ElectronâPhonon Coupling in Cyanobacterial Photosystem I - The

Subscriber access provided by University of South Dakota

B: Biophysics; Physical Chemistry of Biological Systems and Biomolecules

Electron-Phonon Coupling in Cyanobacterial Photosystem I Dmitry A. Cherepanov, Georgy E. Milanovsky, Oksana A. Gopta, Ramakrishnan Balasubramanian, Donald A. Bryant, Alexey Yu. Semenov, and John H. Golbeck J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b03906 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Electron-Phonon Coupling in Cyanobacterial Photosystem I Dmitry A. Cherepanov,†,∥,* Georgy E. Milanovsky,†,1 Oksana A. Gopta,†,‡,1,2,3 Ramakrishnan Balasubramanian,‡,3 Donald A. Bryant,‡,§ Alexey Yu. Semenov,†,∥,* and John H. Golbeck‡,⊥,* †

A.N. Belozersky Institute of Physical-Chemical Biology, Moscow State University, Russia,

119992, Moscow, Leninskye gory, 1, building 40; ‡

Department of Biochemistry and Molecular Biology, The Pennsylvania State University,

University Park, PA, 16802, USA, 328 Frear Laboratory; §

Department of Chemistry and Biochemistry, Montana State University, PO Box 173400,

Bozeman, MT, 59717, USA, 103 Chemistry and Biochemistry Building; ∥N.N.

Semenov Institute of Chemical Physics, Russian Academy of Sciences, Russia,

117977, Moscow, Kosygina st., 4; ⊥Department

of Chemistry, The Pennsylvania State University, University Park, PA, 16802,

USA, 328 Frear Laboratory KEYWORDS: electron transfer, photosystem I, electron-phonon coupling, temperature dependence.

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 55

ABSTRACT. One of the fundamental problems in biophysics is whether the protein medium at room temperature can be properly treated as a fluid dielectric or whether its dynamics is determined by a highly ordered molecular structure resembling the properties of crystalline and amorphous solids. Here, we measured the recombination between reduced A1 and the oxidized chlorophyll special pair P700 over a wide temperature range using preparations of Photosystem I from the cyanobacterium Synechococcus sp. PCC 7002 depleted of the ironsulfur clusters. We found that the dielectric properties of the protein matrix in early electron transfer reactions of Photosystem I resemble the behavior of solids that require an implicit treatment of electron-phonon coupling even at ambient temperatures. The quantum effects of electron-phonon coupling in proteins could account for a variety of phenomena, such as the weak sensitivity of electron transfer in pigment-protein complexes to changing environmental conditions including temperature, driving force, polarity, and chemical composition.

Introduction One of the fundamental problems in biological electron transport theory is whether the protein medium can be properly treated at room temperature as a fluid dielectric or whether its dynamics is determined by a highly ordered molecular structure resembling the properties of crystalline and amorphous solids. Large photosynthetic pigment-protein complexes, in which many electron transfer (ET) cofactors are located at some distance from the proteinwater interface, represent an appropriate experimental system to address this question. A theoretical analysis of ET reactions in pigment-protein complexes is generally based on two complementary approaches. The first quantitative treatment of redox reactions in solutions was advanced in 1956 by Marcus, who analyzed the coupling of electronic transitions with liquid polarization in the high-temperature limit.1 At the same time, Kubo and Toyozawa developed a quantum nonadiabatic theory of multiphonon transitions in crystalline and amorphous solids2, which was later expanded by Dogonadze and Levich to ET reactions in


2

Page 3 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


condensed phases.3 In this approach, nuclear dynamics are rationalized as a set of "librations", i.e. the translational and rotational harmonic modes that are coupled to cofactor recharging. The ET rate W can be calculated using Fermi’s golden rule:

Wa → b =

2π 2 V ⋅ ∑ pm ρ ( E m ) h m

[1]

Here, V is the electronic coupling between weakly interacting cofactors; the sum over initial quantum states |am〉 is known as the Franck-Condon factor (FC); pm is the Boltzmann weight of |am〉 in the thermal population; ρ(Em) is the density of vibronic transitions per unit of energy:

ρ ( Em ) =

E bn − E am < ∆E

∑

f am , bn

2

∆E

[2]

n

where fam,bn is the vibrational overlap integral between the nuclear wave functions of the initial |am〉 and final |bn〉 states, Eam and Ebn represent the energies of vibronic states, and ∆E is a small energy gap. The nonadiabatic approximation assumes that the electronic coupling of cofactors is small, with the consequence that electron tunneling is possible only in nuclear conformations in which the energies of initial and final states are equal. Polarization dynamics is a combination of different conformational motions for which a detailed description at the quantum level is unrealistic. For the sake of simplicity, a large multitude of fast librations was aggregated by Jortner into an "effective" phonon mode of high self-frequency ωf, whereas slower medium dynamics were approximated by a single low-frequency mode ωs.4 Using this two-mode approximation and assuming that ∆E in Eq. 2 can be taken equal to ħωs (the “coarse grained” approximation), the rate of ET can be expressed as a compact row of modified Bessel functions.4 Several closed-form expressions for the Franck-Condon sums in high- and low-temperature asymptotic regimes in two- and three-mode approximations are considered in a detailed review.5 Parson and Warshel set apart five low-frequency harmonic modes in the power spectrum obtained by molecular


