X-ray Spectroscopies Revealing the Structure and Dynamics of

PERSPECTIVE pubs.acs.org/JPCL

X-ray Spectroscopies Revealing the Structure and Dynamics of Metalloprotein Active Centers Emad F. Aziz* Helmholtz-Zentrum Berlin f€ ur Materialien und Energie, Albert-Einstein-Strasse 15, 12489 Berlin, Germany, and Freie Universit€at Berlin, FB Physik, Arnimallee 14, D-14195 Berlin, Germany

ABSTRACT Reactions catalyzed by metalloproteins occur at the active centers, making them the focus of many spectroscopic investigations in an attempt to determine their structure. Here, an overview of the recent achievements and the current developments of X-ray spectroscopies using synchrotron light sources is presented. Their potential for investigating protein structure and dynamics is discussed.

A

comprehensive understanding of the function of metalloproteins requires measuring their specific atomic structure and electronic configuration, specifically at their metal active center where the function takes place. A wide range of techniques have been used for these purposes, each with advantages and limitations. X-ray crystallography is the primary technique for determining protein structure.1,2 Figure 1 shows the 3D structure of myoglobin (Mb) obtained from diffraction. This method requires preparation of the proteins in the form of crystals. Accordingly, proteins which cannot be crystallized or form inhomogeneous structures cannot be investigated with this method. Furthermore, the crystalline form differs significantly from the physiological medium (solution, pH, counterions, body temperature, and pressure) in which the protein performs its function. This also applies to other diffraction-based techniques such as electron or neutron diffraction. Nuclear magnetic resonance (NMR) spectroscopy reveals detailed information about the folding structure of proteins, within separation distances less than 6 Å.3-10 In contrast to the diffraction methods, with NMR, the molecular structure can

be investigated in solution, thus in the natural environment of the proteins.11 Transmission electron microscopy (TEM) provides a detailed view of cells and subcellular organelles, as has been shown for frozen thin films containing hydrated catalase performed at a temperature of ∼100 K.12-15 The primary drawback of the electron-microscopy-based techniques is that they depend on investigating frozen systems, and the process of freezing can change the structure of the macromolecules before investigation. There is a broad variety of optical spectroscopy techniques, which have significantly contributed to the discovery of protein structures and their ultrafast molecular dynamics. Combining the linear dichroism and the pump-probe technique gives access to the time evolution of polarized states. Using this approach, for example, the CO binding orientation to the active center of Mb (schematic picture of the active center is shown in Figure 1b) can be examined. A specific property of the Mb, in comparison to other heme-centered proteins, is to discriminate strongly against a binding of CO in comparison to the pure oxygen molecule. Lim et al. examined this binding behavior via photodissociation of the CO using polarized laser pulses (pump pulse) and successive probing of the infrared absorption band of CO (probe pulse).16 They showed that the binding discrimination between O2 and CO by the Mb cannot be due to a steric effect of better orbital orientation of the O2 molecule, as had been previously assumed. To our knowledge, up to now, no scientific report has provided a satisfactory explanation for the binding discrimination. We propose that this question can be addressed using X-ray spectroscopy of the protein in solution.17-19 This technique is atom- and state-selective and can probe the

A comprehensive understanding of the function of metalloproteins requires measuring their specific atomic structure and electronic configuration, specifically at their metal active center where the function takes place.

r 2011 American Chemical Society

Received Date: October 31, 2010 Accepted Date: January 20, 2011 Published on Web Date: February 02, 2011

320

DOI: 10.1021/jz1014778 |J. Phys. Chem. Lett. 2011, 2, 320–326


Figure 1. (a) 3D structure of myoglobin (obtained from the protein database 1WLA59) obtained via X-ray diffraction. (b) Heme active center schematically showing the ligand binding via the metal atom.

