A molecular beacon probe1 has its natural fluorescence quenched in solution unless it is hybridized to the target sequence. Consequently, the design of a molecular beacon requires a fluorophore to be in one part of the sequence and the quencher molecule to be in another, with both molecules being separated from the oligonucleotide by a hydrocarbon spacer. The Dabcyl group has been found to be a universal quencher. 3’-Dabsyl CPG and 3’-Dabcyl CPG are used to prepare probes with the quencher blocking the 3’-terminus. 5’-Dabcyl Phosphoramidite locates the quencher at the 5’-terminus and Dabcyl-dT places it within the sequence, leaving the 3’-terminus available for polymerase extension.
(1) S. Tyagi and F.R. Kramer, Nature Biotechnology, 1996, 4, 303-308.