Structural forces in biological macromolecules

Biological macromolecules are usually found in a medium (aqueous buffer, for example), and the nature of the medium typically has a profound effect upon the magnitude of y. This is taken into account by exchanging eo for another constant e known as the permittivity of the medium, as shown  

The potential energy of interaction between dipole moment, and point charge, q2, can be expressed by modifying Equation (1.1) to give 

Weak dipoles are associated with anybonds or functional groups involving carbon or hydrogen and an electronegative heteroatoms, such as peptide, phosphodiester or glycosidic links (see later). These weak dipoles interact in a manner described by expressions in the form of Equations (1.7) and (1.8). 

When a weak dipole is in the presence of a polarisable functional group/molecule, then the electric field of that dipole will induce a temporary dipole in the polarisable functional group/molecule. The electrostatic influence of the weak dipole may be expressed in terms of a permanent dipole moment /r i, and that of the induced dipole in terms of an induced dipole moment The potential energy of interaction may then be defined by 

The term a j is known as the polarisability volume of the functional group/molecule that harbours the induced dipole. Whilst the distance dependency of these induced dipole-weak 

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Interactions between macromolecules (protems, lipids, DNA,.. . ) or biological structures (e.g. membranes) are considerably more complex than the interactions described m the two preceding paragraphs. The sum of all biological mteractions at the molecular level is the basis of the complex mechanisms of life. In addition to computer simulations, direct force measurements [98], especially the surface forces apparatus, represent an invaluable tool to help understand the molecular interactions in biological systems. 

The electrostatic attraction between oppositely charged molecules is an adjustable driving force for structured material construction. The current synthetic routes of polymer production often offer many variations in size, topology, functionality and polydispersity. An electrostatically driven assembly of nanostructures allows for the controlled incorporation of materials available by synthetic routes. Biological macromolecules, nevertheless, offer superior polyfunctionality compared to synthetic macromolecules. We preferentially use them. 

There have already appeared artides reviewing the progress achieved in the fields of intermolecular forces, methods of statistical mechanics, and the structures of gases, liqiuds, and their molecular mixtures, including the critical region. De Voe reviews the theory of the conformations of biological macromolecules in solution, and Yomosa that of charge transfer in the molecular compounds of biolo cal systems. 

The structural and dynamic properties of water may be affected by both purely geometrical confinement and/or interaction forces at the interface. Therefore, a detailed description of these properties must take into account the nature of the substrate and its affinity to form bonds with water molecules, as well as the hydration level or number of water layers. In order to discriminate between these effects, reliable model systems exhibiting hydrophilic or hydrophobic interactions with water are required. This appears to be the appropriate strategy to permit some understanding of the behavior of water close to a biological macromolecule, as presented in the following. 

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