title: Diffusion NMR study of complex formation in membrane-associated peptides.
abstract: 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] ≤ 25 mM), the results self-consistently point to a GAD-2-SDS complex of fixed hydrodynamic size R = (5.5 ± 0.3) nm. At intermediate SDS concentrations (25 mM < [SDS] < 60 mM), the apparent size of a GAD-2-SDS complex shows almost a factor of two increase without a significant change in surfactant-to-peptide ratio within a complex, most likely implying an increase in the number of peptides in a complex. For peptide-free solutions, the self-diffusion coefficients of SDS with and without buffer are significantly different at low SDS concentrations but merge above [SDS] = 60 mM. We find that in order to obtain unambiguous information about the hydrodynamic size of a peptide-surfactant complex from diffusion measurements, experiments must be carried out at or below [SDS] = 25 mM.
authors: Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, NL, Canada. sulimanb@mun.ca
authors: Suliman
authors: S
authors: Barhoum
authors: Valerie
authors: V
authors: Booth
authors: Anand
authors: A
authors: Yethiraj
mesh_terms: D000595:Amino Acid Sequence; D002462:Cell Membrane / Q000737:chemistry*; D004058:Diffusion; D004151:Dipeptides / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008823:Micelles; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry; D013501:Surface-Active Agents / Q000737:chemistry
chemical_list: D004151:Dipeptides; D008823:Micelles; D010455:Peptides; D013501:Surface-Active Agents; D012967:Sodium Dodecyl Sulfate
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