From a biology perspective base amino acids (such as theanine) on their own are not generally major modulators of human function. You want to experiment with small molecules, something that binds with proteins such as enzymes which in turn modulate pathways and cell functions.
Binders need a energy configuration and 3 dimensional structures that "fits" into such proteins, and individual amino acids dont really cut it. Also the amino acids get rapidly used for protein construction so they dont stick around long.
PhDs in the subject can please correct me I am wrong here.