Figure 2. (a) Schematic picture of a synchrotron storage ring combined with a laser as the pump source for dynamic studies (pump-probe technique). Spectroscopy techniques using this light source are presented in (b) X-ray absorption in the transmission mode, (c) X-ray absorption on a flow cell in fluorescence yield, and (d) X-ray absorption and emission on a liquid jet.

occupied and empty states of transition metals that are located at the active center of metalloproteins. Furthermore, using X-rays in the hard and soft region of the energy spectrum, these electronic states can be probed from the inner core levels (a few thousand eV) up to the valence levels (few tens of eV). Because the metal active centers are involved


in the chemical reaction, their local electronic structure manipulates the functionality of the protein.17-19 X-ray absorption spectroscopy (XAS), using monochromatic control of the photon energy at synchrotron light sources (a schematic picture of the synchrotron is shown in Figure 2), is mostly applied for probing protein structure

321



Figure 3. L-edge X-ray absorption study on the iron active center of (a) hemin,17,29 (b) hemoglobin,17 and c) catalase.18 The spectra obtained from the powder form are presented n green, the sample dissolved in alcohol is in blue, and the sample dissolved in water is in red. The black spectra show the respective multiplet simulations.

through the K-edge (hard X-ray).20-27 X-ray spectroscopies have the ability to probe ultrafast dynamics. One of the techniques of investigating such dynamics is the core-hole clock. In general, the excitation leads to a molecule in a nonequilibrium state affected by electronic rearrangements and nuclear motion. The full width of the resonant excitation is determined by the lifetime of the final state. According to a simplistic model, the total lifetime consists of the core-hole lifetime and the electron lifetime of the excited state.28 Whereas for bound-state resonances of light elements (carbon, oxygen, nitrogen) the lifetime varies between 10-15 and 10-14 seconds, it drops for continuum resonances down to 10-17-10-16 s. Accordingly, using core-hole X-ray spectroscopy allows probing of the electronic structure and dynamics within the time scale of a few femtoseconds.29-32 XAS at the K-edge is based on exciting the electron from s-type orbitals. According to the dipole selection rule, it will mainly go to p-type orbitals of the probed atoms. For metalloproteins, the d-type orbitals of the transition metal are actually of greater interest because they are the main contributors to the chemical bond. Nevertheless, through the K-edge direct structure, information about the coordinated atoms can be obtained using the extended part of the absorption spectrum.23 The technique is named extended X-ray absorption fine structure spectroscopy (EXAFS), which describes the oscillation caused from back scattering of the photoelectron by atoms surrounding the absorb-


ing atom. Bianconi et al. use the near-edge part of XAS (NEXAFS) technique on the Fe center of Mb in an angular-resolved fashion to determine the Fe-ligand bond angles.20 The NEXAFS technique (near edge X-ray absorption fine structure) incorporates the oscillation at low energies just beyond the absorption edge. This opens the way to identify subtle structural features due to bond angle variations in proteins in solution. The polarized NEXAFS technique has been used by Della Longa et al. for investigating the dissociation process of CO from the Mb active center.24 In this work, they correlate the pre-edge of the absorption band to the quadruple transition from the s- to the d-type orbital of the Fe center. Note that quadruple transitions are not strongly allowed compared to dipole transitions, which impedes a direct mapping of the d-type orbitals. EXAFS has been used for determining the iron-nitrogen distances in the hemoglobin (Hb) active center (the structure of the active center is presented in Figure 3) as shown by Eisenberger et al.33 Their results were later confirmed using X-ray diffraction.34 Solomon and co-workers made a detailed structure analysis of the hemocyanin active center via K-edge XAS of copper and compared the obtained results with variable energy photoelectron spectroscopy as well as with density functional theory calculations.35 Technical requirements and typical set-ups for biological XAS experiments have been reviewed elsewhere.25 For light elements like oxygen or nitrogen, the K-edge lies in the soft X-ray regime, with photon energies ranging from a

