MEASUREMENT OF SDS MICELLE-PEPTIDE ASSOCIATION USING H-1-NMR CHEMICAL-SHIFT ANALYSIS AND PULSED-FIELD GRADIENT NMR-SPECTROSCOPY

The binding of two simple tripeptides, glycyl-histidyl-glycine (GHG) a nd phenylalanyl-histidyl-phenylalanine (FHF) with SDS micelles was exa mined using H-1 NMR chemical shift analysis and self-diffusion coeffic ients measured with pulsed-field gradient NMR spectroscopy, The presen ce of GHG or FHF did not appear to significantly affect the critical m icelle concentration (cmc) or the average size of the SDS micelles for med. The chemical shifts of several of the GHG resonances change as a function of SDS concentration, indicating an interaction between the p eptide and the micelles, In addition, the concentration-dependent decr ease observed for the GHG diffusion coefficients suggests association of the peptide with SDS micelles. The free and micelle-associated GHG are in fast exchange on both the H-1 chemical shift and diffusion time scales, The equilibrium constant for the binding of GHG to SDS micell es was determined from the analysis of the concentration dependence of the histidine C2 and C4 resonances to be 17 +/- 5 and 24 +/- 6 M-1, r espectively, The precision of the equilibrium constants obtained by an alysis of the chemical shift data is limited by the small chemical shi ft changes observed. Analysis of the concentration dependence of the d iffusion coefficients produced an equilibrium constant of 17 +/- 1 M-1 . The more hydrophobic peptide, FHF is strongly associated with the SD S micelles, Because the fraction of free FHF is small in these solutio ns, it was not possible to determine a formation constant for the inte raction of FHF with the SDS micelles by analysis of either the H-1 che mical shift or diffusion coefficient data, The cmc of SDS in 0.10 M Na 2C2O4 buffer was determined to be 5.4 +/- 0.1 mM by analysis of the SD S diffusion coefficients in the absence of the peptides, The SDS cmc c ould also be extracted from the GHG and FHF diffusion coefficients mea sured as a function of the SDS concentration, The cmc determined from the GHG diffusion data, 5.7 +/- 0.2 mM, is in good agreement with the value determined from analysis of the SDS diffusion coefficients in th e 5.0 mM GHG solution, 5.2 +/- 0.1 mM, The smaller cmc determined from the FHF diffusion data, 4.1 +/- 0.1 mM, may reflect some association of the SDS with the peptide prior to micelle formation in bulk solutio n.