Molecular-level construction

In the mid-1980s, virtually simultaneous reports on two new precise molecular level constructions of sub-nanoscopic/nanoscopic size appeared in the literature. In 1984 we (DAT) reported the synthesis, isolation and characterization of the first iterated series of Starburst/cascade dendrimers based on genealogical synthesis [2, 77-83]. The following year Smalley, Curl and Kroto described the first observation of a 60-carbon fullerene by mass spectroscopy [43a]. More recently the synthesis of buckminsterfullerene has been achieved by physicists at the Max Planck Institute in Heidelberg [44] to give macroscopic quantities of the third allotropic and first molecular form of carbon, named after Buckminster Fuller. Similarities between these two constructions were not initially apparent, but in retrospect they deserve comment in so far as they each involve molecular level synthesis leading to closed geometrical architecture". 

The belief that molecular-level prototypes can be constructed to imitate familiar macroscopic objects has been the inspiration for many intensive and successful synthetic efforts [51-57], Molecular-level construction of Platonic solids such as the tetrahedron [54], the cube [55], and the dodecahedron [56], as well as non-Euclidian objects such as molecular knots [57a], Mobius strips [57b], and even molecular machinery [57d] exemplifies such efforts (cf. Chapter 5 of this book). Synthetic organic chemists have developed a rich legacy in this area as described in more detail by Nickon and Silversmith [57c]. 

Other than the above structural features there are two important and exclusive properties that make DNA suitable for molecular level constructions. These are molecular recognition and self-assembly. The nucleotide bases A and T on two different ss-DNA have affinity towards each other, so do G and C. Effective and stable cis-DNA structures are only formed if the base orders of the individual strands are complementary. Hence, if two complementary single strands of DNA are in a solution, they will eventually recognize each other and hybridize or zip-up forming a cis-DNA. This property of molecular recognition and self-assembly has been exploited in a number of ways to build complex molecular structures [ 114-121]. In the mechanical perspective, if the free energy released by hybridization of two complementary DNA strands is used to lift a hypothetical load, a force capacity of 15 pN can be achieved [122], comparable to that of other molecular machines like kinesin (5 pN) [123],... 

While the methods discussed represent appHed modalities that promise to be implemented within years, the scope of nanobiotechnology encompasses research which, though currently on the very frontiers of modern science, present innumerable possibihties for the future of nanomedicine. The prophesied abihty of nanoscale machines, or nanobots, to provide molecular level construction and repair to exterminate disease and erase genetic defects represents the pinnacle of nanomedicine aspirations. Although the inherent difficulties in the design and manufacture of such devices raise questions as to their feasibihty [153], research continues to explore nature s nanomachines and issues crucial to the development of nanobots. In cells, proteins are nanomachines that act as transporters, actuators, and motors, and are responsible for meticulous monitoring and repair processes [154,155],... 

In parallel, the challenge continues for the chemist to prepare compounds whose properties are similar to objects belonging to other fields of human science or culture. Platonic solids have been a target in synthetic chemistry for decades. Among them, the dodecahedron [7] is certainly one of the most beautiful examples. Molecular-level construction of a tetrahedron [36] or a cube [37] also represent remarkable accomplishements. Buckminsterfullerene Cgq and all its derived structures represent an explosive field of research in which aesthetics and applications are intricately Iinked.[38]... 

Any mathematical function that adequately represents experimental rate data can be used in the rate law. Such a rate law is called an empirical orphenomenologicd rate law. In a broader sense, a rate law may be constructed based, in addition, on concepts of reaction mechanism, that is, on how reaction is inferred to take place at the molecular level (Chapter 7). Such a rate law is called a fundamental rate law. It may be more correct in functional form, and hence more useful for achieving process improvements. 

We have shown that redox chromophores organized in LB films with resped to their orientation, alignment, or electronic interactions make very useful and specific photoresponses such as amplified fluorescence quenching, photocurrents controlled at the molecular level, photoinduced anisotropic eledrochromism, and photochemically modulated second harmonic generation. These results may contribute to facilitate the design and construction of novel photonic devices in the near future. 

Using the LB technique, one can control the film thickness and the molecular arrangement along the film normal at molecular level. These features are very useful for the construction of sophisticated nonlinear waveguide with high... 

FIGURE 3.4 Molecular level alignment diagrams constructed using the HOMO and vacuum levels measured using UPS. The lowest unoccupied molecular orbital LUMO positions are inferred assuming a HOMO/LUMO gap equal to the onset of optical absorption. The chemical structure of CuPc is shown. (From Hill, I.G. and Kahn, A., J. Appl. Phys., 86, 2116, 1991. With permission.)... 

The structure of an alphavirus particle is simpler than that of all known cellular organelles, but it is built according to the same principles. This is because the viral genome is small and the virus must use for its construction those cellular components normally engaged in the biogenesis of host cell membranes. This means that studies of viral replication can be exploited to study cellular functions at the molecular level. Naturally viral infections also perturb cellular physiology, but there is usually enough time early in infection for studies to be carried out before cellular malfunction becomes a source of error. 

Starting from simple statistical considerations concerning the coupling between similar or dissimilar chains, a model was constructed for the architecture of a class of IPN where interaction among phases occurs on a molecular level. It was shown(Equation 13) that the contributions to the modulus due to crosslinks are subject to linear additivity, while a square root additivity rule holds for the contributions due to entanglements. 

Construction of devices based on molecular electronics will require connections at the molecular level. One of the biggest challenges is to construct wires or electrodes... 

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