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Kimoto K, Satoh T, Iwamura M, Nozaki K, Horikoshi T, Suzuki S, Kozaki M, Okada K. Very Long-Lived Photoinduced Charge-Separated States of Triphenylamine–Naphthalenediimide Dyads in Polymer Matrices. J Phys Chem A 2016; 120:8093-8103. [DOI: 10.1021/acs.jpca.6b07705] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Kenshi Kimoto
- Department
of Chemistry, Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
| | - Tsubasa Satoh
- Department
of Chemistry, Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
| | - Munetaka Iwamura
- Department
of Chemistry, Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
| | - Koichi Nozaki
- Department
of Chemistry, Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
| | - Takafumi Horikoshi
- Department
of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
| | - Shuichi Suzuki
- Department
of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
| | - Masatoshi Kozaki
- Department
of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
| | - Keiji Okada
- Department
of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
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2
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Sensitization of ultra-long-range excited-state electron transfer by energy transfer in a polymerized film. Proc Natl Acad Sci U S A 2012; 109:15132-5. [PMID: 22949698 DOI: 10.1073/pnas.1213646109] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Distance-dependent energy transfer occurs from the Metal-to-Ligand Charge Transfer (MLCT) excited state Ru(bpy)3(2+*) to an anthracene-acrylate derivative (Acr-An) incorporated into the polymer network of a semirigid poly(ethyleneglycol)dimethacrylate monolith. Following excitation, Ru(bpy)3(2+*) to Acr-An triplet energy transfer occurs followed by long-range, Acr-(3)An-Acr-An → Acr-An-Acr-(3)An, energy migration. With methyl viologen dication (MV(2+)) added as a trap, Acr-(3)An + MV(2+) → Acr-An(+) + MV(+) electron transfer results in sensitized electron transfer quenching over a distance of approximately 90 Å.
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3
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Structural and dynamic aspects of electron transfer in proteins — highly organized natural nanostructures. Russ Chem Bull 2012. [DOI: 10.1007/s11172-011-0199-3] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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4
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Gray HB, Winkler JR. Electron flow through metalloproteins. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2010; 1797:1563-72. [PMID: 20460102 DOI: 10.1016/j.bbabio.2010.05.001] [Citation(s) in RCA: 183] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/10/2010] [Revised: 04/21/2010] [Accepted: 05/03/2010] [Indexed: 01/23/2023]
Abstract
Electron transfers in photosynthesis and respiration commonly occur between metal-containing cofactors that are separated by large molecular distances. Understanding the underlying physics and chemistry of these biological electron transfer processes is the goal of much of the work in our laboratories. Employing laser flash-quench triggering methods, we have shown that 20A, coupling-limited Fe(II) to Ru(III) and Cu(I) to Ru(III) electron tunneling in Ru-modified cytochromes and blue copper proteins can occur on the microsecond timescale both in solutions and crystals; and, further, that analysis of these rates suggests that distant donor-acceptor electronic couplings are mediated by a combination of sigma and hydrogen bonds in folded polypeptide structures. Redox equivalents can be transferred even longer distances by multistep tunneling, often called hopping, through intervening amino acid side chains. In recent work, we have found that 20A hole hopping through an intervening tryptophan is several hundred-fold faster than single-step electron tunneling in a Re-modified blue copper protein.
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Affiliation(s)
- Harry B Gray
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.
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5
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Weik M, Colletier JP. Temperature-dependent macromolecular X-ray crystallography. ACTA CRYSTALLOGRAPHICA SECTION D: BIOLOGICAL CRYSTALLOGRAPHY 2010; 66:437-46. [PMID: 20382997 PMCID: PMC2852308 DOI: 10.1107/s0907444910002702] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/09/2009] [Accepted: 01/21/2010] [Indexed: 11/10/2022]
Abstract
X-ray crystallography provides structural details of biological macromolecules. Whereas routine data are collected close to 100 K in order to mitigate radiation damage, more exotic temperature-controlled experiments in a broader temperature range from 15 K to room temperature can provide both dynamical and structural insights. Here, the dynamical behaviour of crystalline macromolecules and their surrounding solvent as a function of cryo-temperature is reviewed. Experimental strategies of kinetic crystallography are discussed that have allowed the generation and trapping of macromolecular intermediate states by combining reaction initiation in the crystalline state with appropriate temperature profiles. A particular focus is on recruiting X-ray-induced changes for reaction initiation, thus unveiling useful aspects of radiation damage, which otherwise has to be minimized in macromolecular crystallography.
