The domain analysis of human apotransferrin (apo hTF) upon interaction with sodium n-dodecyl sulphate (SDS) was studied by differential scanning calorimetry (DSC) and circular dichroism (CD) using Hepes buffer, 100 mM, at pH = 7. The DSC profile for apo hTF depicts two distinct peaks, while when interacted with SDS, the DSC peaks are shifted to the left as well as the area under the peaks are reduced. The DSC profile without the presence of SDS has two dissimilar peaks including two melting points (T-m = 60degreesC, T-m = 70degreesC). This profile in the presence of low concentrations of SDS shows two nearly similar peaks and decrement of T-m values about 5 degreesC relative to profile in the absence of SDS. The DSC excess heat capacity was deconvoluted for apo hTF in the presence of SDS. The interaction of SDS with apo hTF induced two dissimilar subdomains for each domain, but each domain having two subdomains similar to each other. The CD experiment of apo hTF-SDS complexes shows the decrement of alpha-helix content and the increment of beta-sheet structure relative to protein in the absence of SDS. The alpha-helix tendency calculation shows more alpha-helix content for N domain relative to C domain. Here SDS at low concentration plays a role as a good probe to define the electrostatic moiety for N domain relative to C domain that is initiated from dissimilarity of alpha-helicity of two domains. (C) 2003 Elsevier B.V. All rights reserved.