Water-Mediated Electron Transfer between Protein Redox Centers

Adenine Fine-Tunes DNA Photolyase's Repair Mechanism

The comparative study of DNA repair by mesophilic and extremophilic photolyases helps us understand the evolution of these enzymes and their role in preserving life on our changing planet. The mechanism of repair of cyclobutane pyrimidine dimer lesions in DNA by electron transfer from the flavin adenine dinucleotide cofactor is the subject of intense interest. The role of adenine in mediating this process remains unresolved. Using microsecond molecular dynamics simulations, we find that adenine mediates the electron transfer in both mesophile and extremophile DNA photolyases through a similar mechanism. In fact, in all photolyases studied, the molecular conformations with the largest electronic couplings between the enzyme cofactor and DNA show the presence of adenine in 10-20% of the strongest-coupling tunneling pathways between the atoms of the electron donor and acceptor. Our theoretical analysis finds that adenine serves the critical role of fine-tuning rather than maximizing the donor-acceptor coupling within the range appropriate for the repair function.

Mechanism of Side Chain-Controlled Proton Conductivity in Bioinspired Peptidic Nanostructures

Bioinspired peptide assemblies are promising candidates for use as proton-conducting materials in electrochemical devices and other advanced technologies. Progress toward applications requires establishing foundational structure-function relationships for transport in these materials. This experimental-theoretical study sheds light on how the molecular structure and proton conduction are linked in three synthetic cyclic peptide nanotube assemblies that comprise the three canonical basic amino acids (lysine, arginine, and histidine). Experiments find an order of magnitude higher proton conductivity for lysine-containing peptide assemblies compared to histidine and arginine containing assemblies. The simulations indicate that, upon peptide assembly, the basic amino acid side chains are close enough to enable direct proton transfer. The proton transfer kinetics is determined in the simulations to be governed by the structure and flexibility of the side chains. Together, experiments and theory indicate that the proton mobility is the main determinant of proton conductivity, critical for the performance of peptide-based devices.

Conductance and configuration of molecular gold-water-gold junctions under electric fields

Water molecules can mediate charge transfer in biological and chemical reactions by forming electronic coupling pathways. Understanding the mechanism requires a molecular-level electrical characterization of water. Here, we describe the measurement of single water molecular conductance at room temperature, characterize the structure of water molecules using infrared spectroscopy, and perform theoretical studies to assist in the interpretation of the experimental data. The study reveals two distinct states of water, corresponding to a parallel and perpendicular orientation of the molecules. Water molecules switch from parallel to perpendicular orientations on applying an electric field, producing switching from high to low conductance states, thus enabling the determination of single water molecular dipole moments. The work further shows that water-water interactions affect the atomic scale configuration and conductance of water molecules. These findings demonstrate the importance of the discrete nature of water molecules in electron transfer and set limits on water-mediated electron transfer rates.

Mechanically Controlled Electron Transfer in a Single-Polypeptide Transistor

Proteins are of interest in nano-bio electronic devices due to their versatile structures, exquisite functionality and specificity. However, quantum transport measurements produce conflicting results due to technical limitations whereby it is difficult to precisely determine molecular orientation, the nature of the moieties, the presence of the surroundings and the temperature; in such circumstances a better understanding of the protein electron transfer (ET) pathway and the mechanism remains a considerable challenge. Here, we report an approach to mechanically drive polypeptide flip-flop motion to achieve a logic gate with ON and OFF states during protein ET. We have calculated the transmission spectra of the peptide-based molecular junctions and observed the hallmarks of electrical current and conductance. The results indicate that peptide ET follows an NC asymmetric process and depends on the amino acid chirality and α-helical handedness. Electron transmission decreases as the number of water molecules increases, and the ET efficiency and its pathway depend on the type of water-bridged H-bonds. Our results provide a rational mechanism for peptide ET and new perspectives on polypeptides as potential candidates in logic nano devices.

The mechanism of water/ion exchange at a protein surface: a weakly bound chloride in Helicobacter pylori apoflavodoxin

Binding/unbinding of small ligands, such as ions, to/from proteins influences biochemical processes such as protein folding, enzyme catalysis or protein/ligand recognition. We have investigated the mechanism of chloride/water exchange at a protein surface (that of the apoflavodoxin from Helicobacter pylori) using classical all-atom molecular dynamics simulations. They reveal a variety of chloride exit routes and residence times; the latter is related to specific coordination modes of the anion. The role of solvent molecules in the mechanism of chloride unbinding has been studied in detail. We see no temporary increase in chloride coordination along the release process. Instead, the coordination of new water molecules takes place in most cases after the chloride/protein atom release event has begun. Moreover, the distribution function of water entrance events into the first chloride solvation shell peaks after chloride protein atom dissociation events. All these observations together seem to indicate that water molecules simply fill the vacancies left by the previously coordinating protein residues. We thus propose a step-by-step dissociation pathway in which protein/chloride interactions gradually break down before new water molecules progressively fill the vacant positions left by protein atoms. As observed for other systems, water molecules associated with bound chloride or with protein atoms have longer residence times than those bound to the free anion. The implications of the exchange mechanism proposed for the binding of the FMN (Flavin Mononucleotide) protein cofactor are discussed.

