Distance measurement in the nanometer range by electron paramagnetic resonance spectroscopy (EPR) in combination with site-directed spin labeling is a very powerful tool to monitor the structure and dynamics of biomacromolecules in their natural environment. However, in-cell application is hampered by the short lifetime of the commonly used nitroxide spin labels in the reducing milieu inside a cell. Here, we demonstrate that the Gd(III) based spin label Gd-PyMTA is suitable for in-cell EPR. Gd-PyMTA turned out to be cell compatible and was proven to be inert in in-cell extracts of Xenopus laevis oocytes at 18 °C for more than 24 h. The proline rich peptide H-AP10CP10CP10-NH2 was site-directedly spin labeled with Gd-PyMTA at both cysteine moieties. The resulting peptide, H-AP10C(Gd-PyMTA)P10C(Gd-PyMTA)P10-NH2, as well as the model compound Gd-spacer-Gd, which consists of a spacer of well-known stiffness, were microinjected into Xenopus laevis oocytes, and the Gd(III)-Gd(III) distances were determined by double electron-electron resonance (DEER) spectroscopy. To analyze the intracellular peptide conformation, a rotamer library was set up to take the conformational flexibility of the tether between the Gd(III) ion and the Cα of the cysteine moiety into account. The results suggest that the spin labeled peptide H-AP10C(Gd-PyMTA)P10C(Gd-PyMTA)P10-NH2 is inserted into cell membranes, coinciding with a conformational change of the oligoproline from a PPII into a PPI helix.
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