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Affiliation(s)
- Martin Weik
- CEA, IBS, Laboratoire de Biophysique Moléculaire, F-38054 Grenoble, France.
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6
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Ghorai PK, Matyushov DV. Solvent reorganization of electron transitions in viscous solvents. J Chem Phys 2006; 124:144510. [PMID: 16626217 DOI: 10.1063/1.2185102] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
We develop a model of electron transfer reactions at conditions of nonergodicity when the time of solvent relaxation crosses the observation time window set up by the reaction rate. Solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile is studied by molecular dynamics simulations at varying temperatures. We observe a sharp decrease of the reorganization energy at a temperature identified as the temperature of structural arrest due to cage effect, as discussed by the mode-coupling theory. This temperature also marks the onset of the enhancement of translational diffusion relative to rotational relaxation signaling the breakdown of the Stokes-Einstein relation. The change in the reorganization energy at the transition temperature reflects the dynamical arrest of the slow, collective relaxation of the solvent related to the relaxation of the solvent dipolar polarization. An analytical theory proposed to describe this effect agrees well with both the simulations and experimental Stokes shift data. The theory is applied to the analysis of charge-transfer kinetics in a low-temperature glass former. We show that the reorganization energy is substantially lower than its equilibrium value for the low-temperature portion of the data. The theory predicts the possibility of discontinuous changes in the dependence of the electron transfer rate on the free energy gap when the reaction switches between ergodic and nonergodic regimes.
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Affiliation(s)
- Pradip K Ghorai
- Department of Chemistry, Arizona State University, Tempe, Arizona 85287-1604, USA
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7
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Sakamoto M, Tachikawa T, Fujitsuka M, Majima T. Two-color two-laser fabrication of gold nanoparticles in a PVA film. Chem Phys Lett 2006. [DOI: 10.1016/j.cplett.2005.12.053] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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8
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Ghorai PK, Matyushov DV. Reorganization Energy of Electron Transfer in Viscous Solvents above the Glass Transition. J Phys Chem B 2006; 110:1866-71. [PMID: 16471757 DOI: 10.1021/jp055235h] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We present a molecular-dynamics study of the solvent reorganization energy of electron transfer in supercooled water. We observe a sharp decrease of the reorganization energy at a temperature identified as the temperature of structural arrest due to cage effect as discussed by the mode coupling theory. Both the heat capacity and dielectric susceptibility of the pure water show sharp drops at about the same temperature. This temperature also marks the onset of the enhancement of translational diffusion relative to rotational relaxation signaling the breakdown of the Stokes-Einstein relation. The change in the reorganization energy at the transition temperature reflects the dynamical arrest of the slow, collective relaxation of the solvent related to Debye relaxation of the solvent dipolar polarization.
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Affiliation(s)
- Pradip K Ghorai
- Department of Chemistry and Biochemistry and the Center for the Early Events in Photosynthesis, Arizona State University, P.O. Box 871604, Tempe, Arizona 85287-1604, USA
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9
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Ghorai PK, Matyushov DV. Dynamical Arrest of Electron Transfer Reorganization in Super-Cooled Water. J Am Chem Soc 2005; 127:16390-1. [PMID: 16305214 DOI: 10.1021/ja0535920] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
This paper reports the results of Molecular Dynamics (MD) simulations of the solvent reorganization energy of electron transfer (ET) reactions in low-temperature solvents. Simulations are carried out for a model charge-transfer optical dye (p-nitroaniline) in SPC/E water in a range of temperatures down to the point of solvent ideal glass transition. We show a significant departure of the solvent reorganization energy, measured on a given time window, from its thermodynamic limit obtained by averaging over long simulation trajectories. Our results thus indicate that optical solvatochromism and activation parameters of ET reactions measured in viscous solvents will be significantly affected by the dynamical arrest of nuclear solvation when the experimental time scale becomes comparable to the characteristic relaxation time of the solvent.
