Secondary structure of a human growth hormone-releasing factor fragment (Leu27-hGRF(15-32)NH2) in aqueous/SDS micelle...

The secondary structure of a human growth hormone-releasing factor (hGRF) fragment (Leu27-hGRF(15-32)NH2) has been studied by 1H NMR at 500 MHz in aqueous solutions containing varying concentrations of d25-sodium dodecyl sulfate (SDS). Chemical shifts, coupling constants and NOESY data show that the secondary structure of the peptide is random in aqueous solution in the absence of SDS. At relatively low molar ratios of SDS to peptide (1.3:1 to 3.3:1 SDS:peptide) the 1D 1H spectrum of the peptide changes as the peptide resonances are broadened significantly. NOESY patterns consistent with helical structure are present in the region of residues 22-29 when the SDS:peptide molar ratio is 1.3:1 and the SDS concentration is slightly below the critical micelle concentration (CMC). At higher molar ratios of SDS to peptide (16:1 to 72:1), where the SDS concentration is significantly above the CMC, the lineshape of the peptide's 1H NMR spectrum is sharpened. In these environments an alpha-helical conformation is induced in residues 19-32 of the hGRF fragment, as shown both by NOESY and by chemical shift data. Thus, the well-known tendency of this region of the GRF peptide to form alpha-helix in isotropic mixed-solvent systems (e.g., methanol/water, trifluoroethanol (TFE)/water) is seen also in SDS/aqueous systems.