L-Homoproline or L-pipecolic acid, also abbreviated Pip or Hpr. The proline homolog Hpr is a degradation product of L-lysine. Both enantiomers can be detected in human plasma.
As proline residues strongly influence the conformation of a peptide, replacement of Pro by Hpr (or azetidine-2-carboxylic acid (Aze)) can modulate its secondary structure.