The sample was infused with a flow rate of 10 μl/min. MAS NMR sample preparation Selectively isotope-enriched Synechocystis cells were harvested by centrifugation and washed once with buy Palbociclib standard BG-11 medium. The pellet was resuspended in a 100 μl of standard BG-11 under low light conditions. The sample was bubbled shortly with nitrogen to remove oxygen and quinone reduced by adding sodium dithionite to a final concentration of 100 mM under oxygen free and near dark conditions. After 30 min of incubation in the dark at room temperature, the sample was loaded into an optical transparent 4-mm sapphire MAS rotor under oxygen free conditions.

The sample was inserted into the NMR spectrometer right away. The isolated samples of PS1 and PS2 from spinach (Spinacia oleracea) at natural abundance have been prepared following the procedures described in Matysik et al. (2000) and Alia et al. (2004). Photo-CIDNP MAS NMR experiments

13C-MAS NMR experiments were performed on a DMX-200 NMR spectrometer (Bruker Biospin GmbH, Karlsruhe, Germany). All spectra have been obtained at a sample temperature of 235 K and a spinning speed of 8 kHz. The spectra were collected with a spin echo pulse sequence with the CYCLOPS phase cycle of (π/2) pulse under TPPM carbon-proton decoupling. Photo-CIDNP MAS NMR spectra have been obtained under continuous illumination with a 1,000-W xenon arc lamp. Results and discussion Determination of the 13C label incorporation The biosynthetic route from [4-13C]-ALA to Chl a is depicted in Fig. 2. Two molecules of [4-13C]-ALA are asymmetrically condensed to form the pyrrole porphobilinogen (PBG). Kinase Inhibitor Library Four molecules of PBG tetramerize,

and prior to macrocycle ring closure, the last pyrrole ring is inverted via a spiro-intermediate (Schulten et al. 2002). Upon incorporation of [4-13C]-ALA, a maximum of 8 13C can be pair wise incorporated into each Chl a molecule, resulting into the specific labeling pattern shown in Fig. 2 with 13C isotopes incorporated on position C-1/C-3, C-6/C-8, C-11/C-13, and C-17/C-19. The level of [4-13C]-ALA incorporation was determined quantitatively by LC-MS Sodium butyrate analysis. Chl a pigments were extracted from Synechocystis cells grown in [4-13C]-ALA supplemented BG-11 (labelled sample), and normal BG-11 medium (reference sample). Figure 3 shows the LC-MS spectra observed in the region of m/z = 893.5 ([M]+; C55H72O5N4Mg) from the reference (A) and the labelled sample (B). The total level of incorporation (P tot) was determined through an iterative procedure as described earlier in (Schulten et al. 2002) making use of a weighted sum according to the formula: $$ P_\texttot = \sum\limits_n\; = \;0^8 \fracn8 \times P_n $$ (1)where n stands for the number of labels present in an isotopomer and P 0 is the corresponding fraction of unlabelled Chl a estimated from the isotopic labeling pattern detected from the reference sample (Fig. 3a).