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Over the last 30 years empirical evidence in favour of the Maximum Entropy Production (MEP) principle for non-equilibrium systems has been accumulating from studies of phenomena as diverse as planetary climates, crystal growth morphology, bacterial metabolism and photosynthesis. And yet MEP is still regarded by many as nothing other than a curiosity, largely because a theoretical justification for it has been lacking. This chapter offers a non-mathematical overview of a recent statistical explanation of MEP stemming from the work of Boltzmann, Gibbs, Shannon and Jaynes. The aim here is to highlight the key physical ideas underlying MEP. For non-equilibrium systems that exchange energy and matter with their surroundings and on which various constraints are imposed (e.g., external forcings, conservation laws), it is shown that, among all the possible steady states compatible with the imposed constraints, Nature selects the MEP state because it is the most probable one, i.e., it is the macroscopic state that could be realised by more microscopic pathways than any other. That entropy production is the extremal quantity emerges here from the universal constraints of local energy and mass balance that apply to all systems, which may explain the apparent prevalence of MEP throughout physics and biology. The same physical ideas also explain self-organized criticality and a result concerning the probability of violations of the second law of thermodynamics (the Fluctuation Theorem), recently verified experimentally. In the light of these results, dissipative structures of high entropy production, which include living systems, can be viewed as highly probable phenomena. The prospects for applying these results to other types of non-equilibrium system, such as economies, are briefly outlined. [ABSTRACT FROM AUTHOR]

This paper describes the management and evolution of a large collection of 1200 documents detailing the functionality in NAG Library products. This provides a case study addressing many of the issues which concern the "MKM" project, involving conversion of legacy formats (SGML and LATEX) to XML, and inferring semantic content from mainly presentational mathematical expressions. [ABSTRACT FROM AUTHOR]

Single nucleotide polymorphisms exist at a high frequency in the human genome. Estimates of variability suggest that up to 1 in 1000 bp within coding regions of the genome are polymorphic, and the frequency of polymorphism within noncoding regions is even higher at about 1 in 500. Because of redundancy in amino acid coding, not all single-base substitutions result in an altered amino acid sequence within a gene. However, even degenerate polymorphisms can potentially alter transcriptional or translational activity. Polymorphisms within promoter or enhancer regions can interfere with transcription factor binding sites and, therefore, can also alter transcriptional activity. [ABSTRACT FROM AUTHOR]

Polymorphisms of the β2-adrenoceptor (β2AR) have been the focus of much interest as part of the search to elucidate the genetic basis of asthma and allergic disease. More recently, these polymorphisms have also been implicated in the genetic etiology of essential hypertension (1,2) and obesity (3,4). This chapter describes in detail the method of allele-specific-oligonucleotide hybridization (ASO), a technique that has been used extensively by the authors' group to determine β2AR genotype. This method should prove useful not only to those intending to analyze β2AR polymorphisms, but also for the analysis of other candidate genes, given that this technique can be adapted and applied to any known single-base mutation. [ABSTRACT FROM AUTHOR]

The unimolecular reactions of ions in a mass spectrometer are remarkably well correlated with both photochemical and thermochemical reactions of the corresponding neutrals. The reactivity correlations are seldom quantitative and exceptions to the correlation should be expected when heteroatoms or delocalization effects convey unusual stability to the ions in the mass spectrometer as compared to their neutral analogs. In spite of the special effects on ionic stability and the relatively large amount of energy that is available in 70 eV electron impact, it appears that mass spectra will be an increasingly useful guide to new photochemical and thermochemical reactions. [ABSTRACT FROM AUTHOR]

Benzenoid (chemical) isomers are, in a strict sense, the benzenoid systems compatible with a formula CnHs≡(n; s). The cardinality of CnHs, viz. CnHs≡n, s is the number of isomers pertaining to the particular formula. The generation of benzenoid isomers (aufbau) is treated and some fundamental principles are formulated in this connection. Several propositions are proved for special classes of benzenoids defined in relation to the place of their formulas in the Dias periodic table (for benzenoid hydrocarbons). Constant-isomer series for benzenoids are treated in particular. They are represented by certain CnHs formulas for which n; s=n1; s1=n2; s2=…, where (nk; sk) pertains to the k times circumscribed CnHs isomers. General formulations for the constant-isomer series are reported in two schemes referred to as the Harary-Harborth picture and the Balaban picture. It is demonstrated how the cardinality n; s for a constant-isomer series can be split into two parts, and explicit mathematical formulas are given for one of these parts. Computational results are reported for many benzenoid isomers, especially for the constant-isomer series, both collected from literature and original supplements. Most of the new results account for the classifications according to the symmetry groups of the benzenoids and their Δ values (color excess). [ABSTRACT FROM AUTHOR]

