Volume 75, Issue 5

The macroscopic concepts of a machine can be extended to the molecular level. Molecular-level machines are constructed by the molecule-by-molecule bottom-up approach following the guidelines of supramolecular chemistry. Like macroscopic machines, molecular machines are characterized by (i) the kind of energy supplied to make them work, (ii) the kind of movement performed by their components, (iii) the way in which their operation can be controlled and monitored, (iv) the possibility to repeat the operation, (v) the time needed to complete a cycle of operation, and (vi) the function performed. The most convenient way to supply energy to a molecular machine is through photochemical energy inputs. Photochemical techniques offer, indeed, several advantages: (i) photons can make a machine work without formation of waste products, (ii) light can be switched on/off easily and rapidly, (iii) lasers provide the opportunity of working in very small space and very short time domains, (iv) photons can be used to “read” the state of the system and to monitor the operation of the machine. The extension of the concepts of a machine to the molecular level is of interest not only for basic research, but also for the growth of nanoscience and the development of nanotechnology.

The structurally intriguing [4]- and [5]triangulanes have been prepared in enantiomerically pure form. Their surprisingly high specific rotations are well reproduced by DFT/SCI computations and stem from the fact that these hydrocarbons essentially are σ-helicenes (i.e., rigidly held helical arrangements of σ-bonds). Some light is shed on the properties of radical cations derived from [3]- and [4]rotanes. While the former adopts C s or C 2v symmetry, the latter retains the D 4h symmetry of the neutral hydrocarbon, according to highlevel computations. Experimental and computational evidence is also presented that the antiaromatic cyclopentadienyl cation is stabilized as a singlet ground state by five cyclopropyl substituents. Yet, the three cyclopropyl groups in tricyclopropylamine do not favor the formation of its radical cation, because they are not in the proper orientation. When this amine radical cation is generated by cobalt γ-irradiation in a Freon matrix, evidence for a significant conformational change is obtained by EPR spectroscopy. Finally, the conformational dynamics of the newly prepared crowded molecules tetracyclopropyl- and tetraisopropylmethane are discussed.

Nature’s solutions to control organization, switching, and linear and rotary motion are not only extremely elegant, but fascinating if one considers the design and synthesis of artificial molecular systems with such functions in order to add components to the nanotoolbox. The synthesis of chiroptical molecular switches and their application in the control of the organization of liquid crystalline materials is outlined. The first light-driven unidirectional molecular motor is described, and the second-generation motor as well as approaches to control the speed of the rotary motion are discussed.

Catalytic control of electron-transfer processes is described for a number of photoinduced and thermal electron-transfer reactions, including back electron transfer in the charge-separated state of artificial photosynthetic compounds. The intermolecular and intramolecular electron-transfer processes are accelerated by complexation of radical anions, produced in the electron transfer, with metal ions that act as Lewis acids. Quantitative measures to determine the Lewis acidity of a variety of metal ions are given in relation with the promoting effects of metal ions in the electron-transfer reactions. The mechanistic viability of metal ion catalysis in electron-transfer reactions is demonstrated by a variety of examples of both thermal and photochemical reactions that involve metal ion-promoted electron-transfer processes as the rate-determining steps, which are made possible to proceed by complexation of radical anions with metal ions.

Since their introduction by E. L. Eliel nearly four decades ago, derivatives of 1,3-dioxane have proved useful in conformational analysis. Examples are discussed, where 5-polar substituents permit the evaluation of fundamental phenomena such as attractive and repulsive gauche effects, electrostatic interactions, and stereoelectronic effects. By the same token, 2-substituted 1,3-dioxanes, 1,3-dithianes, and 1,3-oxathianes are useful frameworks for the study of the anomeric effect and the associated structural and spectroscopic manifestations, such as the so-called Perlin effects. In view of the varied and essential involvement of metal ions and inorganic salts in biological processes, 5-substituted 1,3-dioxanes are presently being examined in the presence of Li + , Na + , K + , Ag + , Mg 2+ , Ca 2+ , Ba 2+ , and other cations, with the aim to increase the understanding of biomolecular properties in vivo.

