The present invention relates to a use of non-cofactor compounds, represented by formulas (I) or (II) wherein R and Z are independently selected from H, D, C1-C12-alkyl, preferably C1-C4-alkyl, alkenyl, alkinyl, phenyl or -LX, wherein X represents a functional group or a reporter group attached via a linker group L, and QH is selected from —SH, —SeH, —NHNH2 or —ONH2, for a targeted modification or derivatization of a biomolecule by covalent coupling to the biomolecule in the presence of a directing methyltransferase. Further development of the method of targeted modification and derivatization are the method for targeted labeling a biomolecule and method for detecting unmethylated target sites in a biomolecule comprising modification of the biomolecule according to the present invention.