Table 1.

Measurements of the Hill coefficient n and ΔΓ₂₊ for A-riboswitch folding^a

	UV titrations			RNA-HQS titrations
Condition	Wavelength (nm)	Midpoint (µM)	Avg n	ΔΓMg+2
11 µM purine	260	152 ± 5	2.00 ± 0.13	2.65 ± 0.48
	295	217 ± 8	1.62 ± 0.16
11 µM adenine	260	41.9 ± 1.4	1.94 ± 0.08	2.51 ± 0.26
	295	45.9 ± 5.9	1.59 ± 0.22
82 µM adenine	260	28.1 ± 2.4	1.65 ± 0.02	2.30 ± 0.22
	280	31.2 ± 4.5	1.44 ± 0.15
	295	25.9 ± 0.4	1.53 ± 0.20
82 µM 2-AP	260	20.9 ± 1.2	1.49 ± 0.05	2.06 ± 0.21
11 µM DAPb	260	10.0	0.73 ± 0.05	1.52 ± 0.17

^aAll titrations with MgCl₂ were performed in 20 mM KMOPS buffer, 50 mM K⁺, at 20 °C. The ligand identity and concentration was the only variable among the data sets, as listed in the first column of the Table. RNA folding was monitored by absorbance changes at one to three different wavelengths (see Materials and Methods for details). The midpoints and Hill coefficients of the titration curves are the parameters (1/K)^1/n and n, respectively, obtained from independent fits of eq 12a to each data set. The errors reported are the standard deviation of values obtained from three repetitions of each experiment. ΔΓ₂₊ values were calculated at the average of the titration midpoints for each condition from the data shown in Figure 7 (ΔΓ₂₊ = Γ_N−2−Γ_I−2+). The reported errors are standard deviations of four repeated experiments.

^bTitration data were plotted as shown in Figure 4C, fit with eq 12b, and the Hill coefficient was calculated from eq 13 at the chosen Mg²⁺ concentration (10 µM).