Figure 3.

(a) pH dependence of selected A-loop RNA NMR spectra. Aromatic–aromatic region of wet-TOCSY spectra of unmodified (Upper) and modified (Lower) A-loop RNA molecules taken as a function of pH (5.5–7.5). Residues C2556, U2555, and U2554 are indicated. On decreasing solution pH by slow dialysis into isotonic 10 mM Na₂PO₄ buffer, C2556 moves downfield, U2555 moves upfield, and U2554 remains unchanged in both molecules. (b) WATERGATE-NOESY spectra of modified A-loop RNA taken at low (pH 5.5, Upper) and high (pH 7.5, Lower) pH. NOEs from C2556 H5 to its exocyclic amino protons are indicated. These experiments were performed at 500 MHz, 15°C, and a mixing time of 100 ms.