322



few to some hundred eV. Soft X-ray spectroscopy requires an ultrahigh vacuum environment, which impedes investigating samples in solution. Aziz and co-workers have used the benefits of introducing ambient pressure experiments to probe the electronic structure of ions and molecules in solution as a function of temperature, solvent, pH, and so forth.36-45 Very recently, they applied the high-pressure microjet technique for the first time to measure XA in fluorescence yield (schematic picture in Figure 2) as well as X-ray emission spectroscopy (XES).46 L-edge XAS on transition metals also requires soft X-rays and has the advantage of probing directly the d-type orbitals of the investigated transition metal. It is an appropriate tool for investigating the structure of metal centers in proteins because (a) the smaller intrinsic core-hole lifetime width (0.5 eV) of p orbitals results in sharper spectral features than optical spectroscopies, (b) 2p1/2,3/2 f 3d transitions are dipole-allowed (according to the dipole selection rules, excitation goes to a state with l = (1), yielding more intense and more structured spectra than the quadruple transition to the d-type orbitals by the K-edge transitions, (c) L2,3-edge features are directly proportional to the amount of d-character of unoccupied valence orbitals of the metal,47 and (d) ligand field multiplet calculations are wellestablished tools to interpret L-edge spectra, delivering a detailed description of the oxidation state, the charge transfer, and the geometry of the excited atom.48-50 Early work has been done by Cramer and co-workers, investigating the iron L-edge of Mb complexes on solid samples.51 The first reports on the L-edge XAS for proteins in solution were made by Aziz and co-workers, where the active center of methemoglobin was investigated.17 The experimental spectra have been explained by means of multiplet simulations, showing the high spin nature of the iron active center. Although, catalase and methemoglobin have very similar haem groups, both of which are ferric, catalase decomposes hydrogen peroxide to water and oxygen very efficiently, while methemoglobin does not. Using Fe L2,3-edge XAS of these proteins in physiological solutions, clear differences in their electronic structures were revealed. Whereas π back-donation of the Fe atom occurs in catalase, which confers on it a partial ferryl (Fe4þ) character, this is not the case in methemoglobin. The origin of the Fe4þ character stems from the proximal tyrosine residue. It was also found that both methemoglobin and catalase systems are in the high spin state. The active centers of these proteins and hemin (hemin is the porphyrin active center of Mb) are shown in Figure 3 with the respective iron L-edge spectra and the theoretical simulations. Recently, hemocyanin has been investigated in its physiological media (the protein has been extracted from American lobster Homarus americanus) via the L-edge of the copper active center.19 The deoxygenated and the oxygenated states of native hemocyanin were compared with each other, and evidence was found that the oxygenation does not simply switch the copper valence state from Cu I to Cu II, as was assumed classically. Water can replace the dioxygen upon deoxygenation by binding to the active site of the hemocyanin and withdrawing electrons from the d orbital of the copper. Note that UV-vis spectroscopy was used to control the sample state freshly prepared before exposure to X-rays as well as after


exposure to X-rays in order to detect and accordingly avoid any potential sample damage. These recent works open a whole new approach to the detailed electronic description of the active centers of respiratory and photosynthetic metalloproteins.

Recent works open a whole new approach to the detailed electronic description of the active centers of respiratory and photosynthetic metalloproteins. Recently, we reported on a case of extreme deviations in fluorescence yield (FY) measurements for XAS in which, besides additional components showing up in the L3,2 spectra of aqueous ferrous species (as atomic or molecular ions), some components dipped below the background level.29 This effect was shown to depend on the respective solute and its concentration, which suggests a partial extinction of the fluorescence channel from the probed solute. The origin of this extinction was identified as an electron transfer (ET) to the surrounding water molecules, leading to a nonradiative decay of the excited atom, as shown for porphyrin in Figure 3a. The related spectral component is considered to be dark for the FY technique, and the mechanism is named dark channel fluorescence yield (DCFY). The ET occurs from core excited states of the metal ion (such as, e.g., Fe(III)) to a delocalized electronic structure built by the water continuum. Later, we determined that the effect is limited neither to specifically Fe as a solute nor to aqueous environments. We could show that the observation of sub-background features in FY spectra represents a novel and relatively simple way to obtain information on the interaction of solutes with their environment and to determine the direction of the ET, which is applicable in solutions as well as in heterogeneous media, such as interfaces. As shown in Figure 3a, the dip disappears once the water is replaced by alcohol as a solvent for hemin, which is correlated to the metal-to-ligand charge transfer (MLCT) by the excited electron from the Fe(III) p orbitals to mixed states between the Fe(III) and the H2O. The core-hole lifetime of the L2-edge (∼0.7 fs) is significantly shorter than that of the L3edge (∼2 fs), leading to a higher radiative decay rate at the L2edge (no dip appear at the L2-edge). Thus, it is likely that the L2 states are less prone to an ET effect. This DCFY mechanism is currently in use to identify the interaction of the active center of proteins with different ligands, as is the case of Mb with CO versus O2, as well as with water molecules in aqueous solution. I believe that the discovery of this mechanism will open a new perspective for the use of L-edge XAS on active metal centers in proteins. In order to quantify the DCFY, we are currently comparing the transmission XA spectra, which are directly proportional to the absorption cross section and are not influenced by the DCFY, to the FY spectra.

