= H. A. Heus
{c}
\b[H. A. Heus] is an author of a 1991 solution-structure study of <RNA hairpins> containing <GNRA tetraloops>. The paper identifies a Chemistry and Biochemistry affiliation at the University of Colorado, Boulder. Its structures for the GCAA and GAAA sequences helped explain how loop contacts can stabilize a short RNA fold.<Structural features that give rise to the unusual stability of RNA hairpins containing GNRA loops by H. A. Heus and others>[byline, affiliation and Abstract]{ref}
The experiment used <NMR spectroscopy> to determine the structures in solution. Both reported loops contain an unusual pair between <guanine> and <adenine> between the first and last loop residues, together with a base–phosphate <hydrogen bond>, a sugar–base <hydrogen bond> and extensive <base stacking>. The proposed stability therefore involves a network of interactions rather than only the count of canonical pairs in the stem.<Structural features that give rise to the unusual stability of RNA hairpins containing GNRA loops by H. A. Heus and others>[Abstract, shared structural features of GCAA and GAAA]{ref}
An original sequence-counting example clarifies the family name. In GNRA, G denotes <guanine>, N permits any of the four ordinary RNA bases, R permits either <purine> base, and A denotes <adenine>. Thus the motif admits $1\times4\times2\times1=8$ nominal sequences. GCAA and GAAA differ at N while retaining the initial G and final A. Membership in that sequence family does not establish equal folding stability for all eight sequences; the paper reports structures of two particular members.
A separate thermodynamic illustration shows why a strong folding preference can still be sensitive to a modest energy change. In a stipulated two-state equilibrium, let the folded fractions be $0.90$ and $0.99$ under two different conditions. Their folding <equilibrium constants> are
$$
K_1=\frac{0.90}{0.10}=9,\qquad K_2=\frac{0.99}{0.01}=99.
$$
At $T=298\,\mathrm K$, the corresponding difference in folding <Gibbs free energy> is
$$
\Delta G_2-\Delta G_1=-RT\ln(K_2/K_1)\approx-5.94\,\mathrm{kJ/mol},
$$
where $R$ is the <molar gas constant>. These are original illustrative populations, not measured values attributed to Heus's paper. They connect a molecular-interaction explanation with an experimentally measurable equilibrium preference.
The reported <GNRA tetraloop> structures give a molecular interpretation of sequence conservation: bases and sugar–phosphate groups have to support a compatible set of contacts. A sequence change can preserve the stem yet alter those loop interactions.
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