Effects of surfactants on the dynamical behavior of acrylodan-labeled bovine serum albumin

We investigate the effects of surfactants on acrylodan-labeled bovine serum albumin (BSA-Ac). Steady-state fluorescence measurements are used to compare the average effects that surfactants have on BSA-Ac and the BSA/2,8-ANS (2-anilinonaphthalene-8-sulfonic acid) complex as a function of the surfactant head group and alkyl tail length. The head group and alkyl tail of the surfactant monomers influence the site-specific environment surrounding Ac and the global environment of the adsorbed 2,8-ANS molecules. We also show that sodium dodecyl sulfate (SDS) influences the dynamical behavior of BSA-Ac. Time-resolved decay of anisotropy measurements show that BSA-Ac gains segmental mobility at SDS concentrations as low as 0.3 mM. The interaction between SDS and BSA-Ac disrupts the salt bridging which causes global structural changes within the protein. The semiangle, θ, through which the Ac residue is able to precess during its excited-state lifetime shows that there are also local changes in the protein structure on adding SDS. Specifically, we have determined that θ increases 15° from its native structure to a more "open" structure when the SDS concentration is 1 mM. Time-resolved fluorescence intensity decay experiments reveal a dipolar relaxation process between the Ac residue and its immediate microenvironment. This process is best described by a two-term rate law where the overall relaxation kinetics are slowed in the presence of SDS. Together these results demonstrate that SDS (1) causes domain I of BSA to reorient independently of domains II and III; (2) forces the cybotactic region surrounding Ac to open and allows the Ac residue greater motional freedom; and (3) actually dehydrates the local microenvironment surrounding the Ac reporter group.