Volume 77, Issue 11

The 9th International Chemistry Conference in Africa (ICCA-9), Africa's continent-wide premier conferencing event in the chemical sciences, was this year hosted in Tanzania. Organized under the umbrella of the continent's African Association of Pure and Applied Chemistry (AAPAC), the event takes place every third year on a rotating basis, in a country so designated at the preceding meeting, with the national affiliate as hosts. This year's event was held for the first time since AAPAC instituted the series in 1990, in the scenic tourist setting of Arusha in northern Tanzania from 2-7 August 2004. That event, hailed as among the continent's most successful, was by coincidence, befittingly held at this panoramic location which has been designated as the exact Cape-to-Cairo midpoint and has as its backdrop, Mt. Kilimanjaro, the highest point on the continent. The event brought together participants from over 60 countries from the five continents, and the scientific sessions included a total of over 100 lectures, presentations, and posters. The social program was structured to afford participants the opportunity to sample nature's unique and spectacular wildlife heritage in the proximity, including the world famous game parks of the Serengeti and the Ngorongoro Crater, which have no parallel elsewhere. The scientific coverage included topics in analytical, physical, organic, environmental, industrial, and natural products chemistry. Delegates heard from the International Union of Pure and Applied Chemistry (IUPAC), who sponsored the event; the Royal Society of Chemistry, from whom generous conferencing input was sourced; and the American Chemical Society, who sent representatives to grace the event; plus the local partners of the Tanzania Chemical Society (TCS), the national affiliate to IUPAC and AAPAC. The official theme of the conference, "Chemistry towards disease and poverty eradication", was more than adequately targeted by the scientific conference content, which did it justice by exploring, evaluating, and demonstrating how advances in the chemical sciences and technology form vital partnerships toward those goals across the continent. This issue of Pure and Applied Chemistry features a small selection of papers arising from the main lecture program, and serves to exemplify important features of the conference theme. After a general assembly to elect new AAPAC office bearers for the coming triennium, the conference wound up business amidst unprecedented optimism that the road to success is always under construction, and that through this gathering we had cast off doubt, demonstrating and providing solid evidence that this activity is alive and well in all corners of the continent. With that upbeat note, delegates bade farewell to each other and to Arusha 2004, promising to gather again in three years time in Botswana for the 10th International Chemistry Conference in Africa. Donnati M. S. Mosha Conference Editor

The 17th International Conference on Physical Organic Chemistry (ICPOC-17) is the latest event in a regular IUPAC-sponsored series, and took place in Shanghai, China on 15ñ20 August 2004 under the local auspices of the Chinese Academy of Sciences and the Chinese Chemical Society. About 200 delegates attended ICPOC-17 from 21 countries, to participate in a scientific program comprising 14 plenary and 17 invited lectures, 41 contributed lectures, and 53 posters. The ongoing importance of the conference theme is reflected in a range of topics that serve to underpin fundamental principles of the subject, whilst also addressing new and evolving challenges posed by the growing interdisciplinary interfaces. This interdependence is exemplified by the centrality of mechanistic insight in modern synthesis, the interpretive and predictive power of theoretical studies, and the indispensable role of physical organic principles in studies of organized matter and biological systems, all of which featured prominently in the program. Proceedings of this series of conferences are traditionally published in Pure and Applied Chemistry , and it is pleasing to introduce a representative selection of outstanding works based upon plenary lectures delivered at ICPOC-17. Mechanistic studies form the subject of papers by H. Mayr (Ludwig-Maximilians University, Munich, Germany), V. D. Parker (Utah State University, Logan, UT, USA), and M. Eckert-Maksic (Rudjer Boskovic Institute, Zagreb, Croatia), whilst those of F. Diederich (ETH, Zurich, Switzerland) and C. J. Easton (Australian National University, Canberra, Australia) address fundamental aspects of materials sciences. Biological themes feature in works by F. M. Menger (Emory University, Atlanta, GA, USA), Zhong-Li Liu (Lanzhou University, China), and A. Pross (Ben Gurion University of the Negev, Beer Sheva, Israel). Collectively, these papers exemplify the vigor and topicality of the overall scientific proceedings of ICPOC-17, and convey something of the diversity of themes that characterize current and future challenges in physical organic chemistry. This important forum will reconvene in Warsaw on 20ñ25 August 2006 for the 18th International Conference on Physical Organic Chemistry (ICPOC-18), and it is confidently expected that this record will serve the needs of practicing scientists, and offer inspiration and insight for future advances that may be reported on that occasion. Min-Bo Chen Conference Editor

Benzhydrylium ions (Ar 2 CH + ) and structurally related quinone methides are employed as reference electrophiles for comparing the nucleophilicities of a large variety of compounds, e.g., alkenes, arenes, alkynes, allylsilanes, allylstannanes, enol ethers, enamines, diazo compounds, carbanions, transition-metal π-complexes, hydride donors, phosphanes, amines, alkoxides, etc., using the correlation equation log k (20 °C) = s ( N + E ), where s and N are nucleophile-dependent parameters and E is an electrophilicity parameter. The same equation was employed to derive the electrophilicity parameter E for different types of carbocations, cationic transition-metal π-complexes, typical Michael acceptors, and electron-deficient arenes. The E , N , and s parameters thus obtained can be used for predicting rates and selectivities of polar organic reactions.

