The Type of Group
The incorporation of polar functional groups, such as the alcohol, amine, amide, carboxylic acid, sulfonic acid and phosphate groups, which either ionize or are capable of relatively strong intermolecular forces of attraction with water (hydrogen bonding), will usually result in analogues with an increased water solubility. Acidic and basic groups are particularly useful, since these groups can be used to form salts, which would give a wider range of dosage forms for the final product. However, the formation of zwitterions by the introduction of either an acid group into a structure containing a base or a base group into a structure containing an acid group can reduce water solubility. Introduction of weakly polar groups, such as carboxylic acid esters, aryl halides and alkyl halides, will not significantly improve water solubility and can result in enhanced lipid solubility.
The incorporation of acidic residues into a lead structure is less likely to change the type of activity, but it can result in the analogue exhibiting haemolytic properties. Furthermore, the introduction of an aromatic acid group usually results in anti-inflammatory activity, whilst carboxylic acids with an alpha functional group may act as chelating agents. Basic water solubilizing groups have a tendency to change the mode of action, since bases often interfere with neurotransmitters and biological processes involving amines. However, their incorporation does mean that the analogue can be formulated as a wide variety of acid salts. Non-ionizable groups do not have the disadvantages of acidic and basic groups.
Read more about this topic: Molecular Modification
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