhibit the action of pathogenic tyrosine kinases. Treatment of appropriately selected patients with these drugs can alter the natural history of their disease and improve survival. The clinical validation of these "first-generation" tyrosine kinase inhibitors has been the prelude to a second wave of advances in molecular targeting that is expected to further change the way we classify and treat cancer. Efforts are now being directed at identifying the tumor subtypes and patients who will benefit the most from these drugs. In addition, new compounds that circumvent acquired resistance to the first-generation tyrosine kinase inhibitors are being tested in patients with refractory disease. Agents directed against new molecular targets are also being explored.     

Quinoxalinediones: potent competitive non-NMDA glutamate receptor antagonists

The N-methyl-D-aspartate (NMDA)-subtype of glutamate receptors has been well described as a result of the early appearance of NMDA antagonists, but no potent antagonist for the "non-NMDA" glutamate receptors has been available. Quinoxalinediones have now been found to be potent and competitive antagonists at non-NMDA glutamate receptors. These compounds will be useful in the determination of the structure-activity relations of quisqualate and kainate receptors and the role of such receptors in synaptic transmission in the mammalian brain.     

PHYSIOLOGY OF THE AMINO ACIDS

We have followed the amino acids from their entrance into the alimentary tract in the form of food proteins through the successive steps of digestion, absorption into the blood stream and passage from the blood stream into the tissues, where they are concentrated by some unknown mechanism to many times their concentration in the blood plasma. We have seen something of the way in which certain of the amino acids can be transformed into one another in the body or synthesized from ammonia and keto acids. However, we have had to admit that our bodies can form in such ways only about half of the different amino acids that are required, and that the other half must be made for us by plants, bacteria or other organisms which have greater synthetic powers than we. And finally we have seen something of the manifold fates of the amino acids after they have entered our tissues; how they may be destroyed and their nitrogenous parts turned into urea in the liver before it is possible to put them to their more specialized uses, how their carbon fractions can be used to form glucose, how they may sacrifice themselves to protect us from toxic products, how they can serve as source material for certain vitamins, hormones and other compounds with physiological functions still to be identified, and how finally those amino acids which are not deflected to these various fates may enter into the proteins of the tissues and become for a time parts of our living structures.     

Coordination polymers of osmium: the nature of osmium black

The design of cytochemical reagents that yield osmiophilic products from which an osmium black may be derived on exposure to osmium tetroxide has resulted in new methods described previously for the ultrastructural demonstration of enzyme activity and functional groups of macromolecules with the electron microscope. Attempts to determine the nature of the osmium black end products have been frustrated by their insolubility. The preparation of watersoluble analogs and their characterization as polymers suggest that the insoluble osmium blacks are coordination polymers. This is consonant with the unusually favorable properties of osmium black in electron microscopy. Although polymers of osmium have frequently been postulated as the end products of reaction of osmium tetroxide with tissue conistituents or with other organic compounds, this is the first example of their characterization.