Multifrequency pulsed electron paramagnetic resonance study of the S2 state of the photosystem II manganese cluster

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is employed to measure the strength of the hyperfine coupling of magnetic nuclei to the paramagnetic (S = 1/2) S2 form of photosystem II (PSII). Previous X-band-frequency ESEEM studies indicated that one or more histidine nitrogens are electronically coupled to the tetranuclear manganese cluster in the S2 state of PSII. However, the spectral resolution was relatively poor at the approximately 9 GHz excitation frequency, precluding any in-depth analysis of the corresponding bonding interaction between the detected histidine and the manganese cluster. Here we report ESEEM experiments using higher X-, P-, and Ka-band microwave frequencies to target PSII membranes isolated from spinach. The X- to P-band ESEEM spectra suffer from the same poor resolution as that observed in previous experiments, while the Ka-band spectra show remarkably well-resolved features that allow for the direct determination of the nuclear quadrupolar couplings for a single I = 1(14)N nucleus. The Ka-band results demonstrate that at an applied field of 1.1 T we are much closer to the exact cancellation limit (alpha iso = 2nu(14)N) that optimizes ESEEM spectra. These results reveal hyperfine (alpha iso = 7.3 +/- 0.20 MHz and alpha dip = 0.50 +/- 0.10 MHz) and nuclear quadrupolar (e(2)qQ = 1.98 +/- 0.05 MHz and eta = 0.84 +/- 0.06) couplings for a single (14)N nucleus magnetically coupled to the manganese cluster in the S 2 state of PSII. These values are compared to the histidine imidazole nitrogen hyperfine and nuclear quadrupolar couplings found in superoxidized manganese catalase as well as (14)N couplings in relevant manganese model complexes.