In recent years, we have reported that a number of organic reactions generally believed to follow simple second-order kinetics actually follow a more complex mechanism. This mechanism, the reversible consecutive second-order mechanism, involves the reversible formation of a kinetically significant reactant complex intermediate followed by irreversible product formation. The mechanism is illustrated for the general reaction between reactant and excess reagent under pseudo-first-order conditions in eq. i where k f ' is the pseudo-first-order rate constant equal to k f [Excess Reagent]. Reactant + Excess reagent = Reactant complex = Products (i) The mechanisms are determined for the various systems, and the kinetics of the complex mechanisms are resolved by our "non-steady-state kinetic data analysis". The basis for the non-steady-state kinetic method will be presented along with examples. The problems encountered in attempting to identify intermediates formed in low concentration will be discussed.

An overview of our recent work on cycloproparenyl anions is given. Preparation, the electronic structure, and the properties of the progenitor of the series, cyclopropabenzenyl anion, are discussed. It is shown that the cyclopropabenzenyl anion is by ca. 145 kJ mol -1 more stable than the parent cyclopropenyl anion according to results of the MP2/6-31+G(d) calculations. This finding was attributed to a delicate balance of two opposing effects: (a) propensity of the aromatic ring to alleviate unfavorable 4π electron interaction within the three-membered ring by the anionic resonance effect and (b) a pyramidalization of the anionic center, which tends to maximize the s-character of the lone pair. We have also shown that stability of the cyclopropabenzenyl anion could be considerably enhanced by substitution of the aromatic ring with fluorine and cyano groups, as well as by a linear extension of the aromatic backbone. Finally, the impact of the fusion of additional cyclopropenyl ring to the benzene moiety to acidity of the benzylic position in cyclopropabenzene is discussed.

Functional π-systems with unusual opto-electronic properties are intensively investigated from both fundamental research and technological application viewpoints. This article reports on novel π-conjugated systems obtained by acetylenic and fullerene scaffolding. Linearly conjugated acetylenic nanorods, consisting of monodisperse poly(triacetylene) (PTA) oligomers and extending up to 18 nm length, were prepared to investigate the limits of effective conjugation and to explore at which length a monodisperse oligomer reaches the properties of an infinite polydisperse polymer. With the cyanoethynylethenes (CEEs), a powerful new class of electron acceptors is introduced that undergo intense intramolecular charge-transfer (CT) interactions with appended donors. Macrocyclic scaffolds with unusual opto-electronic properties are perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes bearing peripheral dialkylanilino donor groups. Extended porphyrin-fullerene conjugates are investigated for their novel photophysical and efficient multicharge storage properties. Self-assembly of fullerenes and porphyrins, governed by weak interactions between the two components, leads to unprecedented nanopatterned surfaces that are investigated by scanning tunneling microscopy (STM).

The naturally occurring cyclodextrins and their derivatives have been developed as miniature reaction vessels, to manipulate the outcomes of chemical transformations at the molecular level. In this manner, the rate of hydrolysis of a phosphate triester has been enhanced by almost five orders of magnitude, and the ratios of products obtained from electrophilic aromatic substitution reactions, from competing reactions to give indigoid dyes, and from nitrile oxide cycloadditions have all been changed, by factors of up to 3500 times.

This paper begins with a brief review of theories and concepts that have influenced today's view of enzyme catalysis: transition-state stabilization, entropy, orbital steering, proximity, and intramolecularity. The discussion then launches into the "spatiotemporal" model of enzyme catalysis in which fast intramolecular and enzymatic rates are ascribed to short distances that are imposed rigidly upon the reacting entities. An equation relating rate and distance is set forth, as are experimental and computational data supporting this relationship. Finally, enzyme systems themselves are analyzed in terms of the distance parameter and the so-called "split-site" model in which ground-state geometries play a crucial role. Among the many surprising conclusions is a transition-state stabilization by noncovalent forces (e.g., hydrogen-bonding) that are positioned far away from the actual transition-state chemistry. The model also confronts and dismisses the claim in classical enzymology that the ubiquitous enzyme-substrate complex is either inconsequential or inhibitory to the overall reaction rate.