Hydrogen-transfer processes are expected to show appreciable quantum mechanical behavior. Intensive investigations of enzymes under their physiological conditions show this to be true in practically every example investigated. Initially, tunneling was treated either as a tunneling correction [cf. Bell, The Tunnel Effect in Chemistry , Chapman & Hall, New York, (l980)], or as corner-cutting [Truhlar et al., J. Chem. Phys . 100, 12771 (l996)]. This worked well as long as the observed properties could be explained by “corrections” to transition-state theory. However, over the past several years, enzymatic behaviors have been observed that are so deviant as to lie outside of transition-state theory. This phenomenon is discussed in the context of the enzyme, soybean lipoxygenase. An environmentally coupled hydrogen-tunneling model is presented that derives from the treatments of Kuznetsov and Ullstrup [ Can. J. Chem. 77, 689 (l999)], and includes heavy-atom reorganization (temperature-dependent and largely isotope-independent), together with heavy-atom gating (temperature- and isotope-dependent). This treatment can explain a wide range of behaviors and leads to a new view of the origin of kinetic isotope effects in hydrogen-transfer reactions. These properties link enzyme fluctuations to the hydrogen-transfer reaction coordinate, making a quantum view of H-transfer necessarily a dynamic view of catalysis.

Self-replicating nanorobots were foreseen in technological dreams and visions whose scientific ground found solid criticism from chemical and physical reasoning. If one, however, views such constructs as three-dimensionally defined noncovalent nanoscaffolding of a multitude of modular functions whose array is replicatable in a nonautonomous way, many pieces of technology needed for their implementation became recently available. Gold cluster-labeled molecules were remotely controlled by GHz radio frequency causing local and selective inductive heating, and monoconjugable thermostable gold clusters will become commercially available soon. Charged molecules were electrophoretically steered and manipulated on the surface of microelectrode array chips. Surface-promoted replication and exponential amplification of DNA analogs (SPREAD) may find particular applications for the cloning and copying of informational nanostructures on the surface of such chips. Synthetic trisoligonucleotidyl junctions were reported as covalent building blocks for noncovalent DNA nanostructures, and it was shown that kinetic control during noncovalent synthesis favors small and defined nanostructures instead of polymeric networks. Very recently, it was demonstrated that functionalized DNA nanoscaffolds with stiff tensegrity such as tetrahedra self-assemble from maximally instructed sets of 3- or [3+1]-arm junctions, and that the connectivity information in such nanoscaffolds can be copied. The implications of these developments are discussed with respect to a possible implementation scheme for the issue of the title.

Self-assembly is a concept familiar to chemists. In the molecular and nanoscale regimes, it is often used as a strategy in fabricating regular 3D structures—that is, crystals. Self-assembly of components with sizes in the µm-to-mm range is less familiar to chemists; this type of self-assembly may, however, become technologically important in the future. In this size range, self-assembly offers methods to form regular 3D structures from components too small or too numerous to be manipulated by other means, and methods to incorporate function into these structures; it also offers simplicity and economy. This paper focuses on the use of self-assembly to build functional systems of components with sizes in the range from microns to millimeters. It compares the principles of selfassembly at the molecular and millimeter scales, reviews the possible applications of mesoscale, self-assembled systems, and outlines some of the most important issues in the use of self-assembly to build functional systems.

Many chemical substances of natural or anthropogenic origin are suspected or known to be endocrine disruptors, which can influence the endocrine system of life. This observation has led to increased interest on the part of the public and the media, as well as to a steep rise of research activities in the scientific community. New papers and results are presented so fast that it is impossible to give a complete review of this emerging research field. Therefore, this paper tries to give insight into some topics of the great scope of endocrine disruptors in the environment. To get a general idea of the biochemical and biological background, some parts of the endocrine systems of mammalians and nonmammalians are explained. The sections that follow describe important mechanisms of endocrine disruption such as interactions with hormone receptors. Test strategies for anthropogenic chemicals on various organisms are critically reviewed with respect to their problems and gaps concerning endocrine disruptors. The main emphasis of the paper is on the chemical substances suspected or known to be endocrine disruptors. To get a better comprehension of their behavior in the environment, physicochemical data such as water solubility or K ow , as well as information about their use and/or function are reviewed and compared. The main routes of exposure for most chemicals are shortly described, and data about concentrations in the environment (soil/sediment, water) are detailed.