The effects of modification of the phosphodiester backbone or the guanine bases on G-quadruplex formation have been widely investigated. Only a few studies have investigated the effects of deoxyribose or ‘sugar’ modifications on G-quadruplex structure. Here, we evaluated the structural, thermodynamic, and kinetic properties of the parallel quadruplexes formed by the sequence d(TGGGGT) in which each guanine base was substituted, one at a time, with acyclic threoninol nucleic acid (aTNA). We found that all sequences were able to form G-quadruplexes; however, the presence of aTNA resulted in the formation of a mixture of quadruplex structures in some cases. Furthermore, the presence of a single substitution at any position resulted in destabilization of the G-quadruplex relative to that formed by the unmodified sequence. The introduction of the aTNA in terminal quartets was the most detrimental to stability. In addition, kinetic experiments showed that, compared to its unmodified counterpart sequence d(TGGGGT), the substitution of a normal guanine nucleotide by aTNA decelerated quadruplex formation except when the aTNA was at the 5′ most guanine of the sequence. In summary, our studies indicate that the deoxyribose sugar affects the properties of G-quadruplex structures.
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