In photosystem II (PSII), one-electron oxidation of the most stable oxidation state of the Mn4CaO5 cluster (S1) leads to formation of two distinct states, the open-cubane S2 conformation [Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV)] with low spin and the closed-cubane S2 conformation [Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III)] with high spin. In electron paramagnetic resonance (EPR) spectroscopy, the open-cubane S2 conformation exhibits a g = 2 multiline signal. However, its protonation state remains unclear. Here, we investigated the protonation state of the open-cubane S2 conformation by calculating exchange couplings in the presence of the PSII protein environment and simulating the pulsed electron-electron double resonance (PELDOR). When a ligand water molecule, which forms an H-bond with D1-Asp61 (W1), is deprotonated at dangling Mn4(IV), the first-exited energy (34 cm-1) in manifold spin excited states aligns with the observed value in temperature-dependent pulsed EPR analyses, and the PELDOR signal is best reproduced. Consequently, the g = 2 multiline signal observed in EPR corresponds to the open-cubane S2 conformation with the deprotonated W1 (OH-).
Long term monotonous driving has been often found to decrease the driver's arousal level and effect his/hers property of perception, cognition and judgment. It is necessary to support driving according to driver's state. In this study, we examined the influence of seat characteristics on the driver's psychosomatic state using a excitation machine by measuring biological. measurement including electrocardiogram and finger plethysmograph. As a result, It has been understood that sheet structure and posture influence driver fatigue.
Rhodopsin is one of G protein-coupled receptors {GPCRs) Ihat werks as a photereceptor protein in our eyes, When it absorbs a photon, rhodopsin converts to its G protein-activating state, metarhodopsin II (MII).Previous study showed that MII can activate not only retinal G protein, Gt but also Gi and its alpha-subunit {Gicr) mutant whese C-terminal 11 amino acids were replaced with those of Go(r.In addition, the activation eficiency of this mutant was about three tlmes less than those of Gt and Gi.However, we recently found that thi$ mutant exhibits 1arger activation ethciency than Gt and Gi at lower concentration ofGTPI,S.
Amyloid nitroxyl radical (nitroxide) ligands were used to detect amyloid-β fibrils, the main constituents of senile plaques in Alzheimer's disease, using anisotropic ESR spectra, and were found to affect the aggregation process due to the radical functionality. These compounds have great potential as novel and multifunctional probes, combining spin labels, spin probes, and fluorescence probes.
Electron transfer in the membranes and the type I reaction center (RC) core protein complex isolated from Heliobacterium modesticaldum was studied by optical and ESR spectroscopy. The RC is a homodimer of PshA proteins. In the isolated membranes, illumination at 14 K led to accumulation of a stable ESR signal of the reduced iron−sulfur center FB- in the presence of dithiothreitol, and an additional 20 min illumination at 230 K induced the spin-interacting FA-/FB- signal at 14 K. During illumination at 5 K in the presence of dithionite, we detected a new transient signal with the following values: gz = 2.040, gy = 1.911, and gx = 1.896. The signal decayed rapidly with a 10 ms time constant after the flash excitation at 5 K and was attributed to the FX--type center, although the signal shape was more symmetrical than that of FX- in photosystem I. In the purified RC core protein, laser excitation induced the absorption change of a special pair, P800. The flash-induced P800+ signal recovered with a fast 2−5 ms time constant below 150 K, suggesting charge recombination with FX-. Partial destruction of the RC core protein complex by a brief exposure to air increased the level of the P800+A0- state that gave a lifetime (t1/2) of 100 ns at 77 K. The reactions of FX and quinone were discussed on the basis of the three-dimensional structural model of RC that predicts the conserved FX-binding site and the quinone-binding site, which is more hydrophilic than that in the photosystem I RC.
Reaction center chlorophylls (Chls) in photosystems II and I were studied in the isolated thylakoid membranes of a cyanobacterium, Acaryochloris marina, which contains Chls d and a as the major and minor pigments, respectively. The membranes contained PS I and II complexes at a 1.8:1 molar ratio on the basis of the spin densities on the tyrosine D radical and the photo-oxidized PS I primary donor (P740+). In the presence of ferricyanide, laser excitation induced bleach at 725 nm that recovered with time constants of 25 micros and 1.2 ms. The signal, designated P725, was suppressed by PS II inhibitors DCMU and hydroxylamine. The P725 spectrum was tentatively assigned to the absorption changes of the special pair Chl d, the accessory Chl d, and the acceptor pheophytin a in PS II. The addition of ascorbate induced the additional signal with a slow decay time constant of 4.5 ms. This signal showed a broad bleach at 740 nm and shift-type absorption changes at around 707 and 685 nm, which were assigned to the absorption changes of PS I special pair of Chl d (P740), the accessory Chl d, and the primary acceptor Chl a (A0), respectively. Mechanisms and the evolution of the Chl-d based reaction centers using far-red light are discussed together with the amino acid sequences of PS II D1 and D2 proteins.
Binding of Mn2+ to manganese-depleted photosystem II and electron donation from the bound Mn2+ to an oxidized YZ tyrosine were studied under the same equilibrium conditions. Mn2+ associated with the depleted membranes in a nonsaturating manner when added alone, but only one Mn2+ ion per photosystem II (PS II) was bound to the membranes in the presence of other divalent cations including Ca2+ and Mg2+. Mn2+-dependent electron donation to photosystem II studied by monitoring the decay kinetics of chlorophyll fluorescence and the electron paramagnetic resonance (EPR) signal of an oxidized YZ tyrosine (YZ+) after a single-turnover flash indicated that the binding of only one Mn2+ ion to the manganese-depleted PS II is sufficient for the complete reduction of YZ+ induced by flash excitation. The results indicate that the manganese-depleted membranes have only one unique binding site, which has higher affinity and higher specificity for Mn2+ compared with Mg2+ and Ca2+, and that Mn2+ bound to this unique site can deliver an electron to YZ+ with high efficiency. The dissociation constant for Mn2+ of this site largely depended on pH, suggesting that a single amino acid residue with a pKa value around neutral pH is implicated in the binding of Mn2+. The results are discussed in relation to the photoactivation mechanism that forms the active manganese cluster.
The g-factor shift of the g = 4.1 EPR signal was detected in spinach PsbO/P/Q-depleted PS II. The effective g-factor of the signal shifts up to ∼4.9, depending on the Ca2+ concentration. Hyperfine structure spacing with about 3 mT was detected in this g = 5 (4.9) signal. The shift to g = 5 (4.9) was related to the distortion of the manganese cluster, derived from the modification of the chemical bond or the crystalline field of the Mn4(III) in the manganese cluster. Based on the EPR analysis of the g = 5 (4.9) spin state, another molecular structure of the S2 state, a "distant Mn" structure, was discussed as an intermediate state between the S2 and S3 states.
