Sugar conformations in DNA and RNA-DNA triple helices determined by FTIR spectroscopy: Role of backbone composition

We have studied the effect of the nature of the third-strand sugar (ribose or deoxyribose) on the geometry and stability of triple helices with a pyrimidine motif targeting the polypurine tract of the Friend murine retrovirus. Comparison between triplexes containing a third strand formed by a deoxy 13mer d(TCT5C6), the same oligomer but with C-5-methylated cytosines d((TCT5C6)-C-5me-C-5me), and an analogous modified 13mer RNA (2'Ome)r(UCU5C6) shows that the sugar conformations of the different triple helices, determined by FTIR spectroscopy, differ depending on nature of the third-strand sugar. Pyrimidine*purine-pyrimidine triple-helix formation with the third-strand RNA and the duplex as DNA appears to be associated with a conversion of the duplex part from a B-form secondary structure with S-type sugars to a geometry in which the polypurine strand sugars adopt an N-type conformation. Thermal dissociation of the triplexes was studied by UV absorbance spectroscopy. The most stable triple helix is obtained when the third strand contains 2'-O-methylated ribose sugars.