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. 2004 Jan 20;101(5):1189–1194. doi: 10.1073/pnas.0307966100

Fig. 2.

Fig. 2.

In situ determination of a pKa for A272. (A) Plot of the C2 carbon chemical shifts vs. pH of the five adenosines in the DIS23(GA) stem–loop in the 13C,15N-A,G-labeled DIS23(GA)·DIS23(HxUC) kissing complex formed in the presence of 300 mM NaCl. Note that resonances from residue A272, A280, and A275 become exchange broadened beyond detection as the pH is lowered, and, therefore, the pH-dependent correlation of the C2 chemical shifts for these bases is incomplete. (B) Plot of the C8 carbon chemical shift of A272 from DIS23(GA) vs. pH in the 13C,15N-A,G-labeled DIS23(GA)·DIS23(HxUC) kissing complex formed in the presence of either 300 mM NaCl or 3.3 mM MgCl2. Solid lines are best fits using a Henderson–Hasselbach equation.