Enhancement of the binding ability of a ligand for nucleobase recognition by introducing a methyl group.

The recognition ability of pteridine derivatives for nucleobases opposite an abasic (AP) site in an oligodeoxynucleotide (ODN) duplex is enhanced by using a propylene residue (Spacer-C3) as an AP site. The recognition ability is further enhanced both by attaching methyl groups to a fluorescent ligand and by measuring the fluorescence response at 5 degrees C; 6.2 x 10(6) M(-1) of the binding constant is attained between 2-amino-6,7-dimethyl-4-hydroxypteridine and guanine opposite the AP site in water.