Wiktionary
norleucine
n. An isomer of leucine, not found in natural proteins, used in experimental studies of protein structure and function.
Wikipedia
Norleucine
Norleucine (abbreviated as Nle) is an amino acid with the formula CH(CH)CH(NH)COH. A systematic name for this compound is 2-aminohexanoic acid. The compound is an isomer of the more common amino acid leucine. Like most other α-amino acids, norleucine is chiral. It is a white, water-soluble solid.