Table 1.

Physicochemical properties of α₃Y and α₃Y-His variants^a

Protein	pKapp Y32b	pKapp Hisb	Y32 Em(pH)c	[Θ]222d	% Helixe	ΔGf
α3Y	11.3	–	8.1	−21.9	73.1 ± 0.7 (pH 5.0–10.0)	−3.7
α3Y-V9H	10.8	<5.0	n.d.	−18.7	63.1 ± 0.4 (pH 5.5–9.1)	−3.0
α3Y-L12H	10.4	6.6	n.d.	−16.0	53.3 ± 1.0 (pH 4.6–8.6)	n.d.
α3Y-K29H	10.7	7.1	7.4	−20.4	66.8 ± 2.5 (pH 5.1–9.1)	−2.8
α3Y-K36H	11.0	7.0	7.1	−19.5	64.7 ± 0.8 (pH 5.1–9.1)	−2.4
α3Y-I62H	10.6	7.0	n.d.	−16.5	53.8 ± 1.6 (pH 5.0–8.9)	n.d.

^aEm (nm); [Θ]₂₂₂ × 10³ (deg cm² dmol⁻¹); ΔG (kcal mol⁻¹); n.d., not determined

^bApparent tyrosinate/tyrosine and imidazole/imidazolium pK_a values of Y32 and histidine residues obtained by fitting the pH-titration curves in Fig. 3 to a single pK_a. Statistical errors ≤ 0.1.

^cApparent pK_a obtained by fitting the pH-titration curves in Fig. 4 to a single pK_a. Statistical errors ≤ 0.1.

^dMean residue ellipticity measured at pH 8.2 and 25° C. The α₃ W reference displays a [Θ]₂₂₂ value of −22.6×10³ deg cm² dmol⁻¹ at the same conditions.

^eScaled relative to α₃ W (76 ± 1% α-helical pH 4–10).^16,18

^fGlobal protein stabilities obtained by fitting the urea-denaturation curves in Fig. S8A. Data recorded at pH 8.2 and 25° C. Fitting standard errors < 0.03 kcal mol⁻¹.