Notice the small side chain consisting of three carbons bonded to hydrogens. Each amino acid differs in what functional group it has hanging from the second carbon. Note the large two rings, one of which contains nitrogen. Tryptophan is the starting material for serotonin, an important neurotransmitter and for melatonin, a hormone produced by the pineal gland. The large ring is a benzene ring.
Some amino acids have a functional group that ionizes in solution.
This is aspartate is it might be in solution. The carboxyl group has lost a hydrogen. Also if you note the two oxygens coming from the bottom of the molecule, one was originally a hydroxyl group and has now lost its hydrogen to the solution. Thus the functional group is a negatively charged ion . This will be very hydrophilic .