L-Tryptophan-d8 is the deuterium labeled L-Tryptophan. L-Tryptophan (Tryptophan) is an essential amino acid that is the precursor of serotonin, melatonin, and vitamin B3[1].
References
[1] Russak EM, et al. Impact of Deuterium Substitution on the Pharmacokinetics of Pharmaceuticals. Ann Pharmacother. 2019;53(2):211-216. DOI:10.1177/1060028018797110