In the presence of sodium dodecyl sulfate (SDS), the secondary structure of bovine serum albumin (BSA) was almost protected against thermal denaturation above 50 degreesC, where the structural change became irreversible. Beyond 30 degreesC, the helicity (66%) of the protein sharply decreased with rise of temperature. In response to this, the proportions of beta-structure and random coil increased. The helicity and the beta-structural proportion were 44% and 13% at 65 degreesC, respectively. The protective effect was observed upon the coexistence of SDS of extremely low concentrations:. the molar ratio of [SDS]/[BSA] of 15 was enough to induce the maximal protective effect on the helical structure of the protein. The maximal protected helicity was 58% at 65 degreesC, increasing to 64% upon cooling down to 25 degreesC. This protective effect became greater with an increase of chain length of alkyl sulfate ion. On the other hand, a cationic surfactant did not protect the BSA structure at all against the thermal denaturation. This protective effect was characterized by the specific amphiphilic nature of anionic surfactant. Such an anionic surfactant is considered to protect the protein structure by building bridges between particular nonpolar residues and particular positively charged residues located on different loops of the protein. (C) 2003 Elsevier Science (USA). All rights reserved.