323



The observation of sub-background features in FY spectra represents a novel and relatively simple way to obtain information on the interaction of solutes with their environment and to determine the direction of the ET, which is applicable in solutions as well as in heterogeneous media

The ability to record absorption and emission spectra of an aqueous liquid microjet opens the way for a new perspective of studying proteins in physiological media. The sum of the spectroscopic techniques based on X-ray light sources yields a more comprehensive picture of protein structure and dynamics. Some of these techniques are already well-established, whereas other recently developed methods, with emphasis in the soft X-ray regime, promise to reveal within the next few years information about the microscopic structure of proteins correlated with dynamics in physiological media. Probing the function of proteins in their natural environment will be the focus of the soft X-ray spectroscopies. The dynamics that can be probed via X-ray spectroscopy techniques will be in the range of a few femtoseconds (dynamics during the core-hole lifetime), subpicoseconds, and longer via the laser pump and X-ray probe technique.

A planned technique development for the examination of the dynamics of proteins in solution, as a direct perspective for the XA technique in the soft X-ray regime, is the combination of the laser pump and X-ray probe approach at the L-edge. Using laser light of varying wavelengths as an excitation source, it is possible to excite spin states of the active center52 or to dissociate the CO ligand from the active center of Hb or Mb. The pump-probe technique has been well-established in hard X-ray synchrotron facilities, as shown in Figure 2 (laser pump and X-ray probe), by Chergui and co-workers.53-56 At the Advanced Light Source (ALS), Berkeley Lab, they reported for the first time on the dynamics of iron complexes in acetonitrile as a solvent via laser pump and XAS probe at the L-edge of the iron center.52 Note that these measurements were carried out on a droplet of the sample between two membranes (static cell). We are currently developing this technique further in order to apply it to proteins in physiological media. For this purpose, the static cells that were used at the ALS (shown schematically in Figure 2b) are not appropriate as sample damage can be easily induced by soft X-ray exposure. Alternatives are the flow cell technique (as shown in Figure 2c) or the microjet technique (as it shown in Figure 2d). For emission spectroscopy in the soft X-ray regime (XES), Soldatov et al. have shown experimental and theoretical work on Mb as a powder.57 As mentioned above, for proteins in solution, the pressure gap between the ultrahigh vacuum and the natural ambient pressure of the proteins has to be bridged. To my knowledge, until now, no X-ray emission spectra in the soft X-ray regime were recorded from proteins in solution. Very recently, we reported, for the first time, the XE spectra of a liquid microjet sample in a vacuum environment (a schematic picture for the setup is shown in Figure 2d).46 The ability to record absorption and emission spectra of an aqueous liquid microjet opens the way for a new perspective of studying proteins in physiological media. In principle, the microjet technique is not limited to spectroscopy but can also find application for imaging techniques using the advantages of the free electron laser as a coherent light source.58


AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected].

Biographies Emad F. Aziz is professor of physics at the Freie Universit€ at Berlin and head of the Structure and Dynamics of Functional Materials in Solution department at the Helmholtz Centre Berlin. He is developing techniques to introduce ambient experimental conditions to soft X-ray light sources. His work was recognized with several prizes, most recently, the international Dale-Sayers Prize 2009. For more information, see http://www.helmholtz-berlin.de/forschung/funkma/materialien-loesung/index_de.html and http://www.physik.fuberlin.de/en/einrichtungen/ag/ag-aziz/index.html.

