ENDOR spectroscopy is primarily directed to study the magnetic interactions of the unpaired electron spin with the spins of magnetic nuclei (hyperfine interaction, HFI). These nuclei can belong either to the molecule on which the unpaired electron is localized, or to the surrounding molecules. selleck inhibitor In favorable cases, the GW-572016 supplier nuclear quadrupole interaction (NQI) experienced by nuclei with spin I > 1/2 can be tested by ENDOR. The strength of the HFI and the NQI is intimately related to the electron spin and charge density distribution of the molecule, respectively. Therefore, their detection offers a deep insight into the electronic
structure of the studied systems, which is crucial for understanding their chemical reactivity and function. The two main branches of ENDOR, continuous wave (CW) and pulse, are based on CW and pulse EPR, respectively.
Pulse ENDOR requires the detection of the electron spin echo (ESE) signal, which limits its application to systems with a sufficiently large transverse electron spin relaxation time (T 2 > 100 ns). This makes pulse ENDOR not suitable for studies of liquid samples and generally requires low-temperature experiments. CW ENDOR is free from this limitation and allows the experiments to be performed under physiological conditions. However, the technique requires “fine tuning” of the longitudinal relaxation times of the electron and nuclear spins YAP-TEAD Inhibitor 1 concentration for optimum signal intensities. enough Due to the strong temperature dependence of these relaxation rates, pulse ENDOR is usually superior to CW ENDOR at low temperatures. This article starts with a brief theoretical section, where the most important equations are presented. Then selected examples of ENDOR studies of photosynthetic systems are reviewed. Furthermore, limitations and perspectives of the technique are discussed. Theory Spin system The simplest system for which ENDOR can be used is a radical with the electron spin
S = 1/2 which has one nucleus with nuclear spin I = 1/2. First, we assume that hyperfine coupling between them is isotropic. If the g-tensor is also isotropic, the spin-hamiltonian H of this system is (in frequency units): $$ \fracHh = \fracg\beta_\texte hB_0 S_\textz – \fracg_\textn \beta_\textn hB_0 I_\textz + a(SI). $$ (1)The first term in this equation describes the electron Zeeman interaction, the second term describes the nuclear Zeeman interaction, and the third describes the HFI. Here, h is Planck’s constant, β e is the Bohr magneton, g is the electronic g-value, β n is the nuclear magneton, g n is the nuclear g-value, a is the HFI constant, S and I are the operators of the electron and nuclear spin. We assumed that the constant magnetic field of the EPR spectrometer B 0 is directed along the z-axis of the laboratory frame. The spin-hamiltonian in Eq.