Bioantioxidants, or dietary antioxidants, are dietary substances in food that significantly decrease the adverse effects of reactive oxygen species (ROS) on normal physiological functions in humans. Inhibition of ROS-induced oxidative damages by supplementation of bioantioxidants, the so-called "antioxidant therapy", has become an attractive therapeutic strategy to reduce the risk of ROS-related diseases and has led to flourishing research in the past decade. However, many questions dealing with the correlation between chemical and biological activities, the bioavailability and "non-antioxidant" effects of bioantioxidants are still under debate. This article outlines our current kinetic and mechanistic studies on naturally occurring antioxidants, including vitamin E, green tea polyphenols (GOHs), and resveratrol, as well as their synthetic analogs in micelles, in red blood cells, in low-density lipoprotein (LDL) and in microsomes. The cytotoxicity and apoptosis-inducing activity of these antioxidants against cancer cells were also studied. It was found that there was significant correlation between the chemical and biological antioxidant activities, as well as between the antioxidant activity and the cytotoxic and apoptosis-inducing activities.

Despite the considerable advances in our understanding of biological processes, the physicochemical relationship between living and nonliving systems remains uncertain and a continuing source of controversy. In this review, we describe a kinetic model based on the concept of dynamic kinetic stability that attempts to incorporate living systems within a conventional physicochemical framework. Its essence: all replicating systems, both animate and inanimate, represent elements of a replicator space. However, in contrast to the world of nonreplicating systems (all inanimate), where selection is fundamentally thermodynamic , selection within replicator space is effectively kinetic . As a consequence, the nature of stability within the two spaces is of a distinctly different kind, which, in turn, leads to different physicochemical patterns of aggregation. Our kinetic approach suggests: (a) that all living systems may be thought of as manifesting a kinetic state of matter (as apposed to the traditional thermodynamic states associated with inanimate systems), and (b) that key Darwinian concepts, such as fitness and natural selection , are particular expressions of more fundamental physicochemical concepts, such as kinetic stability and kinetic selection . The approach appears to provide an improved basis for understanding the physicochemical process of complexification by which life on earth emerged, as well as a means of relating life's defining characteristics - its extraordinary complexity, its far-from-equilibrium character, and its purposeful (teleonomic) nature - to the nature of that process of complexification.

Nature has been a source of medicinal agents for thousands of years, and an impressive number of modern drugs have been isolated from natural sources, particularly plants, with many based on their use in traditional medicine. The past century, however, has seen an increasing role played by microorganisms in the production of the antibiotics and other drugs for the treatment of serious diseases, and more recently, marine organisms have proved to be a rich source of novel bioactive agents. Natural products will continue to play a crucial role in meeting this demand through the expanded investigation of the world's biodiversity, much of which remains unexplored. By using medicinal chemistry, and combinatorial chemical and biosynthetic technology, novel natural product leads will be optimized on the basis of their biological activities to yield effective chemotherapeutic and other bioactive agents. With much of the biological diversity found in tropical and subtropical regions, the investigation of these resources requires multidisciplinary international collaboration in the discovery and development process. Such collaboration can result in substantial short-term benefits accruing to source countries, with the potential for the generation of significant longer-term benefits in the select cases of those agents that proceed into advanced development, and possible commercialization.

In this paper, the chemistry of some unusual natural products that were isolated from various Tanzanian indigenous plant species during the past 20 years is reviewed. Among the compounds covered in this review are cyclohexane epoxides; heptane derivatives; oxygenated pyrenes; phenanthrene derivatives; flavonoids with uncommon structural frameworks; mono-, bis-, and dimeric prenylated indoles; and variously substituted mono-, sesqui-, and diterpenoids.

In order to fulfill research objectives around target-based drug discovery in the field of anti-infective agents that are prevalent mainly in poor Third World countries, selection of biological and chemical targets is guided by economic drug discovery and rational medicinal chemistry. Selection of biological targets of therapeutic relevance in multiple disease-causing organisms, as well as the use of natural products and existing drugs as chemical scaffolds for the discovery and design of novel therapeutics should be viable strategies underpinning drug discovery research in poor Third World countries. In this regard, biological targets of interest to our program include disulfide reductases and cysteine proteases (CPs), while chemical scaffolds include existing antimalarial agents and natural products.

The history of the development of potentiometric sensors over the past century demonstrates progress in constructing single, discrete (i.e., separate, to distinguish from sensor arrays) ion sensors, which have been made as selective as possible. Only a few types reveal high selectivity. However, easy measurement procedure, with low cost and availability, give rise to the search for new ways for their successful application. The present document describes a new concept for application of potentiometric multisensor systems, viz., sensor arrays for solution analysis, and the performance of this new analytical tool - the "electronic tongue". The electronic tongue is a multisensor system, which consists of a number of low-selective sensors and uses advanced mathematical procedures for signal processing based on the pattern recognition (PARC) and/or multivariate analysis [artificial neural networks (ANNs), principal component analysis (PCA), etc.]. Definitions of the multisensor systems and their parameters are suggested. Results from the application of the electronic tongue, both for quantitative and qualitative analysis of different mineral water and wine samples, are presented and discussed.