Refolding of β‐lactoglobulin studied by stopped‐flow circular dichroism at subzero temperatures

摘要

>Refolding of bovine β-lactoglobulin was studied by stopped-flow circular dichroism at subzero temperatures. In ethylene glycol 45%–buffer 55% at −15°C, the isomerization rate from the kinetic intermediate rich in α-helix to the native state is approximately 300-fold slower than that at 4°C in the absence of ethylene glycol, whereas the initial folding is completed within the dead time of the stopped-flow apparatus (10 ms). At −28°C, we observed at least three phases; the fastest process, accompanied by an increase of α-helix content, is completed within the dead time of the stopped-flow apparatus (10 ms), the second phase, accompanied by an increase of α-helix content with the rate of 2 s−1, and the third phase, accompanied by a decrease of α-helix content. This last phase, corresponding to the isomerization process at −15°C described above, was so slow that we could not monitor any changes within 4 h. Based on the findings above, we propose that rapid α-helix formation and their concurrent collapse are common even in proteins rich in β-structure in their native forms.