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Eric Westhof is a structural biochemist whose research concerns the architecture and molecular interactions of RNA. His institutionally hosted CV identifies work at Strasbourg's IBMC and a professorship in structural biochemistry.^[ref]

With N. B. Leontis, Westhof developed a geometric nomenclature for RNA base-pair geometry. The classification records the interacting RNA base-pair edges and the relative orientations of the two glycosidic bonds, allowing noncanonical base pairs to be described systematically.^[ref]

An important distinction is between two kinds of orientation. The glycosidic torsion angle $\chi$ describes how one base rotates relative to its own sugar, giving syn glycosidic conformation or anti glycosidic conformation. The pair's cis/trans descriptor instead relates the two glycosidic bonds across the base-pair interaction. Reporting one descriptor does not automatically report the other.

A geometric family also leaves room for different base identities. The question of RNA base-pair isostericity is whether substitutions can preserve the relevant relative placement and shape sufficiently to support a similar structural role. This makes geometry useful when comparing homologous RNA structures even when their sequences differ.^[ref]

Westhof's contribution helped connect structural observations with a reusable language for comparing RNA folds, rather than restricting descriptions to the familiar sequence letters or canonical base pairs.

In a nucleoside or nucleotide, the glycosidic torsion angle $\chi$ describes rotation of the nucleobase relative to the sugar about their N-glycosidic bond. It is a signed dihedral angle defined by four ordered atoms:The middle two atoms specify the glycosidic bond; the outer atoms define the two reference planes. Thus the atom sequence, not the symbol $\chi$ alone, fixes the angle convention.^[ref]

The recommended broad regions are syn glycosidic conformation, centered at $0^{\circ }$ with a90degree range on either side, and anti glycosidic conformation, centered at $180^{\circ }$ with the corresponding range. For example, $\chi =60^{\circ }$ lies in the syn region and $\chi =-160^{\circ }$ in the anti region. These describe base–sugar orientation; they do not by themselves specify sugar-ring shape or the complete RNA conformation.^[ref]

An original angular comparison illustrates periodicity. Measurements of $179^{\circ }$ and $-179^{\circ }$ differ by only2degrees around the circle, not358degrees. A signed shortest difference can be calculated asusing radians inside the trigonometric functions. Interpreting the two near-boundary values as distant conformations would be an artifact of the angle's reporting interval.

The convention is important when comparing structural models or older literature. Alternative choices using C8 for a purine or C6 for a pyrimidine can shift the numerical angle by approximately180degrees. Numerical torsions should therefore be compared only after checking their defining atoms.^[ref]

A 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.^[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 giveswhich 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.^[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.