Progress and challenges in simulating and understanding electron transfer in proteins

This Review presents an overview of the most common numerical simulation approaches for the investigation of electron transfer (ET) in proteins. We try to highlight the merits of the different approaches but also the current limitations and challenges. The article is organized into three sections. Section 2 deals with direct simulation algorithms of charge migration in proteins. Section 3 summarizes the methods for testing the applicability of the Marcus theory for ET in proteins and for evaluating key thermodynamic quantities entering the reaction rates (reorganization energies and driving force). Recent studies interrogating the validity of the theory due to the presence of non-ergodic effects or of non-linear responses are also described. Section 4 focuses on the tunneling aspects of electron transfer. How can the electronic coupling between charge transfer states be evaluated by quantum chemistry approaches and rationalized? What interesting physics regarding the impact of protein dynamics on tunneling can be addressed? We will illustrate the different sections with examples taken from the literature to show what types of system are currently manageable with current methodologies. We also take care to recall what has been learned on the biophysics of ET within proteins thanks to the advent of atomistic simulations.

Direct Observation of Coupling between Structural Fluctuation and Ultrafast Hydration Dynamics of Fluorescent Probes in Anionic Micelles

The coupling of structural fluctuation and the dynamics of associated water molecules of biological macromolecules is vital for various biological activities. Although a number of molecular dynamics (MD) studies on proteins/DNA predicted the importance of such coupling, experimental evidence of variation of hydration dynamics with controlled structural fluctuation even in model macromolecule is sparse and raised controversies in the contemporary literature. Here, we have investigated dynamics of hydration at the surfaces of two similar anionic micelles sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS) as model macromolecules using coumarin 500 (C500) as spectroscopic probe with femtosecond to picosecond time resolution up to 20 ns time window. The constituting surfactants SDS and SDBS are structurally similar except one benzene moiety in the SDBS may offer additional rigidity to the SDBS micelles through π-stacking and added bulkiness. The structural integrity of the micelles in the aqueous medium is confirmed in dynamic light scattering (DLS) studies. A variety of studies including polarization gated fluorescence spectroscopy and quasielastic neutron scattering (QENS) have been used to confirm differential structural fluctuation of SDS and SDBS micelles. We have also employed femtosecond-resolved Förster resonance energy transfer (FRET) in order to study binding of a cationic organic ligand ethidium bromide (EtBr) salt at the micellar surfaces. The distance distribution of the donor (C500)-acceptor (EtBr) in the micellar media reveals the manifestation of the structural flexibility of the micelles. Our studies on dynamical coupling of the structural flexibility with surface hydration in the nanoscopic micellar media may find the relevance in the "master-slave" type water dynamics in biologically relevant macromolecules.

Electron transfer, decoherence, and protein dynamics: insights from atomistic simulations