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Affiliation(s)
- Pradip K Ghorai
- Department of Chemistry and Biochemistry and the Center for the Study of Early Events in Photosynthesis, Arizona State University, PO Box 871604, Tempe, Arizona 85287-1604, USA
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10
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Benten H, Ohkita H, Ito S, Yamamoto M, Tohda Y, Tani K. Photoinduced intramolecular charge separation in a polymer solid below the glass transition temperature. J Chem Phys 2005; 123:084901. [PMID: 16164324 DOI: 10.1063/1.2008249] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Photoinduced intramolecular charge separation (CS) in a polar polymer glass, cyanoethylated pullulan (CN-PUL), was studied below the glass transition temperature (Tg=395 K). A series of three carbazole (Cz: donor)-cyclohexane (S: spacer)-acceptor (A: acceptor) molecules (Cz-S-A) was used as intramolecular donor-acceptor dyads. The photoinduced CS rate was evaluated by the fluorescence decay measurement at temperatures from 100 to 400 K. The CS rate (kCS) increased above 200 K even far below Tg where micro-Brownian motions of the whole polymer chain are frozen. Below 200 K, on the other hand, kCS showed weak dependence on temperature. The temperature dependence of kCS is discussed in terms of the dielectric relaxation time of the polymer matrix. Consequently, CS below Tg was well explained by a thermally nonequilibrium electron transfer (ET) formula above 200 K and by a two-mode quantum-mechanical ET formula below 200 K. The increase in kCS above 200 K is mainly caused by a thermally activated low-frequency matrix mode originating from the side-chain relaxation of polar cyano groups. The weak temperature dependence of kCS can be explained by a nuclear-tunneling effect caused by a high-frequency matrix mode (variant Planck's over 2piomegH=250 cm-1) and an intramolecular vibrational mode (variant Planck's over 2piomegaQ=1300 cm-1). The high-frequency mode of the polymer matrix was attributed to a vibrational or librational motion of polar groups in the CN-PUL glassy solid.
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Affiliation(s)
- Hiroaki Benten
- Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan
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11
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Abstract
Recent investigations have shed much light on the nuclear and electronic factors that control the rates of long-range electron tunneling through molecules in aqueous and organic glasses as well as through bonds in donor-bridge-acceptor complexes. Couplings through covalent and hydrogen bonds are much stronger than those across van der Waals gaps, and these differences in coupling between bonded and nonbonded atoms account for the dependence of tunneling rates on the structure of the media between redox sites in Ru-modified proteins and protein-protein complexes.
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Affiliation(s)
- Harry B Gray
- Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.
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12
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Benten H, Ohkita H, Ito S, Yamamoto M, Tohda Y, Tani K. Photoinduced Intramolecular Charge Separation in Poly(alkyl methacrylate) Solids Studied by Using a Fluorescent Polarity Probe. J Phys Chem B 2004. [DOI: 10.1021/jp047354n] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Affiliation(s)
| | | | | | - Masahide Yamamoto
- Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
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13
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Moran AM, Spears KG. The effect of diffusive solvent relaxation on ultrafast electron transfer within the methyl viologen-hexacyanoferrate complex in trehalose–water glass. Chem Phys Lett 2004. [DOI: 10.1016/j.cplett.2004.06.057] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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14
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Ohkita H, Benten H, Anada A, Noguchi H, Kido N, Ito S, Yamamoto M. Photoinduced electron transfer of carbazole–acceptor dyads in solution and in a polymer solid. Phys Chem Chem Phys 2004. [DOI: 10.1039/b401951b] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
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15
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Srivatsan N, Kolbasov D, Ponomarenko N, Weber S, Ostafin AE, Norris JR. Cryogenic Charge Transport in Oxidized Purple Bacterial Light-Harvesting 1 Complexes. J Phys Chem B 2003; 107:7867-76. [DOI: 10.1021/jp027736y] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
| | - Dmitri Kolbasov
- Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
| | - Nina Ponomarenko
- Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
| | - Stefan Weber
- Institute of Experimental Physics, Free University Berlin, Arnimallee 14, 14195 Berlin, Germany
| | - Agnes E. Ostafin
