S-Adenosyl-L-methionine analogs of formula (I):are disclosed wherein R comprises a carbon-carbon double bond, carbon-sulfur double bond, carbon-nitrogen double bond, -a carbon-carbon triple bond, carbon-nitrogen triple bond or an aromatic carbocyclic or heterocyclic system in β-position to the sulfonium center, X{circle around (−)} is an organic or inorganic anion carrying one or more negative charges, Z is —CR1R2—, —O—, —S— or —NR3— and R1, R2 and R3 are independently selected from H, D and C1—C alkyl; as well as complexes with a methyltransferase, pharmaceutical compositions, methods for modifying a biomolecule, and methods for detecting sequences specific methylation of biomolecules.