STRUCTURE OF DODECYL-SULFATE PROTEIN COMPLEXES AT SUBSATURATING CONCENTRATIONS OF FREE DETERGENT

Earlier neutron small-angle scattering experiments had revealed the lo w resolution structure of the complex between sodium dodecyl sulfate ( SDS) and the single polypeptide (452 amino acid residues) of a water-s oluble enzyme. The saturated complex consists of three globular micell es (.--.) which are connected by short flexible polypeptide segments. New experiments, described here, were performed at subsaturating conc entrations 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 (C MC) to one per four residues at half the CMC. At 0.3 CMC, a two-micell e complex (-.) is formed by the recombination of the small amino-term inal micelle with the middle one; and the center-to-center distance be tween the carboxyl-terminal micelle and the middle one decreases from 7.5 to 6.2 nm. These structural data allow us to better understand ear lier results obtained with high-performance agarose gel chromatography of the same SDS-protein complexes.