- Department of Chemical Engineering, University of Notre Dame, 182 Fitzpatrick, Notre Dame, Indiana 46556
| | - James R. Norris
- Department of Chemistry and Institute of Biodynamics, University of Chicago, 5735 South Ellis Avenue,
Chicago, Illinois 60637
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16
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Srivatsan N, Weber S, Kolbasov D, Norris JR. Exploring Charge Migration in Light-Harvesting Complexes Using Electron Paramagnetic Resonance Line Narrowing. J Phys Chem B 2003. [DOI: 10.1021/jp0255233] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Nagarajan Srivatsan
- Division of Chemistry and Chemical Engineering, Mail Stop 127-72, California Institute of Technology, Pasadena, California 91125
| | - Stefan Weber
- Institute of Experimental Physics, Free University Berlin, Arnimallee 14, 14195 Berlin, Germany
| | - Dmitri Kolbasov
- Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
| | - James R. Norris
- Department of Chemistry and Institute of Biodynamics, University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
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17
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Kotelnikov AI, Ortega JM, Medvedev ES, Psikha BL, Garcia D, Mathis P. Effect of protein relaxation on electron transfer from the cytochrome subunit to the bacteriochlorophyll dimer in Rps. sulfoviridis reaction centers within mixed adiabatic/nonadiabatic model. Bioelectrochemistry 2002; 56:3-8. [PMID: 12009432 DOI: 10.1016/s1567-5394(02)00036-1] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The broad set of nonexponential electron transfer (ET) kinetics in reaction centers (RC) from Rhodopseudomonas sulfoviridis in temperature range 297-40 K are described within a mixed adiabatic/nonadiabatic model. The key point of the model is the combination of Sumi-Marcus and Rips-Jortner approaches which can be represented by the separate contributions of temperature-independent vibrational (v) and temperature-dependent diffusive (d) coordinates to the preexponential factor, to the free energy of reaction DeltaG=DeltaG(v)+DeltaG(d)(T) and to the reorganization energy lambda=lambda(v)+lambda(d)(T). The broad distribution of protein dielectric relaxation times along the diffusive coordinate is considered within the Davidson-Cole formalism.
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Affiliation(s)
- A I Kotelnikov
- Institute of Problems of Chemical Physics RAS, Chernogolovka, 142432 Moscow region, Russia.
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18
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Srivatsan N, Norris JR. Electron Paramagnetic Resonance Study of Oxidized B820 Complexes. J Phys Chem B 2001. [DOI: 10.1021/jp011676z] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Nagarajan Srivatsan
- Division of Chemistry and Chemical Engineering, Mail Stop 127-72, California Institute of Technology, Pasadena, California 91125
| | - James R. Norris
- Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637
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19
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Weik M, Ravelli RB, Silman I, Sussman JL, Gros P, Kroon J. Specific protein dynamics near the solvent glass transition assayed by radiation-induced structural changes. Protein Sci 2001; 10:1953-61. [PMID: 11567086 PMCID: PMC2374210 DOI: 10.1110/ps.09801] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2001] [Revised: 06/22/2001] [Accepted: 07/03/2001] [Indexed: 10/17/2022]
Abstract
The nature of the dynamical coupling between a protein and its surrounding solvent is an important, yet open issue. Here we used temperature-dependent protein crystallography to study structural alterations that arise in the enzyme acetylcholinesterase upon X-ray irradiation at two temperatures: below and above the glass transition of the crystal solvent. A buried disulfide bond, a buried cysteine, and solvent exposed methionine residues show drastically increased radiation damage at 155 K, in comparison to 100 K. Additionally, the irradiation-induced unit cell volume increase is linear at 100 K, but not at 155 K, which is attributed to the increased solvent mobility at 155 K. Most importantly, we observed conformational changes in the catalytic triad at the active site at 155 K but not at 100 K. These changes lead to an inactive catalytic triad conformation and represent, therefore, the observation of radiation-inactivation of an enzyme at the atomic level. Our results show that at 155 K, the protein has acquired--at least locally--sufficient conformational flexibility to adapt to irradiation-induced alterations in the conformational energy landscape. The increased protein flexibility may be a direct consequence of the solvent glass transition, which expresses as dynamical changes in the enzyme's environment. Our results reveal the importance of protein and solvent dynamics in specific radiation damage to biological macromolecules, which in turn can serve as a tool to study protein flexibility and its relation to changes in a protein's environment.
