Complexation of a Globular Protein, β-Lactoglobulin, with an Anionic Surfactant in Aqueous Solution

Abstract

The complexation of a globular protein, β-lactoglobulin (BLG), and an anionic surfactant sodium dodecyl sulfate (SDS) in aqueous media was investigated using capillary zone electrophoresis, electrophoretic, static, and dynamic light scattering, and small-angle X-ray scattering in a considerably high protein concentration range (0.27 mM < C < 3 mM). On increasing the molar concentration C of the surfactant, cooperative binding of SDS to BLG starts at C/C ≈ 1; the BLG-SDS complex consists mainly of the BLG dimer and approximately 20 SDS molecules, where BLG takes a compact conformation similar to that of the native BLG up to C/C ≈ 20. At C/C higher than 30, the BLG dimer in the BLG-SDS complex dissociates into a unimer, but the dissociated BLG unimer still takes a compact conformation at least at 30 < C/C < 65.

Encapsulation of Albumin in Organic Nanotube Channel: Structural Investigation by Small-Angle X-ray Scattering

“…If there were not only the complexes but also the free nanotubes in the solution, the multimodal distribution would be observed. 16,17 That is to say, in the present case, all nanotubes form the complexes with BSA. There are no free nanotubes in the solution.…”

“…The original peaks around = 2.5 × 10 –10 m 2 /(V s) for the nanotube and = −2.5 × 10 –10 m 2 /(V s) for BSA completely disappeared. If there were not only the complexes but also the free nanotubes in the solution, the multimodal distribution would be observed. , That is to say, in the present case, all nanotubes form the complexes with BSA. There are no free nanotubes in the solution.…”

Multiple Association–Dissociation Equilibria in Solutions of Amphiphilic Molecules

“…The hydrophobic cracks may grow by the adsorption of surfactant molecules, and adsorbed surfactant molecules interact both with the hydrophobic crack and already-adsorbed neighboring surfactant molecules. Li and Sato proposed a combination model of cooperative and stepwise adsorptions to describe the complexation of the globular protein and surfactant, where n ′ surfactant molecules are adsorbed simultaneously on the protein and afterward n ″ surfactant molecules are adsorbed stepwise on the protein where K P, n ′ is the cooperative association constant and K P,1 is the stepwise association constant. Thus, we have assumed that all K P m –1, m ( m ≥ n + 1) values in eq are equal to K P,1 .…”

“…To argue for the association constants K P m , m +1 ( m = 0, 1,...) in eq , we have to consider not only interactions among surfactant molecules but also interactions between the surfactant and polymer or colloidal particle. Complexation between the protein and surfactant has been argued in terms of phenomenological association–dissociation equilibria so far, but general molecular-thermodynamic models for the protein–surfactant complexation have not yet been established. − …”

“…Afterward, the theoretical arguments are applied to solutions of amphiphilic molecules, which form micelles, emulsions, and complexes as shown in Figures and . Those systems were studied by our research group ,− mainly using scattering methods. The theoretical background used to analyze the scattering results of each system is given in the Supporting Information (SI), and many symbols, association–dissociation equilibrium formula, and basic equations used to analyze light-scattering data are listed in the last section.…”