L-Tryptophan Overview L-Tryptophan is a heterocyclic amino acid, and is an essential amino acid required for human health. The L-type has a slightly bitter taste, decomposes at 289 °C, and is slightly soluble in water, while being insoluble in ethyl alcohol. It dissolves in diluted acid or diluted alkali. Prolonged exposure to light will cause coloration. Heating together with water can produce a small amount of indole. When heated in the presence of sodium hydroxide and cupric sulfate, it produces a large amount of indole. Heating with acid in a dark environment is relatively more stable.