Figure 9.

Determination of the pK_a of the 2′-OH by the kinetic method. (A) Plot of k_obs as a function of pH in the presence of 3.16 M Na⁺ (◆), 3.16 M K⁺ (◇), and 10 mM Ca²⁺ (■). (B) Plot of log(k_rel) as a function of pH using the same symbols as in panel A. k_rel was obtained by normalizing the data in panel A to the k_max value from the fit. The pK_a values were determined by fitting the rate versus pH plots in panel A to eq 3 and were found to be 13.6, 13.3, and 11.4 in the presence of 3.16 M Na⁺, 3.16 M K⁺, and 10 mM Ca²⁺_, respectively. (Note that for panel A, the pK_a is the pH where k_obs = k_max/2, while for panel B the pK_a is the pH near the flex point.) The Hill coefficients determined from the fits of the data in panel A to eq 3 were found to be 0.8, 1.1, and 1.9 for Na⁺, K⁺, and Ca²⁺, respectively. These Hill coefficients are apparent in the relative slopes in panel B. The value of k_max is ∼3-fold higher for monovalent ions than divalent ions, as revealed in panel A, but the pK_a values are unaffected by k_max differences as they are determined entirely from the shape of the curves. The origin of the k_max difference is unclear, but this difference could reflect different interactions of divalent and monovalent ions with this 22-mer.