Novel benzopyrimidines as widened analogues of DNA bases.

We report on the synthesis, stacking, and pairing properties of a new structural class of size-expanded pyrimidine nucleosides, abbreviated dyT and dyC. Their bases are benzo-homologated variants of thymine and cytosine and have a design that is distinct from a previously described class of size-expanded (xDNA) pyrimidines, with a different vector of expansion relative to the sugar. We term this new base geometry "yDNA" (a mnemonic for "wide DNA"). Both C-glycosides were prepared using Pd-mediated coupling of iodinated base derivatives with a deoxyribose precursor. As free deoxynucleosides, both dyT and dyC displayed robust fluorescence, with emission maxima at 375 and 390 nm, respectively. Both widened pyrimidines could be incorporated readily as protected phosphoramidite derivatives into synthetic oligonucleotides. Experiments in "dangling end" DNA contexts revealed that both yT and yC stack more favorably than their natural counterparts. When opposite natural bases in the context of Watson-Crick DNA were paired, the yT nucleotide formed a pair with A that was equally stable as a T-A pair, despite the mismatch in size with the neighboring natural pairs. The yC nucleotide (paired opposite G) was destabilizing by a small amount in the same context. Despite the large size of the pairs, both yT and yC were selective for their Watson-Crick complementary partners A and G, respectively. The pairing properties and fluorescence of yDNA nucleotides may lead to useful applications in the study of steric effects in DNA-protein interactions. In addition, the compounds may serve as building blocks for a large-sized artificial genetic system.