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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.

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.

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.

A UUCG tetraloop is an RNA tetraloop whose four loop residues have nucleotide sequence U–U–C–G. It is one member of the UNCG tetraloop family, with U occupying the variable N position. The closing stem base pair lies outside those four residues.

The structurally studied sequence 5'-GGACUUCGGUCC-3' illustrates the bookkeeping. Stem pairs 1–12,2–11,3–10 and 4–9 leave UUCG at positions 5–8, giving $2(4)+4=12$ nucleotides. The loop's first uracil and last guanine can themselves contact without changing its four-residue sequence definition.

Solution NMR spectroscopy determined interproton distances and scalar couplings for this hairpin. The resulting restrained structural model showed a loop G–U contact with syn-oriented guanosine, plus a cytosine–phosphate contact and organized base stacking. Mutating conserved loop residues reduced the measured stability.^[ref] The syn description concerns the glycosidic torsion angle of that nucleotide; it must not be confused with the cis/trans classification of relative RNA base-pair geometry.

The original paper describes only two residues as the remaining structural loop after its G–U contact extends the paired region. This is compatible with naming the four-residue sequence motif UUCG: a sequence-based tetraloop designation and a detailed contact-based loop count use different boundaries.^[ref]

A useful comparison keeps the stem and solution conditions fixed while changing one loop position. RNA optical melting tests the effect on a transition, while NMR spectroscopy tests the spatial contacts. Membership in a short sequence family does not fix a context-independent RNA melting temperature or prove that every larger RNA containing those letters adopts the same structure.