2008a). In particular, experiments on the magnetic field dependence (Prakash et al. 2005a, 2006), with different NMR cycle delays (Diller et al. 2007a) and with time-resolution using flash laser (Daviso et al. 2008b) allowed for deeper insight. In these studies, it has been demonstrated that up to three mechanisms are involved to build up photo-CIDNP under continuous illumination, which may run in parallel. In all mechanisms the break of the balance of the Androgen Receptor antagonist opposite nuclear spin populations in the two decay branches of the radical pair states (Fig. 2) leads to net steady-state nuclear polarization, which is detected in the NMR experiment. In time-resolved photo-CIDNP MAS NMR experiments, transient nuclear

polarization, buy NVP-HSP990 due to the different kinetics on the two decay channels of the radical pair (see below), may occur additionally NU7026 concentration (Daviso et al. 2008b). This phenomenon, however, will not be discussed further in the present review. Fig. 2 The mechanisms of photo-CIDNP production in natural RCs of Rb. sphaeroides WT and R26 as established for high-field conditions. From the photochemically excited donor, P*, an electron is transferred to the primary acceptor Φ, a bacteriopheophytin. The radical pair (P+•Φ−•) is initially in a pure singlet state and highly electron polarized. Due to hyperfine interaction,

the radical pair is oscillating between a singlet and a T0 triplet state. During intersystem crossing (ISC), electron polarization is transferred to nuclei by three-spin mixing (TSM). Back-ET from the singlet state of the radical pair leads to the electronic ground-state. Back-ET from the triplet state of the radical pair leads to the donor triplet (3P) state. In the differential decay (DD) mechanism, net photo-CIDNP is produced by different contributions of the two spin states of the spin-correlated radical pair to the spin evolution. In RCs having a long lifetime

of the donor triplet, 3P, as in R26, the differential relaxation (DR) mechanism occurs since nuclear spin relaxation is significant on the triplet branch, causing incomplete cancellation of nuclear polarization Tenoxicam of both branches Initially, the spin-correlated radical pair is formed in a pure singlet state and it is, therefore, highly electron polarized (Fig. 2). This electron polarization can be observed by EPR as photo-CIDEP. There are two transfer mechanisms which transfer this electron polarization to nuclear polarization: (i) Electron–electron–nuclear three-spin mixing (TSM) breaks the balance of the two radical pair decay channels by spin evolution within the correlated radical pair state depending on the signs of the electron–electron and of the electron–nuclear interactions (Jeschke 1997, 1998). This process occurs during ISC in solids. In contrast to Overhauser cross relaxation, it is a coherent process that relies on anisotropy of the hyperfine (hf) coupling.