Table 1.

Effect of Mg²⁺ on the affinity of rescuing Mn²⁺ ions

[Mg2+], mM	KMn,app,* mM
	MnA2+	MnB2+	MnC2+
2	0.16	5	0.058
10	0.82	7	0.28
50	—	13	1.3
100	6.3	—	—

^*The observed dissociation constant of Mn²⁺ from free E at each Mg²⁺ concentration was determined in experiments analogous to those described in the text with 10 mM Mg²⁺. The dissociation constants of Mn_C²⁺ are from ref. 7. As described in the text, the measurements were made with (E⋅S)_o or E⋅P, and the absence of a Mn²⁺ effect on formation of (E⋅S)_o and E⋅P suggests that the same Mn²⁺ affinities hold for free E. This was directly verified in several instances (ref. 7 and unpublished results). The dissociation constants for Mn_A²⁺ and Mn_C²⁺ were determined at pH 7.9, and those for Mn_B²⁺ were determined at pH 6.3 (see text and refs. 7 and 66). The Mn²⁺ dissociation constants obtained have been shown to be unaffected by the following changes in pH: Mn_A²⁺, pH 7.0–7.9; Mn_B²⁺, pH 6.3–7.1; Mn_C²⁺, pH 5.0–7.9 (refs. 7 and 66 and unpublished results), as expected because RNA functional groups typically have pK_a values ≥9 or ≤4. The error associated with each K^Mn,app value is less than ±30%, and specific error limits for the apparent Mn²⁺ affinities at 10 mM Mg²⁺ are in the text.