ACKNOWLEDGMENT I would like to thank Kathrin M. Lange for constructing the figures and for support as well as Dr. Kai F. Hodeck for support. Special thanks to the collaborators in the protein projects from the Max-Delbr€ uck-Center for Molecular Medicine Berlin, Johannes Gutenberg-Universit€ at Mainz, and Ecole Polytechnique F ed erale de Lausanne. Financial support by the Helmholtz-Gemeinschaft through the young investigator fund VH-NG-635 is acknowledged.

REFERENCES (1)

(2) (3)

(4)

324

Perutz, M. F. Myoglobin and Hemoglobin ; Role of Distal Residues in Reactions with Heme Ligands. Trends Biochem. Sci. 1989, 14, 42–44. Perutz, M. F. Refinement of Hemoglobin and Myoglobin. Acta Crystallogr. 1975, A31, S31–S31. Fiorito, F.; Herrmann, T.; Damberger, F. F.; Wuthrich, K. Automated Amino Acid Side-Chain NMR Assignment of Proteins Using 13C- and 15N-Resolved 3D [1H,1H]-NOESY. J. Biomol. NMR 2008, 42, 23–33. Etezady-Esfarjani, T.; Herrmann, T.; Peti, W.; Klock, H. E.; Lesley, S. A.; Wuthrich, K. Letter to the Editor: NMR Structure



(5)

(6)

(7)

(8)

(9)

(10)

(11) (12) (13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

Determination of the Hypothetical Protein TM1290 from Thermotoga Maritima Using Automated NOESY Analysis. J. Biomol. NMR 2004, 29, 403–406. Herrmann, T.; Guntert, P.; Wuthrich, K. Protein NMR Structure Determination with Automated NOE-Identification in the NOESY Spectra Using the New Software ATNOS. J. Biomol. NMR 2002, 24, 171–189. Pervushin, K. V.; Wider, G.; Riek, R.; Wuthrich, K. The 3D NOESY-[1H,15N,1H]-ZQ-TROSY NMR Experiment with Diagonal Peak Suppression. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 9607–9612. Mumenthaler, C.; Guntert, P.; Braun, W.; Wuthrich, K. Automated Combined Assignment of NOESY Spectra and ThreeDimensional Protein Structure Determination. J. Biomol. NMR 1997, 10, 351–362. Otting, G.; Orbons, L. P. M.; Wuthrich, K. Suppression of ZeroQuantum Coherence in NOESY and Soft NOESY. J. Magn. Reson. 1990, 89, 423–430. Leupin, W.; Otting, G.; Amacker, H.; Wuthrich, K. Application of 13C(ω1)-Half-Filtered [1H,1H]-NOESY for Studies of a Complex Formed between DNA and a 13C-Labeled Minor-GrooveBinding Drug. FEBS Lett. 1990, 263, 313–316. Bodenhausen, G.; Wagner, G.; Rance, M.; Sorensen, O. W.; Wuthrich, K.; Ernst, R. R. Longitudinal 2-Spin Order in 2d Exchange Spectroscopy (NOESY). J. Magn. Reson. 1984, 59, 542–550. Yamamoto, Y. 1H NMR Probes for Inter-Segmental Hydrogen Bonds in Myoglobins. J. Biochem. 1996, 120, 126–132. Taylor, K. A.; Glaeser, R. M. Electron-Diffraction of Frozen, Hydrated Protein Crystals. Science 1974, 186, 1036–1037. Taylor, K. A.; Glaeser, R. M. Electron-Microscopy of Frozen Hydrated Biological Specimens. J. Ultrastruct. Res. 1976, 55, 448–456. Taylor, K. A.; Glaeser, R. M. Hydrophilic Support Films of Controlled Thickness and Composition. Rev. Sci. Instrum. 1973, 44, 1546–1547. Marks, R. A.; Taylor, S. T.; Mammana, E.; Gronsky, R.; Glaeser, A. M. Directed Assembly of Controlled-Misorientation Bicrystals. Nat. Mater. 2004, 3, 682–686. Lim, M.; Jackson, T. A.; Anfinrud, P. A. Binding of Co to Myoglobin from a Heme Pocket Docking Site to Form Nearly Linear Fe-C-O. Science 1995, 269, 962–966. Aziz, E. F.; Ottosson, N.; Bonhommeau, S.; Bergmann, N.; Eberhardt, W.; Chergui, M. Probing the Electronic Structure of the Hemoglobin Active Center in Physiological Solutions. Phys. Rev. Lett. 2009, 102, 68103. Bergmann, N.; Bonhommeau, S.; Lange, K. M.; Greil, S. M.; Eisebitt, S.; de Groot, F.; Chergui, M.; Aziz, E. F. On the Enzymatic Activity of Catalase: an Iron L-Edge X-Ray Absorption Study of the Active Centre. Phys. Chem. Chem. Phys. 2010, 12, 4827–4832. Panzer, D.; Beck, C.; Hahn, M.; Maul, J.; Schonhense, G.; Decker, H.; Aziz, E. F. Water Influences on the Copper Active Site in Hemocyanin. J. Phys. Chem. Lett. 2010, 1, 1642–1647. Bianconi, A.; Congiucastellano, A.; Durham, P. J.; Hasnain, S. S.; Phillips, S. The Co Bond Angle of Carboxymyoglobin Determined by Angular-Resolved Xanes Spectroscopy. Nature 1985, 318, 685–687. Bianconi, A.; Dellalonga, S.; Ascone, I.; Fontaine, A.; Congiucastellano, A. Time-Resolved Structure of Geminate States of Myoglobin Co by X-Ray-Absorption near-Edge Structure Spectroscopy. Synchrotron Radiat. Biosci. 1994, 294–301. Liu, T.; Chen, X. H.; Ma, Z.; Shokes, J.; Hemmingsen, L.; Scott, R. A.; Giedroc, D. P. A Cu-I-Sensing ArsR Family Metal Sensor


