The sum over all possible angles θ, as observed on a random sample in the immobilized Emricasan state, results in a powder pattern, the Pake pattern. In solid-state NMR the sample is rotated about an axis that has an angle θ of θMA = 53.4° with respect to the magnetic field. Since the magnitude of cos θMA is zero, the dipolar interactions cancel out and therefore narrow lines
are observed even in the solid state (Matysik et al. 2009; Alia et al. 2009). Electron–electron interactions The primary reactions of photosynthesis comprise single electron transfer reactions; therefore coupled radicals and radical pairs abound. The interactions between electron spins located on different cofactors have revealed a wealth
of information on the distances and relative orientation of the radicals. Over short distances, exchange interactions need to be considered, but in the distance range between most of the cofactors, several nm, the dominant part of the interaction is dipolar. Several experiments have been designed in magnetic resonance to exploit electron–electron interactions in photosynthetic systems (van der Est 2009; Kothe and Thurnauer 2009; Matysik et al. 2009; Alia et al. 2009). Ultimately, complete quantum mechanical understanding of the interactions within the radical pairs should reveal the mechanisms responsible for the high efficiency of photosynthetic electron transfer. Electron–nuclear (hyperfine) interactions The hyperfine interaction between an electron spin and a nuclear XAV-939 solubility dmso spin has two components: the isotropic, Fermi-contact interaction and a dipole–dipole term. The latter can be used to determine the location of protons and
other nuclei in the vicinity of a center carrying spin density. One example for an application is the assignment of the protons hydrogen-bonded to the quinones in bacterial reaction centers (Flores et al. 2007). The Fermi-contact term derives from spin density in the s-orbital of the nucleus in question. For radicals with a delocalized π-electron system, the isotropic hyperfine interaction allows mapping the wavefunction at every position in the radical that has a suitable nucleus. Thereby, the wavefunction containing the unpaired electron is measured. The hyperfine interaction serves as a local probe of the MO coefficients, yielding a Evodiamine wealth of information on the electronic structure. To determine hyperfine couplings of the protons in π-radicals such as the bacteriochlorophyll radicals, EPR is not LY2835219 sufficient. Hyperfine couplings are in the range of several MHz, and EPR spectra are broadened by the interaction with several nuclei. Better resolution is obtained by electron–nuclear double resonance (ENDOR) (Kulik and Lubitz 2009) and pulsed EPR methods (van Gastel 2009). In the bacterial reaction center, the cation or anion radicals of the cofactors have been investigated.