Pulsed ELDOR in spin-labeled polypeptides

The pulsed electron–electron double-resonance (PELDOR) technique was applied to obtain information about the structure of the synthetic polypeptide–biradical in a frozen glassy solution. From the concentration dependence of the PELDOR signal, the effects of intermolecular and intramolecular interactions were separated. It was found that the intramolecular dipole–dipole interactions in the biradical peptide led to the modulation effects in the PELDOR signal decay. This may be attributed to the existence of a conformational population having a distance between the two unpaired electrons of ∼20 Å with a distribution of (∼2 Å). Its fraction is estimated as about 25%.