(23)

(24)

(25)

(26)

(27)

(28) (29)

(30)

(31)

(32)

(33)

(34)

(35)

(36)

(37) (38)

(39)

325

Protein with a Relaxed Metal Selectivity Profile. Biochemistry 2008, 47, 10564–10575. Levina, A.; Armstrong, R. S.; Lay, P. A. Three-Dimensional Structure Determination Using Multiple-Scattering Analysis of XAFS: Applications to Metalloproteins and Coordination Chemistry. Coord. Chem. Rev. 2005, 249, 141–160. Della Longa, S.; Arcovito, A.; Vallone, B.; Castellano, A. C.; Kahn, R.; Vicat, J.; Soldo, Y.; Hazemann, J. L. Polarized X-Ray Absorption Spectroscopy of the Low-Temperature Photoproduct of Carbonmonoxy-Myoglobin. J. Synchrotron Radiat. 1999, 6, 1138–1147. Ascone, I.; Meyer-Klaucke, W.; Murphy, L. Experimental Aspects of Biological X-Ray Absorption Spectroscopy. J. Synchrotron Radiat. 2003, 10, 16–22. Frank, P.; DeTomaso, A.; Hedman, B.; Hodgson, K. O. A New Structural Motif for Biological Iron: Iron K-Edge XAS Reveals a [Fe-4-μ-(OR5(OR)9-10] Cluster in the Ascidian Perophora annectens. Inorg. Chem. 2006, 45, 3920–3931. Alvarez, H. M.; Xue, Y.; Robinson, C. D.; Canalizo-Hernandez, M. A.; Marvin, R. G.; Kelly, R. A.; Mondragon, A.; PennerHahn, J. E.; O'Halloran, T. V. Tetrathiomolybdate Inhibits Copper Trafficking Proteins Through Metal Cluster Formation. Science 2010, 327, 331–334. St€ ohr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, New York, 1992. Aziz, E. F.; Rittmann-Frank, M. H.; Lange, K. M.; Bonhommeau, S.; Chergui, M. Charge Transfer to Solvent Identified Using Dark Channel Fluorescence-Yield L-Edge Spectroscopy. Nat. Chem. 2010, 2, 853–857. Fohlisch, A.; Feulner, P.; Hennies, F.; Fink, A.; Menzel, D.; Sanchez-Portal, D.; Echenique, P. M.; Wurth, W. Direct Observation of Electron Dynamics in the Attosecond Domain. Nature 2005, 436, 373–376. Chen, W.; Wang, L.; Qi, D. C.; Chen, S.; Gao, X. Y.; Wee, A. T. S. Probing the Ultrafast Electron Transfer at the CuPc/Au(111) Interface. Appl. Phys. Lett. 2006, 88, 184102. Thoss, M.; Kondov, I.; Wang, H. B. Correlated Electron-Nuclear Dynamics in Ultrafast Photoinduced Electron-Transfer Reactions at Dye-Semiconductor Interfaces. Phys. Rev. B 2007, 76, 153313. Eisenberger, P.; Shulman, R. G.; Kincaid, B. M.; Brown, G. S.; Ogawa, S. Extended X-Ray Absorption Fine-Structure Determination of Iron Nitrogen Distances in Hemoglobin. Nature 1978, 274, 30–34. Fermi, G.; Perutz, M. F.; Shulman, R. G. Iron Distances in Hemoglobin ; Comparison of X-Ray Crystallographic and Extended X-Ray Absorption Fine-Structure Studies. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 6167–6168. Solomon, E. I.; Szilagyi, R. K.; George, S. D.; Basumallick, L. Electronic Structures of Metal Sites in Proteins and Models: Contributions to Function in Blue Copper Proteins. Chem. Rev. 2004, 104, 419–458. Aziz, E. E.; Vollmer, A.; Eisebitt, S.; Eberhardt, W.; Pingel, P.; Neher, D.; Koch, N. Localized Charge Transfer in a Molecularly Doped Conducting Polymer. Adv. Mater. 2007, 19, 3257. Aziz, E. F. The Solvation of Ions and Molecules Probed via Soft X-Ray Spectroscopies. J. Electron Spectrosc. 2010, 177, 168. Aziz, E. F.; Eisebitt, S.; Eberhardt, W.; Cwiklik, L.; Jungwirth, P. Existence of Oriented Ion-Hydroxide Clusters in Concentrated Aqueous NaCl Solution at pH 13. J. Phys. Chem. B 2008, 112, 1262–1266. Aziz, E. F.; Freiwald, M.; Eisebitt, S.; Eberhardt, W. Steric Hindrance of Ion-Ion Interaction in Electrolytes. Phys. Rev. B 2006, 73, 75120.