3


Page 4 of 55

dynamics (MD) and calculated the rate of primary ET reactions in the bacterial reaction center (bRC) by numeric evaluation of the Franck-Condon factors fam,bn.6 The implementations of the quantum mechanical approach to ET reactions in biological and chemical systems can be found in recent reviews.7,8 The classical treatment of ET reactions in liquid medium was derived from multiphonon theory as a high-temperature limit, when the aggregate frequency of solvent polarization ωs is much lower than kBT/ħ.9 In this approximation, the rate of the ET reaction is determined by the energy parameters of the system: the free energy change ∆G, the reorganization energy λ, the energy of thermal fluctuations kBT, and the energy of electronic coupling of cofactors V: = 2ℏ ∙

||

4

∙ exp −

Δ + " # [3] 4

This equation has been applied extensively to various proteins in which redox transitions occur at ambient temperature near the polar protein-water interface.10 In their pioneering work, DeVault and Chance showed that the rate of cytochrome c oxidation in bRC becomes constant at temperatures below 100 K.11 Most of the ET reactions in the central part of the bRC

from

Rhodobacter

sphaeroides

are

activationless:

the

forward

reactions

P860*HA→P860+HA– and HA–QA→HAQA– 12,13 and the backreaction P860+QA–→P860QA 14 do not slow on freezing to liquid helium temperatures. A systematic investigation of electronphonon coupling in the bRC was undertaken by Dutton and coworkers, who found that the rate of the backreaction P860+QA–→P860QA with chemically modified quinones in the QA site was nearly independent of −∆G in the interval of 0.5-0.7 eV at temperatures from 5 K to 113 K.15 The authors rationalized this behavior by employing the two-mode approximation by Jortner4 and concluded that ET in the bRC is governed by nuclear tunneling at low temperatures.16 An alternative way to change the driving force of the backreaction P860+QA– →P860QA was achieved by point amino acid substitutions in the vicinity of the primary donor P860, where −∆G varied in the interval of 0.4-0.8 eV 17. Although the inverted region was not


4

Page 5 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reached, the data demonstrated a deviation from the behavior predicted by Eq. 3 at room temperature: a high-frequency mode (>1000 cm-1) was required to describe quantitatively the dependency of the ET rate on the driving force within Jortner’s model. The primary forward ET reactions in the bRC also remain activationless after alteration of the redox potential of the P860 bacteriochlorophyll special pair by point mutations.18 A similar approach was used for the analysis of the charge recombination in Photosystem I (PS I) reaction centers.19,20 Any experimental study of electron-phonon coupling of ET reactions with chemically modified pigment-protein complexes faces a number of methodological difficulties. (i) Because the quinone derivatives have different sizes, chemical structures, and polarities, their binding in the active sites may differ from the binding of native quinones. (ii) In bRC complexes, a conformational transition occurs at ~150 K that affects the rates of all observed ET reactions.15 (iii) The recombination reaction in the modified bRC with low-potential quinones in the QA site proceeds at ambient temperatures indirectly via a thermally activated intermediate state (probably, HA–). For these reasons, analysis of ET reactions in the bRC has been limited to the relatively narrow temperature interval from 5 K to 113 K.15 PS I is a large pigment-protein complex whose overall structural motif is generally similar to the bRC. The six chlorophyll (Chl) and two phylloquinone (PhQ) electron transfer cofactors are arranged in two branches, A and B, that are related by 2-fold pseudosymmetry. Contrary to the bRC, both branches of cofactors participate in ET.21 PS I from the rubA mutant of Synechococcus sp. PCC 7002 lacks 95% of the FX iron-sulfur cluster and 100% of the terminal FA and FB iron-sulfur clusters.22 Two kinetic components of the charge recombination reactions A1B–→P700 and A1A–→P700 with lifetimes of ~15 µs and ~100 µs, respectively, have been observed at room temperature in PS I complexes from this mutant.23 Similar to ET reactions in the bRC, the rate slightly depended on the magnitude of the driving force when −∆G was varied in the range from 0.8 to 1.1 eV.20


