(en) Richard DM, Dawes MA, Mathias CW, Acheson A, Hill-Kapturczak N, Dougherty DM (2009). Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications. IJTR. 2: 45–60. PMID20651948.
(en) Paredes SD, Barriga C, Reiter RJ, Rodríguez AB Assessment of the Potential Role of Tryptophan as the Precursor of Serotonin and Melatonin for the Aged Sleep-wake Cycle and Immune Function: Streptopelia Risoria as a Model. IJTR. 2009;2:23–36. PMID22084580. Gratis volledige artikel: PMC3195230. Dit is een open access artikel, beschikbaar onder de licentie Creative Commons Naamsvermelding (CC BY; versie 3.0).
(en) Gowland Hopkins F, Cole SW A contribution to the chemistry of proteids: Part I. A preliminary study of a hitherto undescribed product of tryptic digestion. The Journal of Physiology. Blackwell Publishing; 1901 Dec 23;27(4-5):418. PMID16992614. PubMed Central: PMC1540554
