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A series of nonpolar thymidine analogues of increasing size: DNA base pairing and stacking properties.

[reaction: see text] We describe the properties in DNA of a set of five nonpolar nucleoside mimics in which shape is similar but size is increased gradually. The compounds vary in the size of their exocyclic substituents, which range from hydrogen to iodine, and are designed to test the steric effects of nucleosides, nucleotides, and DNA in biological systems in a systematic way. We describe the conversion of toluene, 2,4-difluorotoluene, 2,4-dichlorotoluene, 2,4-dibromotoluene, and 2,4-diiodotoluene deoxyribosides into suitably protected phosphoramidite derivatives and their incorporation into synthetic DNAs. Studies of their behavior in the context of hexamer and dodecamer duplexes were carried out, with comparison to natural thymine. Thermal melting data with compounds in 5' dangling positions showed that all five compounds stack more strongly than thymine, and all the dihalo-substituted cases stack more strongly than the unsubstituted toluene case. Stacking correlated with surface area and hydrophobicity, both of which increase across the series. In base-pairing studies, all five compounds showed destabilized pairing opposite natural bases (relative to thymine-adenine pairing), as expected. Notably, pairing among the nonpolar base analogues was considerably more stable, and some of the pairs involving the largest analogues showed stability equal to that of a natural thymine-adenine pair. The results establish the base pairing properties of a potentially useful new series of biochemical probes for DNA-protein interactions and also identify a set of new, stable hydrophobic base pairs for designed genetic pairing systems.