Principles of Molecular Structures

An example for such a compound is solanin (1, the structure in Fig. 1) which for many years has been reported to possess two different melting points [631. Only recently was it shown that these two temperatures are the melting and clearing points of a smectic liquid crystal [64]. 

Other compounds which might possess amphotropic behavior, but which have not been studied hitherto, can be found within tannines, saponines, gangliosides, naturally occurring membrane components or biopolymers. Also many synthetic surfactants have never been studied for their lyotropic and thermotropic liquid crystal properties. 

As a result, not all the compounds discussed in the following are already tested for both, lyo- and thermomesomorphism. Thus, in some cases their amphotropic behavior is not yet proved but nevertheless likely. On the other hand, it becomes more and more clear that the strong separation of both kinds of liquid crystallinity is fading [65]. The amphiphilic/amphotropic compounds seem to open a view on an uniform field of liquid crystals, regarding both lyo-and thermotropic behavior of compounds together as one intrinsic principle of nature. 

Most liquid crystals can be classified as being monophilic or amphiphilic in character, but the borderline between these extremes is not sharp, see Table 1. 

The thermotropic properties of amphotropic materials are determined by the equilibrium of the hydro- and lipophilic sections 

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Most of the general principles of molecular structure and the nature of the chemical bond were formulated long ago by chemists by indue-tion from the great body of chemical facts. During recent decades these principles have been made more precise and more useful through the application of the powerful experimental methods and theories of modem physics, and some new principles of structural chemistry have also been discovered. As a result structural chemistry has now become significant not only to the various branches of chemistry but also to biology and medicine. 

The nature of the chemical bond and the principles of molecular structure were formu lated along time ago to systematize an immense body of chemical knowledge. With the advent of quanmm mechanics, it became possible to actually derive the concepts of chemical bonding from more fundamental laws governing matter on the atomic scale. Remarkably, many of the empirical concepts developed by chemists have remained valid when reexpressed in terms of quantum-mechanical principles. 

Reassociation Cot curves for bacteriophage T4 DNA, E. coli DNA, and mouse DNA. [Reproduced with permission from D. M. Prescott Cells Principles of Molecular Structure and Function. Jones and Bartlett, London, 1988.]... 

The chemistry of molecules consists of three major modules molecular architecture (structure) molecular dynamics (conformational analysis) and molecular transformation (chemical reactions). The molecular architecture consists of the basic principles of molecular structure and it deals with the atomic structure, orbitals, hybridization and bonding. Molecular dynamics deals with the molecular motion involving rotation around chemical bonds, steric interactions, torsional strain and properties associated with the conformational changes. Molecular transformation accounts for bond formation and bond breaking within the molecule or between molecules, which is generally called the chemical reaction, and consists of two major aspects, reaction mechanism and kinetics. The third module is one of the major areas of chemistry. This aims to understand the reaction mechanism and its manipulation to reduce the reaction barrier, improve stereoselectivity, increase product yield, or suppress undesirable side reactions. 

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