Cations Modulate Actin Bundle Mechanics, Assembly Dynamics, and Structure.

Actin bundles are key factors in the mechanical support and dynamic reorganization of the cytoskeleton. High concentrations of multivalent counterions promote bundle formation through electrostatic attraction between actin filaments that are negatively charged polyelectrolytes. In this study, we evaluate how physiologically relevant divalent cations affect the mechanical, dynamic, and structural properties of actin bundles. Using a combination of total internal reflection fluorescence microscopy, transmission electron microscopy, and dynamic light scattering, we demonstrate that divalent cations modulate bundle stiffness, length distribution, and lateral growth. Molecular dynamics simulations of an all-atom model of the actin bundle reveal specific actin residues coordinate cation-binding sites that promote the bundle formation. Our work suggests that specific cation interactions may play a fundamental role in the assembly, structure, and mechanical properties of actin bundles.

Charge splitters and charge transport junctions based on guanine quadruplexes.

Self-assembling circuit elements, such as current splitters or combiners at the molecular scale, require the design of building blocks with three or more terminals. A promising material for such building blocks is DNA, wherein multiple strands can self-assemble into multi-ended junctions, and nucleobase stacks can transport charge over long distances. However, nucleobase stacking is often disrupted at junction points, hindering electric charge transport between the two terminals of the junction. Here, we show that a guanine-quadruplex (G4) motif can be used as a connector element for a multi-ended DNA junction. By attaching specific terminal groups to the motif, we demonstrate that charges can enter the structure from one terminal at one end of a three-way G4 motif, and can exit from one of two terminals at the other end with minimal carrier transport attenuation. Moreover, we study four-way G4 junction structures by performing theoretical calculations to assist in the design and optimization of these connectors.

Nonequilibrium Hybridization Enables Discrimination of a Point Mutation within 5-40 °C.

Detection of point mutations and single nucleotide polymorphisms in DNA and RNA has a growing importance in biology, biotechnology, and medicine. For the application at hand, hybridization assays are often used. Traditionally, they differentiate point mutations only at elevated temperatures (>40 °C) and in narrow intervals (ΔT = 1-10 °C). The current study demonstrates that a specially designed multistranded DNA probe can differentiate point mutations in the range of 5-40 °C. This unprecedentedly broad ambient-temperature range is enabled by a controlled combination of (i) nonequilibrium hybridization conditions and (ii) a mismatch-induced increase of equilibration time in respect to that of a fully matched complex, which we dub "kinetic inversion".

Biological charge transfer via flickering resonance.

Biological electron-transfer (ET) reactions are typically described in the framework of coherent two-state electron tunneling or multistep hopping. However, these ET reactions may involve multiple redox cofactors in van der Waals contact with each other and with vibronic broadenings on the same scale as the energy gaps among the species. In this regime, fluctuations of the molecular structures and of the medium can produce transient energy level matching among multiple electronic states. This transient degeneracy, or flickering electronic resonance among states, is found to support coherent (ballistic) charge transfer. Importantly, ET rates arising from a flickering resonance (FR) mechanism will decay exponentially with distance because the probability of energy matching multiple states is multiplicative. The distance dependence of FR transport thus mimics the exponential decay that is usually associated with electron tunneling, although FR transport involves real carrier population on the bridge and is not a tunneling phenomenon. Likely candidates for FR transport are macromolecules with ET groups in van der Waals contact: DNA, bacterial nanowires, multiheme proteins, strongly coupled porphyrin arrays, and proteins with closely packed redox-active residues. The theory developed here is used to analyze DNA charge-transfer kinetics, and we find that charge-transfer distances up to three to four bases may be accounted for with this mechanism. Thus, the observed rapid (exponential) distance dependence of DNA ET rates over distances of ≲ 15 Šdoes not necessarily prove a tunneling mechanism.

Effect of backbone flexibility on charge transfer rates in peptide nucleic acid duplexes.

