Figure 3.

An R-5′-methyl substituent (yellow) in RNA or A-DNA will cause energetically unfavorable, short contacts (red lines) even when sugar-phosphate backbone conformations other than the standard sc⁻, ap, sc⁺, sc⁺, ap, sc⁻ (α to ζ) geometry are considered. (A) Extended backbone variant with α, β and γ in the ap range (seen for residue A5 in the S-6′-Me decamer structure). (B) Backbone conformation in tricyclo-DNA¹⁸ with a compensatory change in β and γ.