The use of sodium dodecyl sulfate to model the apolipoprotein environment. Evidence for peptide–SDS complexes using pulsed-field-gradient NMR spectroscopy

Pulsed-field-gradient NMR spectroscopy was used to measure translational diffusion coefficients (D_s) for a peptide corresponding to a proposed lipid-binding domain of human apolipoprotein C-I, residues 7–24 (apoC-I(7–24)). Diffusion coefficients for apoC-I(7–24) were determined directly by following the decay of the resonance intensity of selected peptide protons at various concentrations of sodium dodecyl sulfate (SDS), a detergent increasingly being used to model the apolipoprotein environment. Previously, diffusion coefficients of peptides in the presence of SDS have been determined indirectly by monitoring the SDS diffusion coefficient. The direct measurement of the diffusion coefficient of the peptide enables one to distinguish whether SDS simply coats the peptide's surface to produce a uniformly charged `rod' or if the peptide associates with a micelle. Using the direct method, at SDS concentrations above 5 mM (which is below the SDS critical micelle concentration (8.1 mM)), apoC-I(7–24) exhibited diffusion coefficients consistent with the formation of a large-molecular-weight complex. Based on the ratio of the diffusion coefficients for free- and SDS-associated peptide, the molecular weight of the peptide–SDS complex was much larger than a factor of 1.4, the increase in molecular weight of the free peptide predicted if apoC-I(7–24) was uniformly surface coated with SDS.