Aspartame is a dipeptide methyl ester. It is written chemically as L-aspartyl-L-phenylalanine methyl ester. Aspartame is comprised of two essential amino acids, aspartic acid and phenylalanine. These are the building blocks of proteins in all living things. Unlike sugar which is much simpler chemically, when you heat amino acids they break down to simpler things. Sugar is already a simple hydrocarbon chain and is more heat stable than aspartame, although it does break down, and this is how you get caramel--by heating sugar.