Structure of dodecyl sulfate-protein complexes at subsaturating concentrations of free detergent

Earlier neutron small-angle scattering experiments had revealed the low resolution structure of the complex between sodium dodecyl sulfate (SDS) and the single polypeptide (452 amino acid residues) of a water-soluble enzyme. The saturated complex consists of three globular micelles which are connected by short flexible polypeptide segments. New experiments, described here, were performed at subsaturating concentrations of free SDS in equilibrium with the complex. The data show a decrease in stoichiometry from one bound dodecyl sulfate (DS) anion per two amino acid residues near the critical micelle concentration (CMC) to one per four residues at half the CMC. At 0.3 CMC, a two-micelle complex is formed by the recombination of the small amino-terminal micelle with the middle one; and the center-to-center distance between the carboxyl-terminal micelle and the middle one decreases from 7.5 to 6.2 nm. These structural data allow us to better understand earlier results obtained with high-performance agarose gel chromatography of the same SDS-protein complexes.