In these, glycine (NH2 --CH2 --COOH) has been replaced by another amino acid of the structure NH2 --CHR1 --COOH in which R1 is a lower alkyl group, a lower hydroxyalkyl group, a phenyl group or a 4-hydroxyphenyl group. 4-Amidinophenylalanine (Aph) can be methylated to give N-methyl-Aph.