Some Pragmatic Aspects 141 Definitions and Symbols

At this point, we should make an explanatory comment concerning the expressions "hydrophobicity" and "lipophilicity". Their usage is not uniform. Semantically, they seem to stand for the same feature or object, and are therefore frequently considered to be synonymous. In the scientific use, however, their meaning is quite different. The following operational definitions have been suggested by the IUPAC [15, 16]:

• Hydrophobicity is the association of nonpolar groups or molecules in an aqueous environment which arises from the tendency of water to exclude nonpolar molecules.

• Lipophilicity represents the affinity of a molecule or a moiety for a lipophilic environment. It is commonly measured by its distribution behavior in a biphasic system, either liquid-liquid (e.g., partition coefficient in 1-octanolAvater) or solid-liquid (e.g. retention on RP-HPLC orTLC, see section 1.4.2) systems.

Such definitions are by no means unambiguous and noncontroversial, as our knowledge of molecular mechanisms underlying these phenomena, although continuously growing, is still far from being complete. There are, however, pragmatic reasons for their (albeit tentative) differentiation, and it is therefore not astonishing that they occur with different frequency in languages used in different research disciplines. The term "hydrophobicity" is familiar to biophysicists working with X-ray diffraction, NMR spectroscopy and molecular models. It is used in connection with the description of the molecular surface of a compound in contact with an aqueous environment. "Lipophilicity" is a term mainly employed by medicinal chemists to describe transport processes of a compound in biological systems. Much confusion also exists in the symbols of lipophilicity parameters. To bring some clarity, we offer in Table 1 a compilation of useful symbols.

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