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Dietary Supplement 50/50 Blend of D-Phenylalanine & L-Phenylalanine Product Benefits DL-Phenylalanine plays a key role in the biosynthesis of tyrosine and helps form some of the neurotransmitters that work in the mood centers of the brain. DL-Phenylalanine is an essential amino acid that cannot be made by the body and must be obtained from food. Amino acids are considered the building blocks of protein and are necessary for life.