6.4: Solubility Classification for Biochemical Compounds
- Explain the terms hydrophilic , hydrophobic , and amphipathic and with examples of compounds that belong to each category.
The biochemical compounds found in living organisms may be placed into one of the three solubility classes. Those soluble in water are hydrophilic , those insoluble in water are hydrophobic , and those in between are classified as amphipathic .
Hydrophilic
Glucose and other monosaccharides are classified as hydrophilic. Monosaccharides contain a large number of alcohol functional groups that are able to hydrogen bond to water. The aldehyde functional group in glucose is also polar due to the polar carbonyl and therefore is able to hydrogen bond to water. Amino acids are building blocks of proteins. Two examples of amino acids shown here are serine and tyrosine. They have the functional groups carboxylic acid and 1 o -amine that are able to hydrogen bond to water molecules and so they are soluble in water.
Hydrophobic
Oleic acid is a fatty acid with 18 carbon atoms. One end of the molecule has a polar carboxylic acid however the rest of the molecule is a chain of carbon atoms that is nonpolar. Therefore, oleic acid and other fatty acids are hydrophobic. Triglycerides (oils and fats) similarly are hydrophobic due to the large nonpolar areas in the molecule.
Amphipathic
Soaps and bile salts carry a negative charge on the oxygen atom making the polar end very hydrophilic. This is enough to make the molecules amphipathic. Amphipathic compounds have a hydrophilic part and a hydrophobic part.
Prodrugs
A drug is a chemical agent which can affect living processes. Occasionally, a drug is administered as an inactive (a pro-drug ) compound. The inactive compound in the body is converted to a fully active drug. In our example of the antibiotic chloramphenicol (in red), a prodrug chloramphenicol palmitate (in blue) is used to overcome problems with solubility of the drug in water.
Chloramphenicol has a bitter taste when given orally in solution form . To overcome the bitter taste, it is converted to an ester of palmitic acid to make it water-insoluble and tasteless. The hydrophobic areas of the palmitic acid makes the prodrug- insoluble in saliva and hence almost tasteless. Once in the body, it is undergoes ester hydrolysis to chloramphenicol (drug) and palmitic acid.