5


Page 6 of 55

In this work, we measured and quantitatively analyzed the temperature dependence of the two charge recombination reactions A1B–→P700+ and A1A–→P700+ in PS I core complexes isolated from the rubA mutant of Synechococcus sp. PCC 7002. In contrast to previous studies on bRCs that were performed within a limited temperature range, we report for the first time the kinetics of ET reactions over a wide temperature range of 5 K to 300 K. Combining the results of low-temperature measurements, molecular dynamics simulations and quantum multiphonon analysis, we show that the above ET reactions in PS I are significantly controlled by nuclear tunneling even at ambient temperatures and therefore should be treated in terms of quantum electron-phonon interactions. Our results indicate that the physical properties of proteins resemble more than superficially the properties of structurally ordered amorphous bodies rather than a structurally unorganized liquid bath. Materials and Methods Cell Growth and Isolation of Thylakoids and PS I Trimers. Conditions for the growth of the rubA mutant strains of Synechococcus sp. PCC 7002, the biochemical and physiological properties of the strains employed in the studies reported here, and the preparation of trimeric PS I fractions are described in detail in ref. 22. Isolation of Thylakoids and PS I Trimers. Thylakoid membranes were prepared in buffer A (50 mM MES-NaOH, pH 6.5, 0.4 M sucrose, 5% (w/v) glycerol, 5 mM CaCl2, 5 mM MgCl2, 10 mM NaCl) at a concentration of 0.15 mg Chl ml-1. The trimeric PS I complexes were collected, resuspended in 50 mM Tris, pH 8.0, containing 0.03% (w/v) n-dodecyl-β-Dmaltopyranoside (DM) and 15% (v/v) glycerol at a Chl concentration of 1.5–2 mg ml-1, and stored at -80 °C until required. Flash-induced transient absorption spectroscopy in the near-IR region. Measurements were made with a laboratory-built spectrometer at 820 nm on PS I complexes from the rubA mutant.24 The transient absorption kinetics of PS I cores isolated from the rubA mutant were


6

Page 7 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


measured in the temperature range from 5 K to 300 K. PS I complexes were suspended in ca. 75% (v/v) glycerol/water mixture to ensure the formation of a transparent glass at low temperatures. Two independent sets of measurements were made of PS I from the same rubA mutant to provide a mean and standard deviation of the rates of recombination. Low-temperature measurements. For low-temperature measurements of the transient absorption kinetics, PS I was diluted in a buffer containing 50mM Tris-HCl pH 8.0, 0.04% βdodecyl maltoside and ca. 75% (v/v) glycerol. Glycerol was added to ensure a transparent glass formation of the sample at low temperatures. The glass transition properties of the glycerol-water mixture used in our experiments have been previously characterized.22,25 Ascorbate was added to a final concentration of 0.5 mM. Temperature-dependence experiments were performed in a polycarbonate cuvette that was modified and sealed to fit the sample cryostat (Janis, MN). Liquid helium was used to cool the samples and the temperature was varied using a Lakeshore auto-sensor temperature controller. A diode laser at 820 nm (5 mW output power) was used as the measuring beam and the samples were excited by a 1 mJ/cm2 energy pulse from a Nd-YAG laser (Spectra physics, CA) operating at 532 nm with a 7 ns pulse duration. CONTIN analysis of PS I charge recombination kinetics. Recombination kinetics of PS I were discretized as arrays yt (t=t1,…,tM) and analyzed with CONTIN software.26 The method implements the inverse Laplace transform to deconvolute monotonous decay kinetics yt into a spectrum of exponential components with the characteristic time τk (k=1,…,Nτ) evenlyspaced in the logarithmic timescale. CONTIN employs Tikhonov-Phillips regularization27, which minimizes the sum of the squares of the discretized second derivatives of the solution, resulting in a quasi-continuous spectrum with the local smoothness determined by the regularizing parameter α. Namely, CONTIN suggests an array of solutions Fν (ν=1,…,N)


7


Page 8 of 55

() = * +,,. ∙ / 0/23 [4] .

where aν,k is the amplitude of exponential component exp(-t/τk) obtained at the given value αν of the regularizing parameter. The residuals of solution ν is 4,,0 = 50 − * +,,. ∙ / 0/23 [5] .

In the absence of prior information regarding the shape of kinetic spectrum, e.g. the number of its principal components, CONTIN suggests an “optimal” solution with the most statistically reliable α value. However, in the case of current study, an additional criterion was used to select the appropriate α value, namely, recombination kinetics at a given temperature interval Tn ± 5 K (n=1,…,NT) should have the minimal dispersion of the spectral profiles. The vector of optimal solutions [ν ν]=(ν1,…, νN) was found thereby by minimizing the discrepancy function

7[,] = * 8,9 + : ∙ *;+,9,. − +

ElectronâPhonon Coupling in Cyanobacterial Photosystem I - The

Recommend Documents

ElectronâPhonon Coupling in Cyanobacterial Photosystem I - The