 Anti glycosidic conformation

An anti glycosidic conformation is a base–sugar orientation in the anti region of the glycosidic torsion angle $\chi$. Under the IUPAC-IUB atom convention, this region is centered at $180^{\circ }$ and extends90degrees on either side. Equivalently, for an angle reported in $[0^{\circ },360^{\circ })$, the broad anti region lies between90 and270degrees.^[ref]

For a purine nucleoside, $\chi$ uses O4′–C1′–N9–C4; for a pyrimidine, it uses O4′–C1′–N1–C2. The ordered atoms specify the dihedral angle. An angle reported using different outer atoms cannot be assigned to this region by its numerical value alone.^[ref]

For an original example, $\chi =-160^{\circ }$ and $\chi =200^{\circ }$ describe the same orientation modulo one full turn, and both are anti. Moving to $\chi =60^{\circ }$ changes the orientation into the syn glycosidic conformation region while retaining the base–sugar bond connectivity. Conformational change and a change in covalent constitution are different descriptions.

The word anti here does not describe how two bases pair with one another. RNA base-pair geometry can separately classify the relative orientations of two glycosidic bonds. Nor does a single anti torsion determine the entire nucleotide or RNA structure: sugar geometry and the other backbone angles remain to be specified.