Described is a compound for substituting amino acids of proteins or peptides, wherein the compound has the formula (1)wherein the
stereocenter C1* represents CH that either results in the D- or L-form of the compound of formula (1); A− is a negatively charged
ion compensating the positive charge; X1 is H or an amine
protecting group; X2 is OH, an ester residue or an
amide residue or an activated
carboxylic acid derivate to be used for the formation of an
amide bond; Y1, Y2 and Y3 are independently from each other a C1-C6
alkyl, and Z1 and Z2 are independently from each other a
nucleobase, a C1 to C16
alkyl group, an aromatic residue or a heteroaromatic residue, wherein the aromatic residue or the heteroaromatic residue contains one 5, 6, or 7-membered ring or two condensed 5, 6, or 7-membered rings sharing two carbon atoms, wherein the aromatic residue or the heteroaromatic residue can be substituted by a linear or branched C1 to C16
alkyl group and / or a further aromatic group. Furthermore, a method for preparing this compound and proteins and peptides containing this compound are disclosed.