Pyroglutamic acid (also known as PCA, 5-oxoproline, pidolic acid) is a ubiquitous but little studied natural amino acid derivative in which the free amino group of glutamic acid or glutamine cyclizes to form a lactam. The names of pyroglutamic acid conjugate base, anion, salts, and esters are pyroglutamate, 5-oxoprolinate, or pidolate. Pyroglutamic acid exists as two distinct enantiomers: * (2R) or D which happens to be (+) or d * (2S) or L which happens to be (–) or l