A synthon model of organic chemistry is described. A synthon is determined as a molecular graph in which a certain part of vertices is distinguished and those vertices are called the virtual vertices. The virtual vertices correspond to functional groups or molecular fragments that do not directly participate in chemical transformations (though these transformations may be substantially influenced by them). The synthon precursors or successors are formed from a given synthon by special sort of reaction graph in such a way that all valence states of atoms (vertices) in a precursor or successor are classified as stable valence states. The program PEGAS represents an implementation of the synthon model. Its basic features, scopes, and algorithms are briefly outlined. The effectiveness of the program PEGAS is illustrated by many examples of interest for organic chemists. [ABSTRACT FROM AUTHOR]

In this article we have described the different types of reactions that have been investigated by means of optically active compounds. It remains to determine the scope of the different mechanisms suggested, and to formulate stereochemical rules for use in further synthetic work. [ABSTRACT FROM AUTHOR]

It is evident from the results presented above that the cis-(−N=N−) bond has powerful coordinating properties towards low valent transition metals. Due to the unique electronic structure of the diazeno group, many different modes of interaction have been observed and it seems promising that this ability of the N=N function may be utilized in catalytic processes. Since diazene itself is a proposed intermediate in biological nitrogen-fixation, the observed modes of coordination are of some relevance to this problem. The results obtained from the cycloaddition reactions are of general importance for the investigation of the factors determining photochemical or thermal reactions of coordinated ligands. The ability of iron carbonyls to activate the diazeno group towards attack by unsaturated molecules like alkynes, offers a convenient synthesis of novel organic heterocycles. Together with the recently reported conversion of coordinated dinitrogen into diazeno and diazenato complexes4), these reactions may be key steps in the incorporation of N2 into organic compounds. [ABSTRACT FROM AUTHOR]

A computer program has been developed which automatically generates sequences of chemical reactions. As reactions are treated quite formally, all conceivable reactions are obtained irrespective of whether they axe already known or are without precedence. This program is especially useful for planning organic syntheses and fox generating the products of starting materials. The system is in very active development but the present version can already serve as a tool for the chemist in studying problems of immediate interest. Examples for applications are given to demonstrate the flexibility and performance of the program. [ABSTRACT FROM AUTHOR]

The above examples of the unique possibilities of stereo chemically fixed aromatic rings in multibridged, multilayered and multistepped hydrocarbon compounds show that these classes of compounds are most appropriate for the study of intramolecular, steric and electronic interactions, from which too little quantitative relations are known. Again, it follows that further synthetic approaches are not only possible, but also necessary for a more accurate differentiation and analysis of the frequently overlapping electronic and steric effects. More progress in this field is sure to be expected, above all when synthetic and spectroscopic chemists, and physicists adhere to interdisciplinary, collaborative work. [ABSTRACT FROM AUTHOR]

While in the early days of organic mass spectrometry the ortho effect was preferentially used as a diagnostic tool for structure elucidation, the recent research activities have concentrated on mechanisms and related aspects. Among these the following were the most important topics: hydrogen transfer and especially the question of concerted or multistep reaction; the influence of the structure on the selectivity of hydrogen migration via higher-membered transition states; the apparent cleavage of carbon/heteroatom bonds induced by hydrogen transfer; the redox processes which formally correspond to photochemical transformations, but which are completely different according to the reaction mechanisms; the different types of cyclization reactions; and the field of exchange processes preceding or accompanying unimolecular decompositions of ions in the gas phase. If a prognosis concerning the future activities in the field of electron impact induced ortho effects may be risked, the following aspects will probably become more important in addition to the topics discussed in this review:(i)Kinetic studies in the time range of nanoseconds to elucidate the time dependence of the different ortho effect reactions;(ii)ion/molecule reactions with the scope to obtain more information concerning the reactivities of open and closed shell systems;(iii)comparative studies in the fields of mass spectrometry, photochemistry and electrochemistry. In principle all these studies may extend our knowledge concerning the inherent properties of isolated ionic species. [ABSTRACT FROM AUTHOR]

The strengths and weaknesses of EXAFS in determining the radial distribution of the first nearest neighbours in coordination compounds are well known. We point out here how local information given by EXAFS upon the first coordination sphere can be used to characterize short or long range order or disorder in low dimensional materials. Low temperature measurements, use of several absorbing species in the same compound, comparison between seemingly contradictory X-Ray diffraction and EXAFS data allow to solve three dimensional structural problems related to physical properties. The materials studied are insulating uni- or bimetallic magnetic chains and pure or intercalated MPS3 layers. [ABSTRACT FROM AUTHOR]