Charge transfer (CT) properties are compared between peptide nucleic acid structures with an aminoethylglycine backbone (aeg-PNA) and those with a γ-methylated backbone (γ-PNA). The common aeg-PNA is an achiral molecule with a flexible structure, whereas γ-PNA is a chiral molecule with a significantly more rigid structure than aeg-PNA. Electrochemical measurements show that the CT rate constant through an aeg-PNA bridging unit is twice the CT rate constant through a γ-PNA bridging unit. Theoretical calculations of PNA electronic properties, which are based on a molecular dynamics structural ensemble, reveal that the difference in the CT rate constant results from the difference in the extent of backbone fluctuations of aeg- and γ-PNA. In particular, fluctuations of the backbone affect the local electric field that broadens the energy levels of the PNA nucleobases. The greater flexibility of the aeg-PNA gives rise to more broadening, and a more frequent appearance of high-CT rate conformations than in γ-PNA.

B-DNA to zip-DNA: simulating a DNA transition to a novel structure with enhanced charge-transport characteristics.

The forced extension of a DNA segment is studied in a series of steered molecular dynamics simulations, employing a broad range of pulling forces. Throughout the entire force range, the formation of a zipper-like (zip-) DNA structure is observed. In that structure, first predicted by Lohikoski et al., the bases of the DNA strands interdigitate with each other and form a single-base aromatic stack. Similar motifs, albeit only a few base pairs in extent, have been observed in experimental crystal structures. Analysis of the dynamics of structural changes in pulled DNA shows that S-form DNA, thought to be adopted by DNA under applied force, serves as an intermediate between B-DNA and zip-DNA. Therefore, the phase transition plateau observed in force-extension curves of DNA is suggested to reflect the B-DNA to zip-DNA structural transition. Electronic structure analysis of purine bases in zip-DNA indicates a several-fold to order of magnitude increase in the π-π electronic coupling among nearest-neighbor nucleobases, compared to B-DNA. We further observe that zip-DNA does not require base pair complementarity between DNA strands, and we predict that the increased electronic coupling in zip-DNA will result in a much higher rate of charge transfer through an all-purine zip-DNA compared to B-DNA of equal length.

Nucleic Acid Charge Transfer: Black, White and Gray.

Theoretical studies of charge transport in deoxyribonucleic acid (DNA) and peptide nucleic acid (PNA) indicate that structure and dynamics modulate the charge transfer rates, and that different members of a structural ensemble support different charge transport mechanisms. Here, we review the influences of nucleobase geometry, electronic structure, solvent environment, and thermal conformational fluctuations on the charge transfer mechanism. We describe an emerging framework for understanding the diversity of charge transport mechanisms seen in nucleic acids.

Evidence for a near-resonant charge transfer mechanism for double-stranded peptide nucleic acid.

We present evidence for a near-resonant mechanism of charge transfer in short peptide nucleic acid (PNA) duplexes obtained through electrochemical, STM break junction (STM-BJ), and computational studies. A seven base pair (7-bp) PNA duplex with the sequence (TA)(3)-(XY)-(TA)(3) was studied, in which XY is a complementary nucleobase pair. The experiments showed that the heterogeneous charge transfer rate constant (k(0)) and the single-molecule conductance (σ) correlate with the oxidation potential of the purine base in the XY base pair. The electrochemical measurements showed that the enhancement of k(0) is independent, within experimental error, of which of the two PNA strands contains the purine base of the XY base pair. 7-bp PNA duplexes with one or two GC base pairs had similar measured k(0) and conductance values. While a simple superexchange model, previously used to rationalize charge transfer in single stranded PNA (Paul et al. J. Am. Chem. Soc. 2009, 131, 6498-6507), describes some of the experimental observations, the model does not explain the absence of an enhancement in the experimental k(0) and σ upon increasing the G content in the duplexes from one to two. Moreover, the superexchange model is not consistent with other studies (Paul et al. J. Phys. Chem. B 2010, 114, 14140), that showed a hopping charge transport mechanism is likely important for PNA duplexes longer than seven base pairs. A quantitative computational analysis shows that a near-resonant charge transfer regime, wherein a mix of superexchange and hopping mechanisms are expected to coexist, can rationalize all of the experimental results.

Steering electrons on moving pathways.

