Effects of redox buffer properties on the folding of a disulfide-containing protein: dependence upon pH, thiol pKa, and thiol concentration

Aliphatic thiols are effective as redox buffers for folding non-native disulfide-containing proteins into their native state at high pH values (8.0-8.5) but not at neutral pH values (6-7.5). In developing more efficient and flexible redox buffers, a series of aromatic thiols was analyzed for its ability to fold scrambled ribonuclease A (sRNase A). At equivalent pH values, the aromatic thiols folded sRNase A 10-23 times faster at pH 6.0, 7-12 times faster at pH 7.0, and 5-8 times faster at pH 7.7 than the standard aliphatic thiol glutathione. Similar correlations between thiol pKa values and folding rates at each pH value suggest that the apparent folding rate constants (kapp) are a function of the redox buffer properties (pH, thiol pKa and [RSH]). Fitting the observed data to a three-variable model (log kapp = −4.216(± 0.030) + 0.5816(± 0.0036)pH − 0.233(± 0.004)pKa + log(1 − e−0.98 (±0.02)[RSH])) gave good statistics: r2 = 0.915, s = 0.10.