Electron transfer in biological systems drives the processes of life. From cellular respiration to photosynthesis and enzymatic catalysis, electron transfers (ET) are chemical processes on which essential biological functions rely. Over the last 40 years, scientists have sought understanding of how these essential processes function in biology. One important breakthrough was the discovery that Marcus theory (MT) of electron transfer is applicable to biological systems. Chemists have experimentally collected both the reorganization energies (λ) and the driving forces (ΔG°), two parameters of Marcus theory, for a large variety of ET processes in proteins. At the same time, theoretical chemists have developed computational approaches that rely on molecular dynamics and quantum chemistry calculations to access numerical estimates of λ and ΔG°. Yet another crucial piece in determining the rate of an electron transfer is the electronic coupling between the initial and final electronic wave functions. This is an important prefactor in the nonadiabatic rate expression, since it reflects the probability that an electron tunnels from the electron donor to the acceptor through the intervening medium. The fact that a protein matrix supports electron tunneling much more efficiently than vacuum is now well documented, both experimentally and theoretically. Meanwhile, many chemists have provided examples of the rich physical chemistry that can be induced by protein dynamics. This Account describes our studies of the dynamical effects on electron tunneling. We present our analysis of two examples of natural biological systems through MD simulations and tunneling pathway analyses. Through these examples, we show that protein dynamics sustain efficient tunneling. Second, we introduce two time scales: τcoh and τFC. The former characterizes how fast the electronic coupling varies with nuclear vibrations (which cause dephasing). The latter reflects the time taken by the system to leave the crossing region. In the framework of open quantum systems, τFC is a short time approximation of the characteristic decoherence time of the electronic subsystem in interaction with its nuclear environment. The comparison of the respective values of τcoh and τFC allows us to probe the occurrence of non-Condon effects. We use ab initio MD simulations to analyze how decoherence appears in several biological cofactors. We conclude that we cannot account for its order of magnitude by considering only the atoms or bonds directly concerned with the transfer. Decoherence results from contributions from all atoms of the system appearing with a time delay that increases with the distance from the primarily concerned atoms or bonds. The delay and magnitude of the contributions depend on the chemical nature of the system. Finally, we present recent developments based on constrained DFT for efficient and accurate evaluations of the electronic coupling in ab initio MD simulations. These are promising methods to study the subtle fluctuations of the electronic coupling and the mechanisms of electronic decoherence in biological systems.

Charge transfer in dynamical biosystems, or the treachery of (static) images

The image is not the thing. Just as a pipe rendered in an oil painting cannot be smoked, quantum mechanical coupling pathways rendered on LCDs do not convey electrons. The aim of this Account is to examine some of our recent discoveries regarding biological electron transfer (ET) and transport mechanisms that emerge when one moves beyond treacherous static views to dynamical frameworks.

Studies over the last two decades introduced both atomistic detail and macromolecule dynamics to the description of biological ET. The first model to move beyond the structureless square-barrier tunneling description is the Pathway model, which predicts how protein secondary motifs and folding-induced through-bond and through-space tunneling gaps influence kinetics. Explicit electronic structure theory is applied routinely now to elucidate ET mechanisms, to capture pathway interferences, and to treat redox cofactor electronic structure effects. Importantly, structural sampling of proteins provides an understanding of how dynamics may change the mechanisms of biological ET, as ET rates are exponentially sensitive to structure. Does protein motion average out tunneling pathways? Do conformational fluctuations gate biological ET? Are transient multistate resonances produced by energy gap fluctuations? These questions are becoming accessible as the static view of biological ET recedes and dynamical viewpoints take center stage.

This Account introduces ET reactions at the core of bioenergetics, summarizes our team’s progress toward arriving at an atomistic-level description, examines how thermal fluctuations influence ET, presents metrics that characterize dynamical effects on ET, and discusses applications in very long (micrometer scale) bacterial nanowires. The persistence of structural effects on the ET rates in the face of thermal fluctuations is considered. Finally, the flickering resonance (FR) view of charge transfer is presented to examine how fluctuations control low-barrier transport among multiple groups in van der Waals contact. FR produces exponential distance dependence in the absence of tunneling; the exponential character emerges from the probability of matching multiple vibronically broadened electronic energies within a tolerance defined by the rms coupling among interacting groups. FR thus produces band like coherent transport on the nanometer length scale, enabled by conformational fluctuations. Taken as a whole, the emerging context for ET in dynamical biomolecules provides a robust framework to design and interpret the inner workings of bioenergetics from the molecular to the cellular scale and beyond, with applications in biomedicine, biocatalysis, and energy science.

Interfacial hydration, dynamics and electron transfer: multi-scale ET modeling of the transient [myoglobin, cytochrome b5] complex

Formation of a transient [myoglobin (Mb), cytochrome b(5) (cyt b(5))] complex is required for the reductive repair of inactive ferri-Mb to its functional ferro-Mb state. The [Mb, cyt b(5)] complex exhibits dynamic docking (DD), with its cyt b(5) partner in rapid exchange at multiple sites on the Mb surface. A triple mutant (Mb(3M)) was designed as part of efforts to shift the electron-transfer process to the simple docking (SD) regime, in which reactive binding occurs at a restricted, reactive region on the Mb surface that dominates the docked ensemble. An electrostatically-guided brownian dynamics (BD) docking protocol was used to generate an initial ensemble of reactive configurations of the complex between unrelaxed partners. This ensemble samples a broad and diverse array of heme-heme distances and orientations. These configurations seeded all-atom constrained molecular dynamics simulations (MD) to generate relaxed complexes for the calculation of electron tunneling matrix elements (T(DA)) through tunneling-pathway analysis. This procedure for generating an ensemble of relaxed complexes combines the ability of BD calculations to sample the large variety of available conformations and interprotein distances, with the ability of MD to generate the atomic level information, especially regarding the structure of water molecules at the protein-protein interface, that defines electron-tunneling pathways. We used the calculated T(DA) values to compute ET rates for the [Mb(wt), cyt b(5)] complex and for the complex with a mutant that has a binding free energy strengthened by three D/E → K charge-reversal mutations, [Mb(3M), cyt b(5)]. The calculated rate constants are in agreement with the measured values, and the mutant complex ensemble has many more geometries with higher T(DA) values than does the wild-type Mb complex. Interestingly, water plays a double role in this electron-transfer system, lowering the tunneling barrier as well as inducing protein interface remodeling that screens the repulsion between the negatively-charged propionates of the two hemes.

