Cyclic amino acids

thumb|Proline ball and stick model spinning Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the protonated form (NH2+) under biological conditions, while the carboxyl group is in the deprotonated −COO− form. The "side chain" from the α carbon connects to the nitrogen forming a pyrrolidine loop, classifying it as a aliphatic amino acid. It is non-essential in humans, meaning the body can synthesize it from the

'(2S,4R)-4-Hydroxyproline, or L-hydroxyproline (C5H9O3N), is an amino acid, abbreviated as Hyp or O', e.g., in Protein Data Bank.

group of stereoisomers

chemical compound

pair of enantiomers

chemical compound

Cycloleucine is a non-proteinogenic amino acid. It could be classified as a cyclopentane derivative of norleucine, having two hydrogen atoms less. The α-carbon atom is not a stereocenter. Cycloleucine is a non-metabolisable amino acid that specifically and reversibly inhibits nucleic acid methylation. It is widely used in biochemical experiments.

Thioproline is a nonproteinogenic amino acid with the formula , although it crystallizes as the zwitterion . It consists of a 1,3-thiazolidine ring ( ) substituted with a carboxylic acid. It is synthesized by reaction of formaldehyde and cysteine. It occurs in nature, but rarely. It forms a coordination complex with cobalt.