6 Tesla, and the enhancement factor is usually the highest at lowest field (Prakash et al. 2005a, 2006; Roy et al. 2006, 2008). Full control over the parameters governing the generation of nuclear polarization may allow for

enhancement by a factor of 100,000 (Jeschke and Matysik 2003). The strong signal enhancement allows for direct observation of the photochemical machinery of RCs in membranes (Roy et al. 2008) or cells (Prakash et al. 2006). Furthermore, the solid-state photo-CIDNP effect also provides new channels for signal recovery allowing to increase the cycle delay and to shorten the measuring time (Diller et al. 2007a). Fig. 1 13C MAS NMR spectra of isolated RCs of Rb. sphaeroides R26 (A, B) and WT (C, D) in the dark (A, C) and under illumination with continuous white CX-6258 solubility dmso light. All spectra were obtained at 4.7 Tesla (200 MHz proton frequency) with a cycle delay of 4 seconds at a temperature of 230 K (Prakash et al.

2005a, b, 2006) The strong increase of NMR signal intensity and selectivity allows for detailed analysis of the electronic structure of the active cofactors. The NMR chemical shifts are related to the electronic structure of the electronic ground state after the photocycle, and the photo-CIDNP intensities are related to local electron spin densities. Hence, photo-CIDNP MAS NMR allows for investigation of both, the electronic ground state and the radical pair state. This method has shown that the special pair of RCs of Rhodobacter (Rb.) sphaeroides wildtype (WT) is already asymmetric in SYN-117 its electronic ground state PtdIns(3,4)P2 (Schulten et al. 2002), although the origin of the asymmetry is not yet understood. In the radical Selleck Tanespimycin cation state, the ratio between the two moieties has been determined to be around 3:2 (Prakash et al. 2005a), which is in good agreement with 1H ENDOR data (Lendzian et al. 1993). Time-resolved photo-CIDNP

MAS NMR experiments allowed for determination of the electron spin density distribution of the radical pair at the atomic resolution and precise kinetic modeling (Daviso et al. 2008b). On the other hand, the donors of the RCs of the green sulfur bacteria Chlorobium tepidum (Roy et al. 2007) and of the Heliobacterium mobilis (Roy et al. 2008) have been shown to be monomeric or highly symmetric. The donor of photosystem II (PS2) has been shown to have a highly asymmetric electron spin distribution (Matysik et al. 2000a) which has been proposed to be caused by involvement of an axial histidine (Diller et al. 2007b). In contrast, the cofactors in the donor of photosystem I (PSI) are undisturbed (Alia et al. 2004). Occurrence and origin of the solid-state photo-CIDNP effect Photochemical induced dynamic nuclear polarization (photo-CIDNP) is a well-known phenomenon in liquid NMR (for reviews: Hore and Broadhurst 1993; Roth 1996; Goez 1997). In this article, the term “polarization” is exclusively used for spin polarization, i.e., the difference in population of α and β nuclear or electron spins.