Aqueous light driven hydrogen production by a Ru-ferredoxin-Co biohybrid.

Herein we report the creation of a novel solar fuel biohybrid for light-driven H2 production utilizing the native electron transfer protein ferredoxin (Fd) as a scaffold for binding of a ruthenium photosensitizer (PS) and a molecular cobaloxime catalyst (Co). EPR and transient optical experiments provide direct evidence of a long-lived (>1.5 ms) Ru(III)-Fd-Co(I) charge separated state formed via an electron relay through the Fd [2Fe-2S] cluster, initiating the catalytic cycle for 2H(+) + 2e(-) → H2.

Spin-dynamics of the spin-correlated radical pair in photosystem I. Pulsed time-resolved EPR at high magnetic field.

Spin-dynamics of the spin-correlated radical pair (SCRP) P(700)(+)A(1A)(-) in the photosystem I (PSI) reaction center protein have been investigated with high-frequency (HF), time-resolved EPR spectroscopy. The superior spectral resolution of HF EPR enables spin-dynamics for both the donor and acceptor radicals in the pair to be monitored independently. Decay constants of each spin were measured as a function of temperature and compared to data obtained at X-band EPR. Relaxation times, T(1), and decay rates, k(S), are the same at both X- and D-band magnetic fields. The spin-dynamics within the radical pair were determined from theoretical simulation of experimental time-resolved HF EPR spectra. At low temperatures, T < 60 K, the decay of the SCRP from the singlet state, k(S), is the predominant process, while at high temperatures, T > 130 K, the T(1) relaxation is much faster than k(S). The recombination rate k(S) was observed to decrease as the temperature is increased. These EPR spectral results are in agreement with previously reported optical measurements of P(700)(+)A(1)(-) radical pair recombination.

Cu2+ site in photosynthetic bacterial reaction centers from Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodopseudomonas viridis.

The interaction of metal ions with isolated photosynthetic reaction centers (RCs) from the purple bacteria Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodopseudomonas viridis has been investigated with transient optical and magnetic resonance techniques. In RCs from all species, the electrochromic response of the bacteriopheophytin cofactors associated with Q(A)(-)Q(B) --> Q(A)Q(B)(-) electron transfer is slowed in the presence of Cu(2+). This slowing is similar to the metal ion effect observed for RCs from Rb. sphaeroides where Zn(2+) was bound to a specific site on the surface of the RC [Utschig et al. (1998) Biochemistry 37, 8278]. The coordination environments of the Cu(2+) sites were probed with electron paramagnetic resonance (EPR) spectroscopy, providing the first direct spectroscopic evidence for the existence of a second metal site in RCs from Rb. capsulatus and Rps. viridis. In the dark, RCs with Cu(2+) bound to the surface exhibit axially symmetric EPR spectra. Electron spin echo envelope modulation (ESEEM) spectral results indicate multiple weakly hyperfine coupled (14)N nuclei in close proximity to Cu(2+). These ESEEM spectra resemble those observed for Cu(2+) RCs from Rb. sphaeroides [Utschig et al. (2000) Biochemistry 39, 2961] and indicate that two or more histidines ligate the Cu(2+) at the surface site in each RC. Thus, RCs from Rb. sphaeroides, Rb. capsulatus, and Rps. viridis each have a structurally analogous Cu(2+) binding site that is involved in modulating the Q(A)(-)Q(B) --> Q(A)Q(B)(-) electron-transfer process. Inspection of the Rps. viridis crystal structure reveals four potential histidine ligands from three different subunits (M16, H178, H72, and L211) located beneath the Q(B) binding pocket. The location of these histidines is surprisingly similar to the grouping of four histidine residues (H68, H126, H128, and L211) observed in the Rb. sphaeroides RC crystal structure. Further elucidation of these Cu(2+) sites will provide a means to investigate localized proton entry into the RCs of Rb. capsulatus and Rps. viridis as well as locate a site of protein motions coupled with electron transfer.

EPR investigation of Cu2+-substituted photosynthetic bacterial reaction centers: evidence for histidine ligation at the surface metal site.

The coordination environments of two distinct metal sites on the bacterial photosynthetic reaction center (RC) protein were probed with pulsed electron paramagnetic resonance (EPR) spectroscopy. For these studies, Cu2+ was bound specifically to a surface site on native Fe2+-containing RCs from Rhodobacter sphaeroides R-26 and to the native non-heme Fe site in biochemically Fe-removed RCs. The cw and pulsed EPR results clearly indicate two spectroscopically different Cu2+ environments. In the dark, the RCs with Cu2+ bound to the surface site exhibit an axially symmetric EPR spectrum with g(parallel) = 2.24, A(parallel) = 160 G, g(perpendicular) = 2.06, whereas the values g(parallel) = 2.31, A(parallel) = 143 G, and g(perpendicular) = 2.07 were observed when Cu(2+) was substituted in the Fe site. Examination of the light-induced spectral changes indicate that the surface Cu2+ is at least 23 A removed from the primary donor (P+) and reduced quinone acceptor (QA-). Electron spin-echo envelope modulation (ESEEM) spectra of these Cu-RC proteins have been obtained and provide the first direct solution structural information about the ligands in the surface metal site. From these pulsed EPR experiments, modulations were observed that are consistent with multiple weakly hyperfine coupled 14N nuclei in close proximity to Cu2+, indicating that two or more histidines ligate the Cu2+ at the surface site. Thus, metal and EPR analyses confirm that we have developed reliable methods for stoichiometrically and specifically binding Cu2+ to a surface site that is distinct from the well characterized Fe site and support the view that Cu2+ is bound at or near the Zn site that modulates electron transfer between the quinones QA and QB (QA-QB --> QAQB-) (Utschig, L. M., Ohigashi, Y., Thurnauer, M. C., and Tiede, D. M (1998) Biochemistry 37, 8278-8281) and proton uptake by QB- (Paddock, M. L., Graige, M. S., Feher, G., and Okamura, M. Y. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 6183-6188). Detailed EPR spectroscopic characterization of these Cu2+-RCs will provide a means to investigate the role of local protein environments in modulating electron and proton transfer.

