A partially unfolded state of equine beta-lactoglobulin at pH 8.7

The urea-induced unfolding transition of equine P-lactoglobulin was studied at pH 8.7 using circular dichroism (CD), ultracentrifugation, and gel filt ration chromatography. The unfolding transition curves showed that at least one intermediate accumulates at moderate concentrations of urea. Furthermo re, analytical ultracentrifugation experiments indicated that the intermedi ate forms a dimer. Thus, the urea-induced unfolding transition was measured by CD at various protein concentrations and was analyzed by a model assumi ng the four conformational states (the native, intermediate, dimeric interm ediate, and unfolded states). The characteristics of the intermediate are m arkedly different from those of the intermediate previously observed at pH 4.0 or 1.5. The intermediate at pH 8.7 does not show the intense far-ultrav iolet CD suggestive of the nonnative a-helix.