What does it mean to characterize a molecule as hydrophilic or as lipophilic? How do they differ in terms of their chemical or physical properties? Are there structural differences between these two groups of molecules that you can use to classify them as hydrophilic or as lipophilic? Consider the molecules below, both minor constituents of Danshen, and classify each molecule as lipophilic or hydrophilic.
Hydrophilic molecules form hydrogen bonds with water and are soluble in water and other polar solvents; not surprisingly, hydrophilic is derived from Ancient Greek for water loving. Lipophilic molecules, where lipos is Ancient Greek for fat, are soluble in fats, oils, lipids, and non-polar solvents. Hydrophilic molecules are more polar than lipophilic molecules, have more ionizable functional groups, and have more sites for hydrogen bonding.
For the eight constituents of Danshen included in this exercise, those that are hydrophilic are soluble, to varying extents, in water. Each hydrophilic compound has one or more ionizable carboxylic acid groups (–COOH) and, as the pKa values for these carboxylic acid functional groups are in the range 2.9–3.6, they are ionized and carry a negative charge at a neutral pH. The lipophilic constituents of Danshen do not have ionizable groups and they are not soluble in water, although they are soluble, to some extent, in polar organic solvents, such as methanol and ethanol.
Each lipophilic molecule in this exercise has three hydrogen bond acceptors and no hydrogen bond donors; the hydrophilic molecules, on the other hand, have between five (danshensu) and 12 (lithospermic acid) hydrogen bond acceptors, and between four (danshensu) or seven (lithospermic acid and salvianolic acid I) hydrogen bond donors.
Based on the structures of the two additional compounds, the one on the left is hydrophilic and the one on the right is lipophilic; the presence or absence of a carboxylic acid function group provides for a definitive classification. The two compounds are salvianolic acid F (left) and dihydroisototanshinone I (right).
Note: The structures for lithospermic acid and salvianolic acid A in the original paper are incorrect in their stereochemistry around the alkene double bonds, which, as shown in this exercise, are all trans; the original paper shows the alkene double bond in lithospermic acid as cis, and shows one of the two alkene double bonds in salvianolic acid A as cis. The structure for lithospermic acid in the original paper incorrectly shows an –OH group on the five-membered ring; as shown in this exercise, it is a –COOH group.