Conformational flexibility of nitroxide biradicals determined by X-band PELDOR experiments

PELDOR (pulsed electron-electron double resonance) experiments have been performed at X-band (9 GHz) frequencies on a linear and a bent nitroxide biradical. All PELDOR time traces were recorded with the pump frequency nu(B) set at the center of the nitroxide spectra to achieve maximum pumping efficiency, while the probe frequency nu(A) was stepped between a frequency offset Delta nu(AB)=nu(A)-nu(B) of +40 to +80 MHz. The modulation frequencies and the damping of the oscillations change as a function Delta nu(AB), whereas the modulation depth l for our investigated systems was only very slightly altered. This can be explained by the selection of different orientations of nitroxide radicals with respect to the external magnetic field as a function of frequency offset. Quantitative simulations of the PELDOR time traces could be achieved for both molecules and for all offset frequencies using a simple geometric model, described by a free rotation of the nitroxide radical around its acetylene bond and a single bending mode of the interconnecting molecular bridge. The results show that the distribution function for the relative orientations of the nitroxides with respect to each other and with respect to the dipolar vector R deviates from a random distribution and thus has to be taken into account to quantitatively simulate the PELDOR traces. Vice versa, a quantitative simulation of PELDOR time traces with variable offset frequencies allows the determination of the conformational freedom of such molecules.