Substitution of C-5 propynyl-dC (pdC) for dC and C-5 propynyl-dU (pdU) for dT are effective strategies to enhance base pairing. Duplex stability and melting temperatures are raised by 2.8° or 1.7° per substitution of pdC or pdU respectively. Such Tm modification is used in Turbo Taqman probes for example. C5-propyne pyrimidine modifications are sometimes added to increase nuclease resistance and binding affinity of antisense oligos. Work reported by Seela and co-workers[Ref: Seela, F.; Budow, S.; Leonard, P. Org. Biomol. Chem., 2007, 5, 1858.] has also described stabilization of cytosine intercalated structures (i-motif) via incorporation of propynyl dC.