Secondary amino acid

Proline is the only proteinogenic secondary amino acids.

Pipecolic acid, a heavier analog of proline, is found in efrapeptin.

Sarcosine is a N-methylized glycine so its methyl group is used in many biochemical reactions.

Proline and its higher homolog pipecolic acid affect the secondary structure of protein.

[1] It suggested that acyclic secondary amino acids are more flexible than cyclic secondary amino acids in protein by replacement of pipecolic acid by N-methyl-L-alanine in efrapeptin C.[1] Ninhydrin tests of proline and hydroxyproline give yellow results.