PELDOR on an exchange coupled nitroxide copper(II) spin pair

Transition metal ions play an important role in the design of macromolecular architectures as well as for the structure and function of proteins and oligonucleotides, which makes them interesting targets for spectroscopic investigations. In combination with site directed spin labelling, pulsed electron-electron double resonance (PELDOR or DEER) could be a well-suited method for their characterization and localization. Here, we report on the synthesis and full characterization of a copper(II) porphyrin/nitroxide model system bearing an extended π-conjugation between the spin centres and demonstrate the possibility to disentangle the dipolar through space interaction from the through bond exchange coupling contribution even in the presence of orientational selectivity and conformational flexibility. The simulations used are based on the known experimental and spin Hamiltonian parameters and on a structural model as previously employed for similar systems. The mean exchange coupling of +4(1) MHz (antiferromagnetic) is in agreement with the value of |J| = 3(1) MHz determined from room temperature continuous wave electron paramagnetic resonance (EPR). Thus, as long as the pulse excitation bandwidths are large versus the spin-spin coupling, X-band PELDOR measurements in combination with explicit time trace simulations allow for disentangling the sign and magnitude of through bond electron-electron exchange from the through space dipolar interaction D.