Tryptophan-Mediated Sodium Dodecyl Sulfate Binding to Proteins

Sodium dodecyl sulfate (SDS) is widely used in protein research. Micellar binding of SDS is accepted as a major mechanism of SDS binding to proteins. Here, we investigated SDS-protein interactions using near-UV circular dichroism (CD), fluorescence spectroscopy, and SDS-gel electrophoresis. Bovine serum albumin (BSA), IgG, and lysozyme showed unfolding by SDS without heating or disulfide reduction as determined by SDS-gel electrophoresis. Near-UV CD and fluorescence spectroscopy showed changes in local structures surrounding tryptophan residues upon addition of 0.05-1% SDS. The fluorescence emission was blue-shifted by SDS, indicating that SDS creates more hydrophobic environments for the fluorescent tryptophan residues. We propose that, as one of the modes of SDS binding, both stoichiometric and cluster-like SDS binding occur around the fluorescent tryptophan residues, leading to changes in aromatic CD and fluorescence spectra.