More recently, the triplet state of electron donors in photosynthesis became amenable to investigation (van Gastel 2009). In this state, the HOMO and the

LUMO coefficients of the electron donor are obtained, revealing the distribution of the MO from which the electron leaves the cofactor (LUMO) and the MO which will accept the electron in the eventual charge recombination event. The relation between the light-induced reactions and the orbitals mentioned are discussed elsewhere in this issue (Carbonera 2009). Electronic structure from EPR and NMR Information from the hyperfine and the G-tensors Advanced methods, XAV-939 solubility dmso such as solid-state NMR (Alia et al. 2009; Matysik et al. 2009), pulsed EPR (van Gastel 2009), and ENDOR (Kulik and Lubitz 2009), yield magnetic resonance parameters

with high accuracy. To link these parameters to the electronic structure, quantum chemistry is used, and in many cases further method development in this area was driven by the desire to interpret magnetic resonance parameters. To describe the development in the interpretation of magnetic resonance parameters is beyond PD-1/PD-L1 inhibitor drugs the scope of this account, but as above we will illustrate the essence using the nitroxide spin labels. Their π-electron system comprises only two atoms, the nitrogen and the oxygen atom, substantially simplifying the discussion compared to a molecule such as the chlorophyll, for example. Hyperfine interaction

The spin-density distribution can be obtained from the hyperfine interaction of the unpaired electron with the nitrogen nuclear spin (I = 1). The interaction gives click here rise to the three lines separated by A zz in Fig. 2. Overlap of the N and O pz-orbitals results in the doubly occupied π-orbital and the singly occupied π*-orbital (MO scheme, Fig. 3). The energy of the N versus the O pz-orbital determines the magnitude of the MO coefficient on N, and thereby the hyperfine coupling of N. If the polarity in the vicinity of the NO group increases, the energy of the pz-orbital on oxygen will decrease relative to the energy of the nitrogen pz-orbital. As a result, the π*-orbital will have a larger N character or, in other words, the MO coefficient on N will be larger, resulting in a larger nitrogen hyperfine coupling. Fig. 3 Top: Schematic representation of the frontier orbitals of the nitroxide group. Left: pz-type orbital on nitrogen; right: pz- and non-bonding (n-) orbitals on oxygen. Polarity changes in the environment will shift the energy of the nitrogen pz relative to the oxygen pz-orbital, selleck kinase inhibitor shifting spin density from nitrogen to oxygen. The spin density at nitrogen determines the electron-nitrogen hyperfine splitting, which therefore is a measure for polarity.