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Affiliation(s)
- M Weik
- Department of Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, 3584 CH Utrecht, The Netherlands.
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20
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Tezcan FA, Crane BR, Winkler JR, Gray HB. Electron tunneling in protein crystals. Proc Natl Acad Sci U S A 2001; 98:5002-6. [PMID: 11296248 PMCID: PMC33153 DOI: 10.1073/pnas.081072898] [Citation(s) in RCA: 104] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 02/13/2001] [Indexed: 11/18/2022] Open
Abstract
The current understanding of electron tunneling through proteins has come from work on systems where donors and acceptors are held at fixed distances and orientations. The factors that control electron flow between proteins are less well understood, owing to uncertainties in the relative orientations and structures of the reactants during the very short time that tunneling occurs. As we report here, the way around such structural ambiguity is to examine oxidation-reduction reactions in protein crystals. Accordingly, we have measured and analyzed the kinetics of electron transfer between native and Zn-substituted tuna cytochrome c (cyt c) molecules in crystals of known structure. Electron transfer rates [(320 s(-1) for *Zn-cyt c --> Fe(III)-cyt c; 2000 s(-1) for Fe(II)-cyt c --> Zn-cyt c(+))] over a Zn-Fe distance of 24.1 A closely match those for intraprotein electron tunneling over similar donor-acceptor separations. Our results indicate that van der Waals interactions and water-mediated hydrogen bonds are effective coupling elements for tunneling across a protein-protein interface.
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Affiliation(s)
- F A Tezcan
- Beckman Institute, MC 139-74, California Institute of Technology, Pasadena, CA 91125, USA
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21
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Ackers GK, Holt JM, Huang Y, Grinkova Y, Klinger AL, Denisov I. Confirmation of a unique intra-dimer cooperativity in the human hemoglobin alpha(1)beta(1)half-oxygenated intermediate supports the symmetry rule model of allosteric regulation. Proteins 2001; Suppl 4:23-43. [PMID: 11013398 DOI: 10.1002/1097-0134(2000)41:4+<23::aid-prot30>3.0.co;2-d] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
The contribution of the alpha(1)beta(1)half-oxygenated tetramer [alphabeta:alphaO(2)betaO(2)] (species 21) to human hemoglobin cooperativity was evaluated using cryogenic isoelectric focusing. The cooperative free energy of binding, reflecting O(2)-driven protein structure changes, was measured as (21)DeltaG(c) = 5.1 +/- 0. 3 kcal for the Zn/FeO(2) analog. For the Fe/FeCN analog, (21)DeltaG(c) was estimated as 4.0 kcal after correction for a CN ligand rearrangement artifact, demonstrating that ligand rearrangement does not invalidate previous conclusions regarding this species. In the context of the entire Hb cooperativity cascade, which includes eight intermediate species, the 21 tetramer is highly abundant relative to the other doubly-ligated species, providing strong support for the previously determined consensus partition function of O(2) binding and for the Symmetry Rule model of hemoglobin cooperativity (Ackers et al., Science 1992;255:54-63). Cooperativity of normal human hemoglobin is shown to depend on site-configuration, and not solely the number of O(2) bound, nor the occupancy of alpha vs. beta subunits. Verification of a unique contribution from the alpha(1)beta(1)doubly-oxygenated species to the equilibrium O(2) binding curve strongly reinforces the Symmetry Rule interpretation that the alpha(1)beta(1)dimer acts both as a structural and functional element in cooperative O(2) binding.
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Affiliation(s)
- G K Ackers
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
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