Gd³⁺ Spin Labels Report the Conformation and Solvent Accessibility of Solution and Vesicle-Bound Melittin

Although Gd³⁺-based spin labels have been shown to be an alternative to nitroxides for double electron–electron resonance (DEER) distance measurements at high fields, their ability to provide solvent accessibility information, as nitroxides do, has not been explored. In addition, the effect of the label type on the measured distance distribution has not been sufficiently characterized. In this work, we extended the applicability of Gd³⁺ spin labels to solvent accessibility measurements on a peptide in model membranes, namely, large unilamellar vesicles (LUVs) using W-band ²H Mims electron–nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) techniques and Gd³⁺-ADO3A-labeled melittin. In addition, we carried out Gd³⁺–Gd³⁺ DEER distance measurements to probe the peptide conformation in solution and when bound to LUVs. A comparison with earlier results reported for the same system with nitroxide labels shows that, although in both cases the peptide binds parallel to the membrane surface, the Gd³⁺-ADO3A label tends to protrude from the membrane into the solvent, whereas the nitroxide does the opposite. This can be explained on the basis of the hydrophilicity of the Gd³⁺-ADO3A labels in contrast with the hydrophobicity of nitroxides. The distance distributions obtained from different labels are accordingly different, with the Gd³⁺-ADO3A yielding consistently broader distributions. These discrepancies are most pronounced when the peptide termini are labeled, which implies that such labeling positions may be inadvisible