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An ambiguous NMR distance restraint represents an observation compatible with several atom-pair assignments rather than one uniquely identified pair. It combines candidate contributions according to a specified rule during an NMR distance restraint calculation. A single overlapping NOESY feature can motivate such a representation; ambiguity concerns which nuclei generated the observation.^[ref]

For an inverse-sixth-power sum model with one molecular copy,where $\mathcal{A}$ contains the candidate pairs. An upper bound applies to this effective quantity, not independently to every $r_{ij}$. Xplor-NIH's optional OR syntax lists alternative pairs and uses sum averaging; it avoids adding unwanted cross-combinations of broad atom selections.^[ref]

For an original example, suppose the possible assignments are pair $(a,b)$ at3angstroms and pair $(c,d)$ at8angstroms. ThenA3.5angstrom upper bound is compatible with this sum even though one candidate pair is8angstroms apart. Enforcing a separate3.5angstrom limit on both candidates would assert stronger information than the ambiguous observation supplies.

The pair list matters too: these alternatives need not include $(a,d)$ or $(c,b)$. Selecting all atoms from $\{a,c\}$ against all atoms from $\{b,d\}$ would introduce those extra combinations and change the calculation. An explicit assignment representation preserves the intended alternatives.^[ref]

Sum normalization, molecular symmetry and the distinction between assignment ambiguity and conformational averaging must remain part of the restraint model. Reducing an effective distance during refinement does not by itself prove that a particular candidate assignment is the uniquely correct one.

Gabriele Varani studies the structural basis of RNA function and recognition. His university laboratory identifies NMR spectroscopy, crystallographic and biochemical methods among its approaches. His named publication list includes the 1991 study of an unusually stable RNA hairpin capped by a UUCG tetraloop.^[ref]

The hairpin study determined a solution structure from interproton distances and NMR scalar coupling measurements, combining structural restraints with a calculation of molecular geometry. Its proposed stabilizing features include a noncanonical pair between guanine and uracil, a cytosine–phosphate contact and base stacking. These features connect the observed fold with the sequence-dependent stability of the RNA.^[ref]

An original geometry example illustrates what an NMR distance restraint contributes. Suppose three labelled sites A, B and C have distance intervals, in ångströangstroms,No molecular geometry can satisfy all three: the triangle inequality requires $d_{AC}\le d_{AB}+d_{BC}\le 7.4$. A proposed conformation, an assignment or an assumed restraint would need reconsideration. The intervals are chosen to demonstrate consistency checking, not quoted experimental distances from the paper.

Compatible distance restraints still need not specify a unique three-dimensional arrangement. Mirror-related arrangements preserve all pairwise distances, while additional stereochemical information can distinguish them. Measurements of NMR scalar coupling provide further conformational information; the original study used them to constrain the backbone rather than relying on interproton distances alone.^[ref]

The motivating experiment therefore does more than establish an RNA sequence. It measures interactions in solution that restrict how the nucleotides can be arranged. The resulting structural model explains specific contacts, while the measurement restraints define what the data actually support.