= RNA isostericity matrix
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An <RNA isostericity matrix> organizes the geometric compatibility of <base pairs> in a specified <RNA base-pair geometry> family. Its purpose is to help identify substitutions that approximately preserve the arrangement of the attached backbones. It is not a table of base-pair <free energies> or a substitution-probability matrix.<The non-Watson-Crick base pairs and their associated isostericity matrices by Neocles B. Leontis and others>[Isostericity matrices, printed p. 3525, Tables 3–5]{ref}
The layout must be checked. In the 2002 classification, a family-specific four-by-four table uses one <nucleobase> identity for each axis; each entry locates an ordered base combination and assigns it to an isosteric subgroup where supported. Parentheses distinguish modeled contacts from observed ones. In later quantitative comparisons, rows and columns can instead both label complete base-pair exemplars, with entries giving their geometric discrepancy. These are different uses of a matrix layout.<The non-Watson-Crick base pairs and their associated isostericity matrices by Neocles B. Leontis and others>[Tables 3–4 and their notes]{ref}
For the canonical cis Watson–Crick family, A–U, U–A, G–C and C–G belong to subgroup I1. As an original teaching encoding, label those four pair types $p_1,\ldots,p_4$ and put $M_{ij}=1$ when the classification regards $p_i,p_j$ as mutually compatible. Their restricted block is
$$
M=\begin{pmatrix}1&1&1&1\\1&1&1&1\\1&1&1&1\\1&1&1&1\end{pmatrix}.
$$
This binary comparison block is not a reproduction of the published identity-grid layout. It encodes six distinct unordered substitutions, with diagonal entries representing self-comparison. It does not assert equal <hydrogen bond> counts, equal folding stability or unrestricted interchange in every <RNA> motif.<The non-Watson-Crick base pairs and their associated isostericity matrices by Neocles B. Leontis and others>[Table 3, Family 1, subgroup I1]{ref}
The 2009 quantitative study uses the <RNA IsoDiscrepancy Index> to distinguish isosteric, near-isosteric and more dissimilar exemplars. In that study, values at most2.0are classified as isosteric and values above2.0through3.3as near-isosteric. These cutoffs accompany the authors' structural comparison method; they are not universal energy thresholds.<Frequency and isostericity of RNA base pairs by Jesse Stombaugh and others>[Results, Figure 2 and Table 3]{ref}
When applying a matrix, state the geometric family, order of the two bases, structural exemplars and meaning of each entry. An unobserved entry is not proof that a contact is impossible. Likewise, a compatible substitution is a geometric hypothesis to test in its structural context, not a guarantee of unchanged biological function.
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