(en) F. Gowland Hopkins & Sydney W. Cole (1901). A contribution to the chemistry of proteids: Part I. A preliminary study of a hitherto undescribed product of tryptic digestion. The Journal of Physiology27 (4-5): 418. PMC1540554.
(en) F. Gowland Hopkins (1912). Feeding Experiments Illustrating the Importance of Accessory Food Factors in Normal Dietaries. The Journal of Physiology44 (4-5): 425-460. PMC1512834.