The present invention relates to modified amino acids of general formula wherein A, Z, X, n, m, R, R2, R3, R4 and R11 are defined as in claims 1 to 5, their tautomers, their diastereomers, their enantiomers, the mixtures thereof and the salts thereof, particularly the physiologically acceptable salts thereof with inorganic or organic acids or bases, pharmaceutical compositions containing these compounds, the use thereof and processes for preparing them as well as their use for the production and purification of antibodies and as labelled compounds in RIA- and ELISA assays and as diagnostic or analytical aids in neurotransmitter research.