Structural characterisation of trimethylsilyl-protected DNA bases

The structures of the silylated DNA bases, bis(trimethylsilyl)thymine (1), bis(trimethylsilyl)cytosine (2), bis(trimethylsilyl)adenine (3) and tris(trimethylsilyl)guanine (4), have been determined. 1 is O-silylated and displays no intermolecular interactions. 2 is silylated at both exocylic O, N positions and forms a chain structure through intermolecular NH⋯O and NH⋯N hydrogen bonds. 3 contains two SiMe₃ groups, on the exocylic NH and endocyclic N⁹ position, respectively; of two independent molecules in the asymmetric unit, one dimerises through complementary NH⋯N hydrogen bonds, while the other forms a strained intramolecular hydrogen bond through the same pair of donor and acceptor centres. 4 incorporates N, N, O-SiMe₃ moieties and forms chains via bifurcated CH⋯O/N hydrogen bonds, while the NH function remains unexploited. The effects of silylation on these pyrimidine and purine ring structures are also discussed in comparison with the native bases.