Electron transfer (ET) reactions provide a nexus among chemistry, biochemistry, and physics. These reactions underpin the "power plants" and "power grids" of bioenergetics, and they challenge us to understand how evolution manipulates structure to control ET kinetics. Ball-and-stick models for the machinery of electron transfer, however, fail to capture the rich electronic and nuclear dynamics of ET molecules: these static representations disguise, for example, the range of thermally accessible molecular conformations. The influence of structural fluctuations on electron-transfer kinetics is amplified by the exponential decay of electron tunneling probabilities with distance, as well as the delicate interference among coupling pathways. Fluctuations in the surrounding medium can also switch transport between coherent and incoherent ET mechanisms--and may gate ET so that its kinetics is limited by conformational interconversion times, rather than by the intrinsic ET time scale. Moreover, preparation of a charge-polarized donor state or of a donor state with linear or angular momentum can have profound dynamical and kinetic consequences. In this Account, we establish a vocabulary to describe how the conformational ensemble and the prepared donor state influence ET kinetics in macromolecules. This framework is helping to unravel the richness of functional biological ET pathways, which have evolved within fluctuating macromolecular structures. The conceptual framework for describing nonadiabatic ET seems disarmingly simple: compute the ensemble-averaged (mean-squared) donor-acceptor (DA) tunneling interaction, , and the Franck-Condon weighted density of states, rho(FC), to describe the rate, (2pi/variant Planck's over 2pi)rho(FC). Modern descriptions of the thermally averaged electronic coupling and of the Franck-Condon factor establish a useful predictive framework in biology, chemistry, and nanoscience. Describing the influence of geometric and energetic fluctuations on ET allows us to address a rich array of mechanistic and kinetic puzzles. How strongly is a protein's fold imprinted on the ET kinetics, and might thermal fluctuations "wash out" signatures of structure? What is the influence of thermal fluctuations on ET kinetics beyond averaging of the tunneling barrier structure? Do electronic coupling mechanisms change as donor and acceptor reposition in a protein, and what are the consequences for the ET kinetics? Do fluctuations access minority species that dominate tunneling? Can energy exchanges between the electron and bridge vibrations generate vibronic signatures that label some of the D-to-A pathways traversed by the electron, thus eliminating unmarked pathways that would otherwise contribute to the DA coupling (as in other "which way" or double-slit experiments)? Might medium fluctuations drive tunneling-hopping mechanistic transitions? How does the donor-state preparation, in particular, its polarization toward the acceptor and its momentum characteristics (which may introduce complex rather than pure real relationships among donor orbital amplitudes), influence the electronic dynamics? In this Account, we describe our recent studies that address puzzling questions of how conformational distributions, excited-state polarization, and electronic and nuclear dynamical effects influence ET in macromolecules. Indeed, conformational and dynamical effects arise in all transport regimes, including the tunneling, resonant transport, and hopping regimes. Importantly, these effects can induce switching among ET mechanisms.

Role of nucleobase energetics and nucleobase interactions in single-stranded peptide nucleic acid charge transfer.

Self-assembled monolayers of single-stranded (ss) peptide nucleic acids (PNAs) containing seven nucleotides (TTTXTTT), a C-terminus cysteine, and an N-terminus ferrocene redox group were formed on gold electrodes. The PNA monomer group (X) was selected to be either cytosine (C), thymine (T), adenine (A), guanine (G), or a methyl group (Bk). The charge transfer rate through the oligonucleotides was found to correlate with the oxidation potential of X. Kinetic measurements and computational studies of the ss-PNA fragments show that a nucleobase mediated charge transport mechanism in the deep tunneling superexchange regime can explain the observed dependence of the kinetics of charge transfer on the PNA sequence. Theoretical analysis suggests that the charge transport is dominantly hole-mediated and takes place through the filled bridge orbitals. The strongest contribution to conductance comes from the highest filled orbitals (HOMO, HOMO-1, and HOMO-2) of individual bases, with a rapid drop off in contributions from lower lying filled orbitals. Our studies further suggest that the linear correlation observed between the experimental charge transfer rates and the oxidation potential of base X arises from weak average interbase couplings and similar stacking geometries for the four TTTXTTT systems.

Solution structure of a peptide nucleic acid duplex from NMR data: features and limitations.

