Table 3. Sources of used substances.

substance	LOT	denoted purity [%]	company
L-tryptophan	113H0290	98*	Sigma-Aldrich (St. Louis, USA)
1-Methyl-L-tryptophan	15399MJ	95*	Sigma-Aldrich (St. Louis, USA)
1-Methyl-L-tryptophan	08116EJ	95*	Sigma-Aldrich (St. Louis, USA)
1-Methyl-L-tryptophan	MKBF4000V	95*	Sigma-Aldrich (St. Louis, USA)
1-Methyl-L-tryptophan	L24579	95	Enzo Life Sciences Inc. (Farmingdale, USA)
1-Methyl-DL-tryptophan	G0411	98	Santa Cruz Biotechnology Inc. (Santa Cruz, USA)
1-Methyl-D-tryptophan	01929PH	95*	Sigma-Aldrich (St. Louis, USA)
1-Methyl-D-tryptophan	MKBF1251V	95*	Sigma-Aldrich (St. Louis, USA)

^*based on elemental analysis.