Enantiomerically pure 2-, 3- or 4-pyridylalanine (pya) and 2-azatyrosine (azatyr) are known to present various biological activities. After incorporation into appropriate peptide sequences, these heterocyclic non natural a-amino acids could behave as new substrates or inhibitors of elastase from Pseudomonas aeruginosa.This enzyme is known to be involved in nosocomial infections and infections related to the cystic fibrosis disease. New efficient chemoenzymatic preparations of those compounds using a-chymotrypsin (a-CT) are presented.