This paper describes the results of a 1D and 2D NMR spectroscopy study of a palindromic 8-base pair PNA duplex GGCATGCC in H2O and H2O-D2O solutions. The (1)H NMR peaks have been assigned for most of the protons of the six central base pairs, as well as for several amide protons of the backbone. The resulting 36 interbase and base-backbone distance restraints were used together with Watson-Crick restraints to generate the PNA duplex structure in the course of 10 independent simulated annealing runs followed by restrained molecular dynamics (MD) simulations in explicit water. The resulting PNA structures correspond to a P-type helix with helical parameters close to those observed in the crystal structures of PNA. Based on the current limited number of restraints obtained from NMR spectra, alternative structures obtained by MD from starting PNA models based on DNA cannot be ruled out and are also discussed.

PNA versus DNA: effects of structural fluctuations on electronic structure and hole-transport mechanisms.

The effects of structural fluctuations on charge transfer in double-stranded DNA and peptide nucleic acid (PNA) are investigated. A palindromic sequence with two guanine bases that play the roles of hole donor and acceptor, separated by a bridge of two adenine bases, was analyzed using combined molecular dynamics (MD) and quantum-chemical methods. Surprisingly, electronic structure calculations on individual MD snapshots show significant frontier orbital electronic population on the bridge in approximately 10% of the structures. Electron-density delocalization to the bridge is found to be gated by fluctuations of the covalent conjugated bond structure of the aromatic rings of the nucleic bases. It is concluded, therefore, that both thermal hopping and superexchange should contribute significantly to charge transfer even in short DNA/PNA fragments. PNA is found to be more flexible than DNA, and this flexibility is predicted to produce larger rates of charge transfer.

Modeling DNA loops using the theory of elasticity.

An elastic rod model of a protein-bound DNA loop is adapted for application in multi-scale simulations of protein-DNA complexes. The classical Kirchhoff system of equations which describes the equilibrium structure of the elastic loop is modified to account for the intrinsic twist and curvature, anisotropic bending properties, and electrostatic charge of DNA. The effects of bending anisotropy and electrostatics are studied for the DNA loop clamped by the lac repressor protein. For two possible lengths of the loop, several topologically different conformations are predicted and extensively analyzed over the broad range of model parameters describing DNA bending and electrostatic properties. The scope and applications of the model in already accomplished and in future multi-scale studies of protein-DNA complexes are discussed.

Structural dynamics of the lac repressor-DNA complex revealed by a multiscale simulation.

A multiscale simulation of a complex between the lac repressor protein (LacI) and a 107-bp-long DNA segment is reported. The complex between the repressor and two operator DNA segments is described by all-atom molecular dynamics; the size of the simulated system comprises either 226,000 or 314,000 atoms. The DNA loop connecting the operators is modeled as a continuous elastic ribbon, described mathematically by the nonlinear Kirchhoff differential equations with boundary conditions obtained from the coordinates of the terminal base pairs of each operator. The forces stemming from the looped DNA are included in the molecular dynamics simulations; the loop structure and the forces are continuously recomputed because the protein motions during the simulations shift the operators and the presumed termini of the loop. The simulations reveal the structural dynamics of the LacI-DNA complex in unprecedented detail. The multiple domains of LacI exhibit remarkable structural stability during the simulation, moving much like rigid bodies. LacI is shown to absorb the strain from the looped DNA mainly through its mobile DNA-binding head groups. Even with large fluctuating forces applied, the head groups tilt strongly and keep their grip on the operator DNA, while the remainder of the protein retains its V-shaped structure. A simulated opening of the cleft of LacI by 500-pN forces revealed the interactions responsible for locking LacI in the V-conformation.

Structural basis for cooperative DNA binding by CAP and lac repressor.

Catabolite gene activator protein (CAP) and lac repressor (LR) are celebrated transcription-regulating proteins that bind to DNA cooperatively forming a ternary complex with the promoter loop. Here we present a multiscale model of the ternary complex derived from crystal structures of the proteins and a continuous structure of the DNA loop built using the theory of elasticity. We predict that the loop is underwound in the binary complex with the LR, whereas in the ternary complex with the LR and CAP, the loop is overwound and extended due to an upstream relocation of a DNA binding hand of LR. The computed relocation distance matches the experimental observations and the energy balance of the system explains the cooperativity effect. Using the multiscale approach, we build an all-atom model of the ternary complex that suggests a series of further experimental investigations.