(40)

(41)

(42)

(43)

(44)

(45)

(46)

(47)

(48) (49)

(50)

(51)

(52)

(53)

(54)

(55) (56)

Aziz, E. F.; Grasjo, J.; Forsberg, J.; Andersson, E.; Soderstrom, J.; Duda, L.; Zhang, W. H.; Yang, J. L.; Eisebitt, S.; Bergstr€ om; et al. Photoinduced Formation of N2 Molecules in Ammonium Compounds. J. Phys. Chem. A 2007, 111, 9662–9669. Aziz, E. F.; Ottosson, N.; Eisebitt, S.; Eberhardt, W.; JagodaCwiklik, B.; Vacha, R.; Jungwirth, P.; Winter, B. Cation-Specific Interactions with Carboxylate in Amino Acid and Acetate Aqueous Solutions: X-ray Absorption and Ab Initio Calculations. J. Phys. Chem. B 2008, 112, 12567–12570. Aziz, E. F.; Ottosson, N.; Faubel, M.; Hertel, I. V.; Winter, B. Interaction between Liquid Water and Hydroxide Revealed by Core-Hole De-Excitation. Nature 2008, 455, 89–91. Aziz, E. F.; Zimina, A.; Freiwald, M.; Eisebitt, S.; Eberhardt, W. Molecular and Electronic Structure in NaCl Electrolytes of Varying Concentration: Identification of Spectral Fingerprints. J. Chem. Phys. 2006, 124, 114502. Ottosson, N.; Aziz, E. F.; Bergersen, H.; Pokapanich, W.; Ohrwall, G.; Svensson, S.; Eberhardt, W.; Bjorneholm, O. Electronic Rearrangement upon the Hydrolyzation of Aqueous Formaldehyde Studied by Core-Electron Spectroscopies. J. Phys. Chem. B 2008, 112, 16642–16646. Ottosson, N.; Aziz, E. F.; Bradeanu, I. L.; Legendre, S.; € Ohrwall, G.; Svensson, S.; Bj€ orneholm, O.; Eberhardt, W. An Electronic Signature of Hydrolysation in the X-Ray Absorption Spectrum of Aqueous Formaldehyde. Chem. Phys. Lett. 2008, 460, 540–542. Lange, K. M.; K€ onnecke, R.; Ghadimi, S.; Golnak, R.; Soldatov, M. A.; Hodeck, K. F.; Soldatov, A.; Aziz, E. F. High Resolution X-Ray Emission Spectroscopy of Water and Aqueous Ions Using the Micro-Jet Technique. Chem. Phys. 2010, 377, 1–5. Aziz, E. F.; Eberhardt, W.; Eisebitt, S. Effect of Cysteine vs. Histidine on the Electronic Structure of Zn2þ Upon Complex Formation. Z. Phys. Chem. 2008, 222, 727–738. de Groot, F. Multiplet Effects in X-Ray Spectroscopy. Coord. Chem. Rev. 2005, 249, 31–63. Aziz, E. F.; Eisebitt, S.; de Groot, F.; Chiou, J.; Dong, C.; Guo, J.; Eberhardt, W. Direct Contact versus Solvent-Shared