Structures of oligonucleotides containing 5-(Methoxymethyl)-2deoxyuridine determined by nmr spectroscopy

Base pairing of 5-(methoxymethyl)-2deoxyuridine (MMdU) opposite either adenine or guanine in a seven base pair oligonucleotide duplex has been studied by NMR spectroscopy. When paired with A, we observe that the MMdU.Abase pair adopts Watson-Crick geometry. The methoxymethyl substituent is not held in a fixed conformation and may rotate around the C5-CH2 and CH2-0 bonds. Examination of the potential energy as a function of rotation around these bonds indicates the presence of four low energy conformations. No hydrogen bonding is indicated for the methoxymethyl substituent, and the four potential minima result from reduced steric clash. For the MMdU.G base pair, the two bases adopt a wobble geometry which does not change with increasing solvent pH. Similarly, we find four low energy conformations for the methoxymethyl substituent in the major groove of the DNA helix.