Current views on the structure and function of biological membranes imply a great variety of dynamical aspects. These range from the dynamic phase behaviour of phospholipids and conformational variations of membrane proteins during membrane-associated processes, to the interactions between membranes in fusion and pore formation. The structural description of these processes, many of which imply the possibility of intermediate structures, calls for fast time-resolved diffraction methods, i.e. the cinematographic approach. The present article gives first an overview on the specific problems and the theories for their solution in X-ray diffraction on membranes, and then reviews the present possibilities of time-resolved structural studies using synchrotron radiation on phospholipid model systems and functional membranes (sarcoplasmic reticulum membrane). It is shown that such studies bear great promise in entering the millisecond time domain and bridging the existing gap between static structural information and the wealth of dynamic data derived from spectroscopic methods. [ABSTRACT FROM AUTHOR]

Small angle X-ray scattering and diffraction methods can provide information about chromatin structure at the levels of nucleosomes, chromatin fibre and whole nuclei. Making use of the high flux and point focusing achieved on synchrotron radiation cameras it has been possible to carry out systematic static and kinetic investigations of chromatin structure with minimum damage or perturbation on fresh samples. The results provide a strong evidence for a preformed 3-D zigzag structure at low ionic strength which folds instantaneously into the "30 nm filament" upon increasing the ionic strength. The extent of folding at a given ionic strength depends on the nature of the cations and can be influenced by other substances which bind to DNA. Models of chromatin fibre structure are discussed in the context of various experimental results. [ABSTRACT FROM AUTHOR]

The use of anomalous X-ray scattering of light elements like sulfur and phosphorus is of particular interest in biological structure research. These elements serve as native labels in proteins, nucleic acids and membranes. Their medium scattering power is drastically changed at their K absorption edges at wavelengths between 5 and 6 Å where X-ray absorption excludes the use of open air diffractometers. The construction of a new diffractometer tunable to wavelengths between 1.2 and 7 Å is presented. First results of anomalous scattering from sulfur in bacteriorhodopsin near the K absorption edge have been obtained recently. Their possible impact on crystallography will be considered. A comparison with nuclear spin dependent neutron scattering is given. [ABSTRACT FROM AUTHOR]

Applications of X-ray absorption spectroscopy in two areas of liquid state studies using synchrotron radiation are reviewed. One area concerns with the determination of the local structure, bonding and dynamics at the metal site of transition metal complexes using Extended X-Ray Absorption Fine Structures (EXAFS) and X-Ray Absorption Near Edge Structures (XANES). Emphasis are placed on the implications of the derived parameters to the reactivity of the complexes in exchange reactions (ligand and electron) in solution and to the subtle electronic and structural change of the complexe upon dissolution. The other area deals with the principles and practices of X-ray induced photo-conductivity of hydrocarbon solutions. The conductivity measurements of EXAFS of organometallics in hydrocarbon solution is described. The special feature of the technique. EXAFS measurement under the condition of total X-ray absorption, is discussed in some details. Current developments is related areas are also noted. [ABSTRACT FROM AUTHOR]

The first observations of an increase of the reactivity between gases in the presence of a metal were made, almost one hundred years ago, by chemists like Davy or Thenard. This pioneer work led Berzelius and Mitscherlich to define the concept of "decomposition of species by contact under a catalytic force". Since then much work has been done in order to understand the behavior of these small transition metal clusters which are able to promote some chemical reactions. In fact, catalysis has become a field of enormous economical interest. It spans a wide variety of areas from oil reforming to the preparation of synthetic fibers or fertilizers. Theoretical research as well as chemical engineering are therefore deeply involved and many current studies deal with the knowledge of such materials. It is not always easy to characterize the electronic and crystallographic structures of very small aggregates. Their size (a few nanometers) is due to the fact that as many atoms as possible must be active and therefore must be at the surface. Moreover, the analysis has to be done in situ, under the true reaction conditions, in order to build a physical model for the role of the catalyst. Then, many experimental techniques have been used, including most recently electron microscopy and X-ray absorption. We focus our attention here on the EXAFS (Extended X-ray Absorption Fine Structure) technique and its possibilities for the study of supported metal catalysts. Most of the examples come from a collaboration between LURE and some public CNRS laboratories (Strasbourg, Meudon) and a private one (IFP — Rueil Malmaison). We begin with some generalities about the technique and the type of catalysts studied, then move to several examples of application. [ABSTRACT FROM AUTHOR]

In this short review of recent advances in XANES we have discussed the use of this spectroscopy to probe the geometry of local structure via multiple scattering effects in the real space and and the local partial unoccupied density of states in metals via the band structure approach. Several important aspects of x-ray absorption spectroscopy have not be considered such as the excitonic effects, the many body infrared singularities in free electron metals and the many body final state configurations in solids with high electronic correlation such as the valence fluctuating metals and correlated oxides as CeO2 and the new high Tc oxide superconsuctors. These effects require to go beyond the one-electron aproximation discussed here but give important information on the local electronic structure of materials. [ABSTRACT FROM AUTHOR]