aromatic compounds play key roles in the biochemistry of all living things. the four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine each serve as one of the 20 basic building blocks of proteins. further, all 5 nucleotides (adenine, thymine, cytosine, guanine, and uracil) that make up the sequence of the genetic code in dna and rna are aromatic purines or pyrimidines. as well as that, the molecule heme contains an aromatic system with 22 π electrons. chlorophyll also has a similar aromatic system.