Di ff usion NMR Study of Complex Formation in 1 Membrane - Associated Peptides

Pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) is used to obtain 6 the true hydrodynamic size of complexes of peptides with sodium dodecyl sulfate SDS 7 micelles. The peptide used in this study is a 19-residue antimicrobial peptide, GAD-2. Two 8 smaller dipeptides, alanine-glycine (Ala-Gly) and tyrosine-leucine (Tyr-Leu), are used for 9 comparison. We use PFG-NMR to simultaneously measure diffusion coefficients of both 10 peptide and surfactant. These two inputs, as a function of SDS concentration, are then fit to 11 a simple two species model that neglects hydrodynamic interactions between complexes. 12 From this we obtain the fraction of free SDS, and the hydrodynamic size of complexes 13 in a GAD-2–SDS system as a function of SDS concentration. These results are compared 14 to those for smaller dipeptides and for peptide-free solutions. At low SDS concentrations 15 ([SDS] ≤ 25 mM), the results self-consistently point to a GAD-2–SDS complex of fixed 16 hydrodynamic size R =(5.5 ± 0.3) nm. At intermediate SDS concentrations (25 mM < [SDS] 17 Suliman Barhoum Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John’s, NL, Canada E-mail: [email protected], Valerie Booth Department of Biochemistry, Memorial University of Newfoundland, St. John’s, NL, Canada Tel.: 709-864-4523 E-mail: [email protected], Anand Yethiraj Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John’s, NL, Canada Tel.: 709-864-2113 E-mail: [email protected] ar X iv :1 21 0. 66 43 v2 [ co nd -m at .s of t] 1 5 Ja n 20 13 2 Suliman Barhoum, Valerie Booth and Anand Yethiraj < 60 mM), the apparent size of a GAD-2–SDS complex shows almost a factor of two increase 18 without a significant change in surfactant-to-peptide ratio within a complex, most likely 19 implying an increase in the number of peptides in a complex. For peptide-free solutions, 20 the self-diffusion coefficients of SDS with and without buffer are significantly different at 21 low SDS concentrations but merge above [SDS]=60 mM. We find that in order to obtain 22 unambiguous information about the hydrodynamic size of a peptide-surfactant complex 23 from diffusion measurements, experiments must be carried out at or below [SDS] = 25 mM. 24