Photochemically induced nuclear spin polarization in bacterial photosynthetic reaction centers: Assignments of the N-15 SSNMR spectra

We report proposed chemical assignments of polarized nitrogen NMR signals from photosynthetic reaction centers of Rb. sphaeroides. These signals, which we previously described, are observed with solid state NMR methods in samples of Q-blocked reaction centers that are enriched in N-15. Th, CIDNP is excited by CW illumination with a Xenon are lamp; we presume that they result from a radical pair mechanism (RPM) involving mixing of the electronic triplet and singlet spin states of P+I.-. In this work selective labeling and comparison with chemical shifts of model compounds were used to assign the signals and were also used to distinguish directly and indirectly polarized signals. Signals at isotropic shifts of 163, 173, 232, and 236 ppm (relative to 1 M (NH4Cl)-N-15 in 2 N HCI) were assigned as arising from the tetrapyrrole nitrogens of the special pair P865, and all appear to be directly polarized from the RPM. An additional small peak at 167 ppm appear to be another bacteriochlorophyll species, either the monomeric ''B'' or the ''other half'' of P865. Signals at 105, 113, and 276 ppm arise from the tetrapyrrole nitrogens. of the bacteriopheophytin acceptor (''I''), and some of these signals (particularly the nitrogens in rings II and IV) seem to be directly polarized, while others are polarized by homonuclear spin diffusion involving a neighboring directly polarized nitrogen. Signals at 147 and 201 ppm arise from the delta and epsilon nitrogens of histidine, presumably from the ligand of P865, and are indirectly polarized. The intensities of the bacteriopheophytin signals are sensitive to the lifetime of P-3, consistent with a RPM mechanism in which P-3 acts as the nuclear relaxant for I; this was concluded from comparisons of samples in which the acceptor Q(A) was prereduced and samples in which is was chemically extracted, which are known from previous work to differ strongly in the lifetime of P-3. Many of the signals have chemical shift values that closely correspond to related model compounds, but moderate deviations (ca. 10 ppm) are seen for a few including the histidine resonances.