Pyroglutamic acid (also known as PCA, 5-oxoproline, pidolic acid, or pyroglutamate for its basic form) is an uncommon and little studied amino acid derivative in which the free amino group of glutamic acid or glutamine cyclizes to form a lactam. It is a metabolite in the glutathione cycle that is converted to glutamate by 5-oxoprolinase. Pyroglutamate is found in many proteins including bacteriorhodopsin. N-terminal glutamic acid and glutamine residues can spontaneously cyclize to become pyroglutamate. This is one of several forms of blocked N-terminals which present a problem for N-terminal sequencing using Edman chemistry, which requires a free primary amino group not present in pyroglutamic acid. The enzyme pyroglutamate aminopeptidase can restore a free N-terminus by cleaving off the pyroglutamate residue.
Pyroglutamic acid, also known as pidolic acid, exists as two distinct enantiomers:

Chemical Name	5-oxo-L-proline
Synonyms	hydroxy-pyridin-3-yl-acetic acid;L-Pyroglutamic acid;L-Pyroglutamicacid;5 oxoproline;L-Proline, 5-oxo-;
CAS No.	98-79-3
Molecular Formula	C5H7NO3
Molecular Weight	129.11400
PSA	66.40000
LogP	-0.32160

MDL number	MFCD00005272

Appearance & Physical State	White fine crystals
Density	1.38 g/cm3
Boiling Point	453.1ºC at 760 mmHg
Melting Point	152-162ºC
Flash Point	227.8ºC
Refractive Index	-10 ° (C=5, H2O)
Water Solubility	10-15 g/100 mL (20 ºC)
Stability	Stable. Incompatible with bases, acids, strong oxidizing agents.
Storage Condition	Store at RT.

RTECS	TW3710000
Safety Statements	S26-S36
HS Code	2933790090
WGK Germany	3
Risk Statements	R36/37/38
Hazard Codes	Xi