Electron nuclear double resonance single crystal

The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. 

ENDOR (Electron Nuclear Double Resonance) involves the simultaneous application of a microwave and a radio frequency signal to the sample. This is a technique invented by Feher in 1956. The original studies were on phosphorous-doped silicon. A description of the experimental results and apparatus used is presented in two Physical Review articles [24, 25], An excellent treatment of EPR double resonance techniques and theory is given in the book by Kevan and Kispert [26], What follows here is the theory and application of ENDOR used the in analysis of single crystal data with the goal of identifying free radical products in DNA constituents. 

Electron Spin Echo Envelope Modulation (ESEEM) and pulse Electron Nuclear Double Resonance (ENDOR) experiments are considered to be two cornerstones of pulse EPR spectroscopy. These techniques are typically used to obtain the static spin Hamiltonian parameters of powders, frozen solutions, and single crystals. The development of new methods based on these two effects is mainly driven by the need for higher resolution, and therefore, a more accurate estimation of the magnetic parameters. In this chapter, we describe the inner workings of ESEEM and pulse ENDOR experiments as well as the latest developments aimed at resolution and sensitivity enhancement. The advantages and limitations of these techniques are demonstrated through examples found in the literature, with an emphasis on systems of biological relevance. 

Scholes CP, Lapidot A, Mascarenhas R, Inubushi T, Isaacson RA, Feher G. 1982. Electron nuclear double resonance (ENDOR) from heme and histidine nitrogens in single crystals of aquometmyoglobin. J Am Chem Soc 104(10) 2724-2735. 

Cu(S2CNEt2)2] adopts a dimeric structure with a nonplanar CUS4 unit, a and electron-nuclear double resonance (ENDOR) study of this complex substituted into single crystals of [Ni(S2CNEt2)2] revealed that the centrosymmetric structure of the nickel host had been adopted by the guest (1675). 

Butler JE and Hutchison, Jr. CA 1981 Electron paramagnetic resonance and electron nuclear double resonance of 237-neptunium hexafluoride in uranium hexafluoride single crystals. J. Chem. Phys. 74(6), 3102-3119. 

Recent Electron-Nuclear-Double and -Triple Magnetic Resonance measurements on single crystals of reaction centers (RC s) of the bacterium Rb. sphaeroides R-26 have revealed an asymmetric spin density distribution of the primary donor radical cation The observed ratio of spin densities, on the monomeric halves Dl and D of... 

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