Four dinucleotide analogs of thymidylyl(3′–5′)thymidine (TpT) have been designed and synthesized with a view to increase the selectivity, with respect to CPD, of efficient UV-induced (6–4) photoproduct formation. The deoxyribose residues of these analogs have been modified to increase north and south conformer populations at 5′- and 3′-ends, respectively. Dinucleotides whose 5′-end north population exceeds ca. 60% and whose 3′-end population is almost completely south display a three-fold selective enhancement in (6–4) adduct production when exposed to UV radiation, compared to TpT. These experimental results undoubtedly provide robust foundations for studying the singular ground-state proreactive species involved in the (6–4) photoproduct formation mechanism.