Modeling and computations of the intramolecular electron transfer process in the two-heme protein cytochrome c(4)

The di-heme protein Pseudomonas stutzeri cytochrome c(4) (cyt c(4)) has emerged as a useful model for studying long-range protein electron transfer (ET). Recent experimental observations have shown a dramatically different pattern of intramolecular ET between the two heme groups in different local environments. Intramolecular ET in homogeneous solution is too slow (>10 s) to be detected but fast (ms-μs) intramolecular ET in an electrochemical environment has recently been achieved by controlling the molecular orientation of the protein assembled on a gold electrode surface. In this work we have performed computational modeling of the intramolecular ET process by a combination of density functional theory (DFT) and quantum mechanical charge transfer theory to disclose reasons for this difference. We first address the electronic structures of the model heme core with histidine and methionine axial ligands in both low- and high-spin states by structure-optimized DFT. The computations enable estimating the intramolecular reorganization energy of the ET process for different combinations of low- and high-spin heme couples. Environmental reorganization free energies, work terms ("gating") and driving force were determined using dielectric continuum models. We then calculated the electronic transmission coefficient of the intramolecular ET rate using perturbation theory combined with the electronic wave functions determined by the DFT calculations for different heme group orientations and Fe-Fe separations. The reactivity of low- and high-spin heme groups was notably different. The ET rate is exceedingly low for the crystallographic equilibrium orientation but increases by several orders of magnitude for thermally accessible non-equilibrium configurations. Deprotonation of the propionate carboxyl group was also found to enhance the ET rate significantly. The results are discussed in relation to the observed surface immobilization effect and support the notion of conformationally gated ET.

Nonorthogonality Problem and Effective Electronic Coupling Calculation: Application to Charge Transfer in π-Stacks Relevant to Biochemistry and Molecular Electronics

A recently proposed method for the calculation of the effective electronic coupling (or charge-transfer integral) in a two-state system is discussed and related to other methods in the literature. The theoretical expression of the coupling is exact within the two-state model and applies to the general case where the charge transfer (CT) process involves nonorthogonal initial and final diabatic (localized) states. In this work, it is shown how this effective electronic coupling is also the one to be used in a suitable extension of Rabi's formula to the nonorthogonal representation of two-state dynamical problems. The formula for the transfer integral is inspected in the regime of long-range CT and applied to CT reactions in redox molecular systems of interest to biochemistry and/or to molecular electronics: the guanine-thymine stack from regular B-DNA, the polyaromatic perylenediimide stack, and the quinol-semiquinone couple. The calculations are performed within the framework of the Density Functional Theory (DFT), using hybrid exchange-correlation (XC) density functionals, which also allowed investigation of the appropriateness of such hybrid-DFT methods for computing electronic couplings. The use of the recently developed M06-2X and M06-HF density functionals in appropriate ways is supported by the results of this work.

Probing deuterium isotope effect on structure and solvation dynamics of human serum albumin

The deuterium isotopic effect on the structure and solvation dynamics of the protein, human serum albumin (HSA), has been studied by using circular dichroism (CD), femtosecond up-conversion, FRET, and single-molecule spectroscopy. The CD spectra suggest that D(2)O affects the structure of HSA, leading to a 20% decrease in the helical structure. The FRET study indicates that the distance of C153 from the lone tryptophan residue of HSA is quite similar (≈21 Å) in H(2)O and D(2)O, and hence, the location of the probe in the protein remains the same in the two solvents. The single-molecule study suggests that coumarin 153 (C153) binds almost exclusively (>96%) to one site of HSA. Solvation dynamics of C153 in HSA is found to be markedly retarded in D(2)O compared with H(2)O. In H(2)O, the solvation of C153 bound to HSA is found to be biexponential with one component of 7 ps (30%) and a long component of 350 ps (70%). In D(2)O, we detected a short component of 4 ps (41%) and a long component of 950 ps (59%). Thus, the ultraslow component of the solvation dynamics of C153 bound to HSA in D(2)O (950 ps) is 2.5-fold slower than that in H(2)O (350 ps). The marked deuterium isotope effect has been ascribed to water molecules confined in the protein environment and to a lesser extent to the structural modification of protein by D(2)O.

