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A series of small-angle neutron scattering (SANS) measurements were carried out on dilute and moderately concentrated bovine serum albumin (BSA) solutions at two different pH values and at t = 35 "C. The amount of bound water to the protein was deduced from the zero-contrast point of dilute BSA solutions, in DzO and H20 solvent mixtures. Detailed analysis of the intensity spectrum from the most dilute BSA solution in D20 yields a prolate ellipsoidal shape (a,b,b) of the protein molecule with a = 70 A and b = 20 A. At moderate concentrations, pH 7, with or without salt (LiC1) added, the intensity spectra can be fitted satisfactorily by taking into account both the ellipsoidal shape of the particle and an interparticle interference factor (S(Q)).Calculation of S(Q) assumes a model of equivalent charged hard spheres interacting through a repulsive potential.For moderately concentrated solutions at pH 5.1, S(Q) can be accounted for by introducing an attractive potential between the particles. IntroductionThe globular protein bovine serum albumin in aqueous solution has been extensively studied in the past by small angle X-ray scattering' (SAXS), quasielastic light scattering (QELS),2 and hydrodynamic techniques. The hydrodynamic size and shape of the BSA molecule are rather well-established. Sedimentation equilibrium experiments on defatted BSA indicate that the molecules behave as prolate ellipsoids with semimajor (a) and -minor (b) axis

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A series of small-angle neutron-scattering (SANS) intensity spectra measurements were carried out on aqueous micellar solutions of ionic detergent lithium dodecyl sulfate (LDS). A systematic study of the system was conducted as a function of both detergent concentration, from 0.008 to 1.107 M, and salt (LiCl) concentration, from 0.0 to 1.0 M, at around 37 °C. The data show that LDS micelles stay "small" within this range of solution conditions, and therefore analysis can be made regarding the micelles as approximately monodisperse macroions suspended in a solution of given ionic strength (/). The SANS spectra exhibit a pronounced interaction peak due to strong intermicellar Coulombic repulsions, especially at low I. Analysis is made by using a mean spherical approximation (MSA) for a model charged hard-sphere system (as developed by Hayter and Penfold1'2 34) to compute the interparticle structure factor S(Q), and a Uniform prolate spheroidal structure for the micellar core to model the intraparticle structure factor P(Q). Excellent agreement is obtained between our data and this model using only two free parameters: the aggregation number ñ, and the degree of ionization of a micelle a. ñ is found to increase with detergent concentration and I. The effective micellar diameter varies from 44 to 52 Á and a ranges from 0.4 at low concentrations to 0.12 at the highest.

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