Diffusion NMR study of complex formation in membrane-associated peptides

Pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) is used to obtain the true hydrodynamic size of complexes of peptides with sodium dodecyl sulfate SDS micelles. The peptide used in this study is a 19-residue antimicrobial peptide, GAD-2. Two smaller dipeptides, alanine-glycine (Ala-Gly) and tyrosine-leucine (Tyr-Leu), are used for comparison. We use PFG-NMR to simultaneously measure diffusion coefficients of both peptide and surfactant. These two inputs, as a function of SDS concentration, are then fit to a simple two species model that neglects hydrodynamic interactions between complexes. From this we obtain the fraction of free SDS, and the hydrodynamic size of complexes in a GAD-2--SDS system as a function of SDS concentration. These results are compared to those for smaller dipeptides and for peptide-free solutions. At low SDS concentrations ([SDS] $\leq$ 25 mM), the results self-consistently point to a GAD-2--SDS complex of fixed hydrodynamic size R =(5.5 $\pm$ 0.3) nm. At intermediate SDS concentrations (25 mM $
Comment: accepted for publication in European Biophysics Journal (2013)