The structure of the complex between sodium dodecyl sulfate (SDS) and a deuterated bifunctional enzyme, N-5'-phosphoribosylanthranilate isomerase/indole-3-glycerol-phosphate synthase (Mr 49 484), has been studied in dilute solution by small-angle neutron scattering. The complex nearly acquired its final size, as shown by molecularsieve chromatography, at the chosen SDS concentration of 1.6 mM, which is slightly below the critical micelle concentration of 1.8 mM (at the ionic strength of 0.1 M). The 452 amino-acid residues of the bifunctional enzyme were combined with 216 detergent molecules. The complex was found to be composed of three protein-decorated SDS micelles of unequal size, connected by short flexible polypeptide segments. The largest of the three micelles was the middle one. The SDS-protein complex contained the dodecyl hydrocarbon moieties in three globular cores. Each core was surrounded by a hydrophilic shell, formed by the hydrophilic and amphiphilic stretches of the polypeptide chain, and by the sulfate head groups of the detergent. The average thickness of these shells was 0.7-0.8 nm.The three-micelle complex was cleaved with trypsin at a single site, possibly in a micelle-connecting segment, into a single-micelle fragment at the carboxyl-terminal which comprised 73 SDS molecules and 163 amino-acid residues, and a dual-micelle fragment. One of the micelles within this larger fragment contained 42 SDS molecules and about 90 amino-acid residues; the other micelle contained 101 SDS molecules and about 190 amino-acid residues. The individual micelle sizes seemed to be determined by the amino-acid sequence.The anionic detergent sodium dodecyl sulfate (SDS) effects solubilization of, for instance, membrane, ribosomal and viral proteins. SDS is widely used for size separations of these proteins as well as of water-soluble proteins or protein subunits, by gel electrophoresis [l -51 and by molecular-sieve chromatography [6 -121. Knowledge of the structure of SDSprotein complexes can be useful for understanding and for further developing these important methods. SDS binds to water-soluble proteins approximately in proportion to the polypeptide length. On average, the saturated complexes contain one SDS molecule per two amino-acid residues at moderate ionic strength [13].The structure of SDS-protein complexes has not yet been determined in detail. Different and partly contradictory structural models have been proposed, mainly for the SDS complexes with water-soluble proteins. Five models (a -e) are briefly described below. b) The 'rodlike-particle model' was proposed on the basis of hydrodynamic measurements [14]. Upon binding of SDS, the polypeptide chain forms a 'rodlike particle', about 3.6 nm in diameter and 0.074 nmlamino-acid residue in length. A reinterpretation has been made: the particle was described as 'short rigid rodlike segment, with intervening regions possessing some flexibility' [13].c) The 'necklace model', which contradicts the 'rodlike particle' model (b) above, was derived mainly from...

A small-angle neutron scattering (SANS) study of slightly sonicated liposomes of binary lipid mixtures is presented. It is demonstrated that the neutron scattering of lipid lamellae may be analyzed in terms of the Kratky--Porod model of scattering by two-dimensional systems. The contrast variation technique may thus be applied in order to study the structure and phase diagrams of lipid layers not disturbed by heavy sonication. The thickness of isolated bilayers is measured, and molar volumes of pure lipid phases are determined. Mixtures of deuterated dimyristoylphosphatidylcholine with (1) protonated dipalmitoylphosphatidylcholine and (2) protonated distearoylphosphatidylcholine, respectively, are studied. Excess volumes of lipid mixtures are determined by the contrast variation. For the first mixture positive excess volumes of +86 A3 in the crystalline phase (5 degrees C) and of +49 A3 in the fluid phase (35 degrees C) are obtained. These large positive excess volumes are interpreted in terms of free volume creation at the interface between the monolayers, which indicates that the polar head groups are rather fixed with respect to the lipid--water interface. We show that the phase boundaries at a given temperature may be determined by performing contrast variation experiments for two mixtures with different initial composition. Good agreement with existing experimental data is observed for the first mixture. A miscibility gap is established in the crystalline state of the second mixture. A most interesting result is the finding of an immiscibility in the fluid state. This is interpreted in terms of critical concentration fluctuations caused by the critical demixing point of the solid-state miscibility gap hidden below the liquidus line.

New strucutral data have been obtained from neutron scattering studies of chromatin. The concentration-dependent meridional peak at 10-11 nm comes from the interparticle spacing of a subunit structure. Peaks at 5.5 and 3.7 nm have a different contrast behaviour to those at 11.0 and 3.7 nm showing that histones and DNA have a different spatial arrangement in the subunit. A globular model in which apolar segments of histones from the core surrounded by DNA complexed with the basic segments of histones agrees with the data.

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The forward scattering of cold neutrons in small-angle scattering experiments on macromolecules dissolved in mixtures of light and heavy water is predominantly determined by the large incoherent cross section of the hydrogen atoms leading to an isotropic background level. The probability per unit solid angle for forward scattering by samples rich in H20 is approximately equal to the fraction of the primary beam which has not been transmitted through the sample, divided by the full solid angle 47t. When using the camera D11 at the high flux reactor in Grenoble [Ibel (1976), J. Appl. Cryst. 9, 296-309-1 this simple relation is fulfilled within +__5~ by a 1 mm thick sample of HzO at 281.8 K using neutrons of 1-0 nm wavelength. In D20, the coherent cross section is of the same order as the incoherent one, and about one half of all those neutrons which had been removed from the primary beam are scattered into large angles. Our experimental results and the derived graphs allow us to determine the effective forward scattering per unit solid angle for any volume fraction of D20 in H20/D20 mixtures around room temperature by a simple and accurate transmission experiment in conjunction with the measurement of the scattering of one single sample of H20 of 1 mm thickness, for neutrons of wavelengths from 0.45 to 1.0 nm.