High resolution neutron scattering on ionic surfactant micelles : sds in water

Abstract

2014 On améliore la résolution obtenue en diffusion des neutrons aux petits angles pour déterminer la structure des micelles de dodécylsulfate de sodium dans l'eau. L'expérience mesure la distribution moyenne des distances entre tous les noyaux d'une micelle, ainsi que les distributions de distances entre des groupes deutérés attachés aux positions 03C9 et 03B3 des chaînes de SDS. A basse résolution (15 Å) on n'observe que la structure moyenne de la micelle : il s'agit d'une sphère contenant N = 74 molécules de SDS; son coeur hydrocarboné a un rayon de 18,4 Å ; il contient très peu d'eau, en tout cas moins qu'une molécule d'eau par molécule de SDS. A haute résolution (5 A) on voit surtout les fluctuations spontanées qui écartent la micelle de cette structure moyenne. Ces fluctuations produisent une dispersion des nombres d'agrégation (03C32/N2 = 0,1) et des déviations par rapport à la forme sphérique. La structure interne du coeur, mesurée par les distances entre groupes deutérés, est également distordue par chacune de ces fluctuations : les queues de chaînes (03C9) ne sont pas concentrées près d'un « centre », et les méthylènes en position 03B3 ne restent pas dans une coquille sphérique. Abstract 2014 An improvement over previous determinations of the structure of sodium dodecyl sulfate micelles has been obtained from small angle neutron scattering. The experiment measures the average distribution of distances between all nuclei within a micelle, and also those between deuterium labels attached at positions 03C9 or 03B3 on the SDS chains. At low resolution (15 Å), only the average structure of the micelle is observed : this is a dense sphere containing N = 74 SDS molecules; the hydrocarbon core has a radius of 18.4 Å and contains less than one water molecule per SDS molecule. At high resolution (5 Å), fluctuations away from the average structure are observed. If the internal structure of the core is not resolved, one sees mainly the dispersion in the aggregation numbers (03C3N/N = 0.33) or radii (03C3R/R = 0.1) of the micelle. When the distributions of distances between deuterium labels are observed, shape fluctuations show up through distortions of the internal structure : indeed the chain ends (03C9) are not concentrated near a « centre » of the micelle, and the 03B3 methylene groups do not remain in a spherical shell.

Two‐Dimensional 1H‐NMR of Transmembrane Peptides from Escherichia Coli Phosphatidylglycerophosphate Synthase in Micelles

“…The peptide/SDS samples were prepared with a peptide/lipid molar ratio of 1 : 100. Since the aggregation number of SDS micelles is estimated to be about 75 (Cabane et al, 1985), the molecular mass of a peptide/fully deuterated SDS aggregate is approximately 26.6 kDa, assuming that there is only one peptide molecule/micelle.…”

“…A final concentration of 300 mM fully deuterated SDS was attained, which included the peptide in a molar ratio of peptide/SDS of 1 : 100. This preparation resulted in an isotropic micellar solution that incorporated approximately 1 peptide/micelle ; the aggregation number of SDS micelles in pure water is about 75 (Cabane et al, 1985). The detergent concentration has been shown to affect NMR spectra of proteins and peptides in micelles (McDonnell and Opella, 1993).…”

Absolute calibration of small-angle neutron scattering data

“…On an arbitrary scale a number of models may fit the data with different sets of structural parameters. Absolute scaling allows the set of parameters to be restricted to those which reproduce the observed intensity (Hayter & Penfold, 1983;Cabane, Duplessix & Zemb, 1985). Similar checks for the internal consistency of absolutely calibrated SANS data with model calculations have been made in studies of the structure of core-shell polymer latexes (Wai, Gelman, Fatica, Hoerl & Wignall, 1986) and phase-separated interpenetrating polymer networks .…”

Spatial correlation functions of radially distributed quantities applied to small-angle scattering

“…where np is the number of particles per unit volume (Hayter & Penfold, 1981, 1983Cabane et al, 1985). The form factor equals the Fourier transform of At32(r), the autocorrelation function of the mean scattering contrast Ap(r) of the individual scatterers:…”