= Syn glycosidic conformation
A \b[syn glycosidic conformation] is a base–sugar orientation in the syn region of the <glycosidic torsion angle> $\chi$. In the IUPAC-IUB convention the region is centered at $0^\circ$ and spans90degrees on either side. In a nonnegative360degree reporting interval, this region crosses the boundary: angles near both0 and360degrees belong to it.<Abbreviations and Symbols for the Description of Conformations of Polynucleotide Chains>[Section2.3, note(a), syn region]{ref}
For an original example, $\chi=30^\circ$ and $\chi=330^\circ$ are both syn, because330degrees is equivalent to−30degrees. Their shortest angular separation is60degrees. Their ordinary arithmetic mean,180degrees, falls in the <anti glycosidic conformation> region and is therefore misleading as a mean orientation. Averaging their unit-circle vectors instead gives
$$
\frac12\bigl((\cos30^\circ,\sin30^\circ)
+(\cos330^\circ,\sin330^\circ)\bigr)
=(\sqrt3/2,0),
$$
which points toward $\chi=0^\circ$.
The numerical convention is essential. Older choices using the base atom C8 for <purines> or C6 for <pyrimidines> can differ from the recommended <glycosidic torsion angle> by approximately180degrees. Comparing the same number under those two definitions could therefore appear to reverse a syn/anti assignment without any molecular motion.<Abbreviations and Symbols for the Description of Conformations of Polynucleotide Chains>[Section2.3, note(b), old and new atom conventions]{ref}
Syn is a local conformational descriptor. It is neither a statement that two bases are paired nor a label for the overall handedness of a nucleic-acid helix. Structural comparisons should retain the defining atoms and angle convention alongside the syn/anti label.
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