A new metal-binding site in photosynthetic bacterial reaction centers that modulates QA to QB electron transfer.

Isolated reaction centers (RCs) from Rhodobacter sphaeroides were found to bind Zn(II) stoichiometrically and reversibly in addition to the 1 equiv of non-heme Fe(II). Metal and EPR analyses confirm that Zn(II) is ligated to a binding site that is distinct from the Fe site. When Zn(II) is bound to this site, electron transfer between the quinones QA and QB (QA-QB --> QAQB-) is slowed and the room-temperature kinetics become distributed across the microsecond to millisecond time domain. This effect of metal binding on the kinetics is similar to the more global effect of cooling RCs to 2 degreesC in the absence of Zn(II). This suggests that Zn(II) binding alters localized protein motions that are necessary for rapid QA-QB --> QAQB- electron transfer. Inspection of the RC crystal structure suggests a cluster of histidine ligands located beneath the QB binding pocket as a potential binding site.

Influence of iron-removal procedures on sequential electron transfer in photosynthetic bacterial reaction centers studied by transient EPR spectroscopy.

Electron spin polarized electron paramagentic resonance (ESP EPR) spectra were obtained with deuterated iron-removed photosynthetic bacterial reaction centers (RCs) to specifically investigate the effect of the rate of primary charge separation, metal-site occupancy, and H-subunit content on the observed P865+QA- charge-separated state. Fe-removed and Zn-substituted RCs from Rb. sphaeroides R-26 were prepared by refined procedures, and specific electron transfer rates (kQ) from the intermediate acceptor H- to the primary acceptor QA of (200 ps)-1 vs (3-6 ns)-1 were observed. Correlation of the transient EPR and optical results shows that the observed slow kQ rate in Fe-removed RCs is H-subunit-independent, and, in some cases, independent of Fe-site occupancy as Zn2+ substitution does not ensure retention of the native kQ. In addition, shifts in the optical spectrum of P865 and differences in the high-field region of the Q-band ESP spectrum for Fe-removed RCs with slow kQ indicate possible structural changes near P865. The experimental X-band and Q-band spin-polarized EPR spectra for deuterated Fe-removed RCs where kQ is at least 15-fold slower at room temperature than the (200 ps)-1 rate observed for native Fe-containing RCs have different relative amplitudes and small g-value shifts compared to the spectra of Zn-RCs which have a kQ unchanged from native RCs. These differences reflect the trends in polarization predicted from the sequential electron transfer polarization (SETP) model [Morris et al. (1995) J. Phys. Chem. 99, 3854-3866; Tang et al. (1996) Chem. Phys. Lett. 253, 293-298]. Thus, SETP modeling of these highly resolved ESP spectra obtained with well-characterized proteins will provide definitive information about any light-induced structural changes of P865, H, and QA that occur upon formation of the P865+QA- charge-separated state.

Mercury-199 NMR of the metal receptor site in MerR and its protein-DNA complex.

Structural insights have been provided by mercury-199 nuclear magnetic resonance (NMR) into the metal receptor site of the MerR metalloregulatory protein alone and in a complex with the regulatory target, DNA. The one- and two-dimensional NMR data are consistent with a trigonal planar Hg-thiolate coordination environment consisting only of Cys side chains and resolve structural aspects of both metal ion recognition and the allosteric mechanism. These studies establish 199Hg NMR techniques as useful probes of the metal coordination environment of regulatory proteins, copper enzymes, and zinc transcription factor complexes as large as 50 kilodaltons.

Magnetic resonance spectroscopy, calcium content, and anion coordination studies of bovine and goat lactoperoxidase.

Bovine lactoperoxidase from two purebred strains and a commercial source as well as lactoperoxidase isolated from Alpine goat milk were examined by proton NMR spectroscopy for structural comparison of the heme site. Hyperfine shifted proton NMR spectra for both the native enzymes and cyanide complexes were equivalent for the protein obtained from the four separate sources. Activity assays (guaiacol and iodide ion oxidations) were also employed to compare the enzyme from various sources. Bovine lactoperoxidase was shown to contain 1.5 +/- 0.1 calcium ions per heme unit. Lactoperoxidase complexes with nitrite ion and thiocyanate ion were characterized for comparison with the cyanide complex. The nitrite complex exhibits a proton NMR hyperfine shift pattern at ambient temperature consistent with a low-spin ferric formulation. Interaction of lactoperoxidase with thiocyanate ion was monitored by NMR and EPR spectroscopy. Proton NMR spectra of lactoperoxidase in the presence of excess thiocyanate ion illustrated the retention of a high-spin ferric configuration consistent with predominant binding of the physiological thiocyanate substrate at a non-heme site at room temperature. However, EPR spectroscopy at cryogenic temperatures revealed the existence of a low-spin lactoperoxidase thiocyanate complex. This result may be explained by low-affinity ambient temperature thiocyanate heme binding that is greatly enhanced at liquid helium temperature.