Full-electron calculation of effective electronic couplings and excitation energies of charge transfer states: Application to hole transfer in DNA pi-stacks

In this work I develop and apply a theoretical method for calculating effective electronic couplings (or transfer integrals) between redox sites involved in hole or electron transfer reactions. The resulting methodology is a refinement and a generalization of a recently developed approach for transfer integral evaluation. In fact, it holds for any overlap between the charge-localized states used to represent charge transfer (CT) processes in the two-state model. The presented theoretical and computational analyses show that the prototype approach is recovered for sufficiently small overlaps. The method does not involve any empirical parameter. It allows a complete multielectron description, therefore including electronic relaxation effects. Furthermore, its theoretical formulation holds at any value of the given reaction coordinate and yields a formula for the evaluation of the vertical excitation energy (i.e., the energy difference between the adiabatic ground and first-excited electronic states) that rests on the same physical quantities used in transfer integral calculation. In this paper the theoretical approach is applied to CT in B-DNA base dimers within the framework of Density Functional Theory (DFT), although it can be implemented in other computational schemes. The results of this work, as compared with previous Hartree-Fock (HF) and post-HF evaluations, support the applicability of the current implementation of the method to larger pi-stacked arrays, where post-HF approaches are computationally unfeasible.

First principles effective electronic couplings for hole transfer in natural and size-expanded DNA

Hole transfer processes between base pairs in natural DNA and size-expanded DNA (xDNA) are studied and compared, by means of an accurate first principles evaluation of the effective electronic couplings (also known as transfer integrals), in order to assess the effect of the base augmentation on the efficiency of charge transport through double-stranded DNA. According to our results, the size expansion increases the average electronic coupling, and thus the CT rate, with potential implications in molecular biology and in the implementation of molecular nanoelectronics. Our analysis shows that the effect of the nucleobase expansion on the charge-transfer (CT) rate is sensitive to the sequence of base pairs. Furthermore, we find that conformational variability is an important factor for the modulation of the CT rate. From a theoretical point of view, this work offers a contribution to the CT chemistry in pi-stacked arrays. Indeed, we compare our methodology against other standard computational frameworks that have been adopted to tackle the problem of CT in DNA, and unravel basic principles that should be accounted for in selecting an appropriate theoretical level.

Long Distance Electron-Transfer Mechanism in Peptidylglycine α-Hydroxylating Monooxygenase: A Perfect Fitting for a Water Bridge

The active sites of copper enzymes have been the subject of many theoretical and experimental investigations from a number of years. Such studies have embraced topics devoted to the modeling of the first coordination sphere at the metallic cations up to the development of biomimetic, or bioinspired, catalytic systems. At least from the theoretical viewpoint, fewer efforts have been dedicated to elucidate how the two copper cations act concertedly in noncoupled dicopper enzymes such as peptidylglycine alpha-hydroxylating monooxygenase (PHM) and dopamine beta-monooxygenase (DbetaM). In these metalloenzymes, an electronic transfer is assumed between the two distant copper cations (11 A). Recent experimental results suggest that this transfer occurs through water molecules, a phenomenon which has been theoretically evidenced to be of high efficiency in the case of cytochrome b5 (Science, 2005, 310, 1311). In the present contribution dedicated to PHM, we overpass the common theoretical approaches dedicated to the electronic and geometrical structures of sites CuM or CuH restricted to their first coordination spheres and aim at directly comparing theoretical results to the experimentally measured activity of the PHM enzyme. To achieve this goal, molecular dynamics simulations were performed on wild-type and various mutants of PHM. More precisely, we provide an estimate of the electron-transfer efficiency between the CuM and CuH sites by means of such molecular dynamics simulations coupled to Marcus theory joined to the Beratan model to approximate the required coupling matrix elements. The theoretical results are compared to the kinetics measurements performed on wild and mutated PHM. The present work, the dynamic aspects of which are essential, accounts for the experimental results issued from mutagenesis. It supports the conclusion that an electronic transfer can occur between two copper(I) sites along a bridge involving a set of hydrogen and chemical bonds. Residue Gln170 is evidenced to be the keystone of this water-mediated pathway.