Ion Pairs in NiCl2 Electrolytes Monitored by Multiplet Effects at Ni(II) L Edge X-ray Absorption. J. Phys. Chem. B 2007, 111, 4440– 4445. Bonhommeau, S.; Ottosson, N.; Pokapanich, W.; Svensson, S.; Eberhardt, W.; Bjorneholm, O.; Aziz, E. F. Solvent Effect of Alcohols at the L-Edge of Iron in Solution: X-Ray Absorption and Multiplet Calculations. J. Phys. Chem. B 2008, 112, 12571–12574. Wang, H. X.; Peng, G.; Miller, L. M.; Scheuring, E. M.; George, S. J.; Chance, M. R.; Cramer, S. P. Iron L-Edge X-Ray Absorption Spectroscopy of Myoglobin Complexes and Photolysis Products. J. Am. Chem. Soc. 1997, 119, 4921–4928. Huse, N.; Kim, T. K.; Jamula, L.; McCusker, J. K.; de Groot, F. M. F.; Schoenlein, R. W. Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy. J. Am. Chem. Soc. 2010, 132, 6809–6816. Bressler, C.; Chergui, M.; Pattison, P.; Wulff, M.; Filipponi, A.; Abela, R. A Laser and Synchrotron Radiation Pump-Probe X-Ray Absorption Experiment with Sub-ns Resolution. Time Struct. X-Ray Sources Its Appl. 1998, 3451, 108–116. Bressler, C.; Saes, M.; Chergui, M.; Abela, R.; Pattison, P. Optimizing a Time-Resolved X-Ray Absorption Experiment. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 467, 1444–1446. Bressler, C.; Chergui, M. Ultrafast X-Ray Absorption Spectroscopy. Chem. Rev. 2004, 104, 1781–1812. Bressler, C.; Chergui, M. Time-Resolved X-Ray Absorption Spectroscopy. Actual. Chim. 2008, 317, 59–61.


(57)

(58)

(59)

326

Soldatov, A. V.; Kravtsova, A. N.; Fedorovich, E. N.; Ankudinov, A.; Moewes, A.; Kurmaev, E. Z. Analysis of the Electronic Structure of Human Hemoglobin from Soft X-ray Emission. J. Electron Spectrosc. 2005, 144, 279–282. Geloni, G.; Saldin, E.; Samoylova, L.; Schneidmiller, E.; Sinn, H.; Tschentscher, T.; Yurkov, M. Coherence Properties of the European XFEL. New J. Phys. 2010, 12, 035021. Maurus, R.; Overall, C. M.; Bogumil, R.; Luo, Y.; Mauk, A. G.; Smith, M.; Brayer, G. D. A Myoglobin Variant with a Polar Substitution in a Conserved Hydrophobic Cluster in the Heme Binding Pocket. Biochim. Biophys. Acta 1997, 1341, 1–13.


X-ray Spectroscopies Revealing the Structure and Dynamics of

Recommend Documents