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 translation al diffusion coefficients (D-s) for a peptide corresponding to a propo sed lipid-binding domain of human apolipoprotein C-I, residues 7-24 (a poC-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 pre sence of SDS have been determined indirectly by monitoring the SDS dif fusion coefficient. The direct measurement of the diffusion coefficien t of the peptide enables one to distinguish whether SDS simply coats t he peptide's surface to produce a uniformly charged 'rod' or if the pe ptide associates with a micelle. Using the direct method, at SDS conce ntrations above 5 mM (which is below the SDS critical micelle concentr ation (8.1 mM)), apoC-I(7-24) exhibited diffusion coefficients consist ent with the formation of a large-molecular-weight complex. Based on t he ratio of the diffusion coefficients for free-and SDS-associated pep tide, 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 pep tide predicted if apoC-I(7-24) was uniformly surface coated with SDS. (C) 1998 Elsevier Science B.V.