 H. A. Heus

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.^[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.^[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 $1\times 4\times 2\times 1=8$ 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 areAt $T=298\mathrm{K}$, the corresponding difference in folding Gibbs free energy iswhere $R$ 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.