Electronic coupling through-space interactions

The previous experiment (COSY) demonstrated the interactions (J coupling) between protons via the bonding electrons. The NOE effect which we described in Section 1.1.6 functions because of the through-space interactions between protons, and we used the NOE difference and selective NOE experiments to demonstrate it. 

Another attractive way to control ET rates in protein-protein complexes is by variations in electronic coupling. Conformational changes conceivably could form or break up pathways involving H-bonds or through-space interactions. Examination of potential ET pathways in selected protein-protein complexes is underway in collaboration with Beratan and Onuchic [76]. The foundation we have built in our investigations on Ru-modified proteins should be helpful in this work. 

The direct contribution (c3)dir arises from the through-space dipolar coupling of the nuclear magnetic moments and, as expected, it decreases as the internuclear distance increases with increasing v, because of its R l dependence. The second contribution (C3)ec is the axial component of the tensorial electron-coupled spin-spin interaction the scalar part of this interaction is given by the value of C4. In the v = 0 level the direct contribution is estimated by English and Zorn [51] to be 1.15 kHz, and the electron-coupled part is -0.23 kHz. 

Two nitroxides, separated by the distance r, are coupled through space via electron-electron dipolar interactions arising from the unpaired electrons. Spin coupling induces line splitting dependent on their separation and their orientation with respect to the magnetic field according to... 

The through-space interaction is a dipolar coupling between the electron and nuclear magnetic moments. When the Zeeman interaction for both the electron and nuclear spins is the dominant term in the spin Hamiltonian of the system, the energy of the dipole-dipole interaction is inversely proportional to the third power of the dipole-dipole distance fis according to... 

The middle Cp ligand effectively couples the electrons between the two CpM units. In other words, there is a strong through-bond rather than through-space interaction. The 18-electron rule obviously cannot be used for these situations. An MO-based, delocalized description like that presented in Figure 20.5 must be utilized. 

If some spin density is transferred to a p or d orbital on a certain nucleus, this spin density contributes to the dipole-dipole interaction between the two spins. Such a contribution to the dipole-dipole interaction may exceed the contribution from through-space interaction between the nuclear spin and the center of electron spin density. The distance between the nucleus and the center of spin density cannot be computed directly from the anisotropic hyperfine coupling in this situation. Distance determination by ENDOR or ESEEM thus requires that either nuclei without significant spin density in p or d orbitals are used (protons, denterons, alkaU, and alkali earth metal ions) or that spin density transfer to the atom under consideration is negligible (intermolecular couplings). 

---