Enzymatic stereospecific preparation of fluorescent S-adenosyl-L-methionine analogs.

S-Adenosyl-L-methionine (SAM) is the preferred cofactor for biological methyl group transfers to various substrates such as nucleic acids, proteins, and lipids. Here we present stereospecific (>95% of the desired enantiomer) and high-yield preparation of four fluorescent and biologically active SAM analogs and demonstrate their usefulness in binding studies. Using a fluorescence titration experiment, we obtained a K(d) of 0.38 microM for the S-2,6-diaminopurinylmethionine-SAM-III riboswitch complex.