Stabilizing contributions of sulfur-modified nucleotides: crystal structure of a DNA duplex with 2'-O-[2-(methoxy)ethyl]-2-thiothymidines

Substitution of oxygen atoms by sulfur at various locations in the nucleic acid framework has led to analogs such as the DNA phosphorothioates and 4′-thio RNA. The phosphorothioates are excellent mimics of DNA, exhibit increased resistance to nuclease degradation compared with the natural counterpart, and have been widely used as first-generation antisense nucleic acid analogs for applications in vitro and in vivo. The 4′-thio RNA analog exhibits significantly enhanced RNA affinity compared with RNA, and shows potential for incorporation into siRNAs. 2-Thiouridine (s²U) and 5-methyl-2-thiouridine (m⁵s²U) are natural nucleotide analogs. s²U in tRNA confers greater specificity of codon–anticodon interactions by discriminating more strongly between A and G compared with U. 2-Thio modification preorganizes the ribose and 2′-deoxyribose sugars for a C3′-endo conformation, and stabilizes heteroduplexes composed of modified DNA and complementary RNA. Combination of the 2-thio and sugar 2′-O-modifications has been demonstrated to boost both thermodynamic stability and nuclease resistance. Using the 2′-O-2-(methoxy)ethyl]-2-thiothymidine (m⁵s²Umoe) analog, we have investigated the consequences of the replacement of the 2-oxygen by sulfur for base-pair geometry and duplex conformation. The crystal structure of the A-form DNA duplex with sequence GCGTAT*ACGC (T* = m⁵s²Umoe) was determined at high resolution and compared with the structure of the corresponding duplex with T* = m⁵Umoe. Notable changes as a result of the incorporation of sulfur concern the base-pair parameter ‘opening’, an improvement of stacking in the vicinity of modified nucleotides as measured by base overlap, and a van der Waals interaction between sulfur atoms from adjacent m⁵s²Umoe residues in the minor groove. The structural data indicate only minor adjustments in the water structure as a result of the presence of sulfur. The observed small structural perturbations combined with the favorable consequences for pairing stability and nuclease resistance (when combined with 2′-O-modification) render 2-thiouracil-modified RNA a promising candidate for applications in RNAi.