CD-resolved secondary structure of bovine plasma albumin in acid-induced isomerization.

Bovine plasma albumin (BPA) showed the acid-induced two-step transition, the N-F transition and acid-expansion. Changes in fractions of alpha-helix (f alpha), beta-form (f beta) and unordered form (fR) in the acid-induced isomerization of BPA were studied using the method of Chen et al. (1972) with two constraints: sigma fi = 1, 0 less than or equal to fi less than or equal to 1. pH-profiles of f alpha and fR showed the two-step change, one corresponding to the N-F transition and the other to the acid-expansion in 0.10 M KCl and in 0.02 M NaClO4. pH-profile of f beta showed one-step change, correlating to the later part (lower pH side) of the N-F transition. The N-F transition might thus involve the helix leads to beta and helix leads to coil transitions.