A dipeptide is a sometimes ambiguous designation of two classes of organic compounds: Its molecules contain either two amino acids joined by a single peptide bond or one amino acid with two peptide bonds.
Two amino acids, one peptide bond 
In this usage, X dipeptide (with X designating an amino acid) must be understood as di-X peptide. This nomenclature is continued by tripeptide (three amino acids, two peptide bonds), tetrapeptide, and so on; longer chains are called oligopeptide, polypeptide, protein.
Dipeptides are produced from polypeptides by the action of the hydrolase enzyme dipeptidyl peptidase. Dietary proteins are digested to dipeptides and amino acids, and the dipeptides are absorbed more rapidly than the amino acids, because their uptake involves a separate mechanism. Dipeptides activate G-cells found in the stomach to secrete gastrin.
- Carnosine (beta-alanyl-L-histidine) is highly concentrated in muscle and brain tissues.
- Anserine (beta-alanyl-N-methyl histidine) is found in the skeletal muscle and brain of mammals.
- Homoanserine (N-(4-aminobutyryl)-L-histidine) is another dipeptide identified in the brain and muscles of mammals.
- Kyotorphin (L-tyrosyl-L-arginine) is a neuroactive dipeptide which plays a role in pain regulation in the brain.
- Balenine (or ophidine) (beta-alanyl-N tau-methyl histidine) has been identified in the muscles of several species of mammal (including man), and the chicken.
- Aspartame (N-L-α-aspartyl-L-phenylalanine 1-methyl ester) is an artificial sweetener.
- Glorin (N-propionyl-γ-L-glutamyl-L-ornithine-δ-lac ethyl ester) is a chemotactic dipeptide for the slime-mould Polysphondylium violaceum.
- Barettin (cyclo-[(6-bromo-8-en-tryptophan)-arginine]) is a cyclic dipeptide from the marine sponge Geodia barretti.
One amino acid, two peptide bond 
In this usage, X dipeptide is taken literally: One amino acid X is equipped with two minimal peptide bonds: The C terminus COOH becomes COCH3, the N terminus NH2 becomes NHCH3. For instance, alanine dipeptide is CH3CONHCH(CH3)CONHCH3.
- M. Bergmann et al., Ann. 449, 277 (1926)
- drugfuture.com Link
- T. Head-Gordon et al, A theoretical study of alanine dipeptide and analogs. International Journal of Quantum Chemistry 36, 311 (1989).
- An introduction to dipeptides at PeptideGuide.