David N. Beratan

Approaches for Optimizing the First Electronic Hyperpolarizability of Conjugated Organic Molecules

A two-state, four-orbital, independent electron analysis of the first optical molecular hyperpolarizability, β, leads to the prediction that |β| maximizes at a combination of donor and acceptor strengths for a given conjugated bridge. Molecular design strategies that focus on the energetic manipulations of the bridge states are proposed for the optimization of β. The limitations of molecular classes based on common bridge structures are highlighted and more promising candidates are described. Experimental results supporting the validity of this approach are presented.

Electron tunneling through covalent and noncovalent pathways in proteins

A model is presented for electron tunneling in proteins which allows the donor–acceptor interaction to be mediated by the covalent bonds between amino acids and noncovalent contacts between amino acid chains. The important tunneling pathways are predicted to include mostly bonded groups with less favorable nonbonded interactions being important when the through bond pathway is prohibitively long. In some cases, vibrational motion of nonbonded groups along the tunneling pathway strongly inluences the temperature dependence of the rate. Quantitative estimates for the sizes of these noncovalent interactions are made and their role in protein mediated electron transport is discussed.

Biochemistry and Theory of Proton-Coupled Electron Transfer

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The Nature of Aqueous Tunneling Pathways Between Electron-Transfer Proteins

Structured water molecules near redox cofactors were found recently to accelerate electron-transfer (ET) kinetics in several systems. Theoretical study of interprotein electron transfer across an aqueous interface reveals three distinctive electronic coupling mechanisms that we describe here: (i) a protein-mediated regime when the two proteins are in van der Waals contact; (ii) a structured water-mediated regime featuring anomalously weak distance decay at relatively close protein-protein contact distances; and (iii) a bulk water–mediated regime at large distances. Our analysis explains a range of otherwise puzzling biological ET kinetic data and provides a framework for including explicit water-mediated tunneling effects on ET kinetics.

A predictive theoretical model for electron tunneling pathways in proteins

A practical method is presented for calculating the dependence of electron transfer rates on details of the protein medium intervening between donor and acceptor. The method takes proper account of the relative energetics and mutual interactions of the donor, acceptor, and peptide groups. It also provides a quantitative search scheme for determining the important tunneling pathway(s) [specific sequence(s) of localized bonding and antibonding orbitals of the protein which dominate the donor–acceptor electronic coupling] in native and tailored proteins, provides a tool for designing new proteins with prescribed electron transfer rates, and provides a consistent description of observed electron transfer rates in existing redox labeled metalloproteins and small molecule model compounds.

Fluctuations in Biological and Bioinspired Electron-Transfer Reactions

Central to theories of electron transfer (ET) is the idea that nuclear motion generates a transition state that enables electron flow to proceed, but nuclear motion also induces fluctuations in the donor-acceptor (DA) electronic coupling that is the rate-limiting parameter for nonadiabatic ET. The interplay between the DA energy gap and DA coupling fluctuations is particularly noteworthy in biological ET, where flexible protein and mobile water bridges take center stage. Here, we discuss the critical timescales at play for ET reactions in fluctuating media, highlighting issues of the Condon approximation, average medium versus fluctuation-controlled electron tunneling, gated and solvent relaxation controlled electron transfer, and the influence of inelastic tunneling on electronic coupling pathway interferences. Taken together, one may use this framework to establish principles to describe how macromolecular structure and structural fluctuations influence ET reactions. This framework deepens our understanding of ET chemistry in fluctuating media. Moreover, it provides a unifying perspective for biophysical charge-transfer processes and helps to frame new questions associated with energy harvesting and transduction in fluctuating media.

DNA: insulator or wire?

DNA-based electron transfer reactions are seen in processes such as biosynthesis and radiation damage/repair, but are poorly understood. What kinds of experiments might tell us how far and how fast electrons can travel in DNA? What does modern theory predict?

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.

The chiroptical signature of achiral metal clusters induced by dissymmetric adsorbates

Using a dissymmetrically-perturbed particle-in-a-box model, we demonstrate that the induced optical activity of chiral monolayer protected clusters, such as Whettens Au28(SG)16 glutathione-passivated gold nanoclusters (J. Phys. Chem. B, 2000, 104, 2630-2641), could arise from symmetric metal cores perturbed by a dissymmetric or chiral field originating from the adsorbates. This finding implies that the electronic states of the nanocluster core are chiral, yet the lattice geometries of these cores need not be geometrically distorted by the chiral adsorbates. Based on simple chiral monolayer protected cluster models, we rationalize how the adsorption pattern of the tethering sulfur atoms has a substantial effect on the induced CD in the NIR spectral region, and we show how the chiral image charge produced in the core provides a convenient means of visualizing dissymmetric perturbations to the achiral gold nanocluster core.

Donor-bridge-acceptor energetics determine the distance dependence of electron tunneling in DNA

Electron transfer (ET) processes in DNA are of current interest because of their involvement in oxidative strand cleavage reactions and their relevance to the development of molecular electronics. Two mechanisms have been identified for ET in DNA, a single-step tunneling process and a multistep charge-hopping process. The dynamics of tunneling reactions depend on both the distance between the electron donor and acceptor and the nature of the molecular bridge separating the donor and acceptor. In the case of protein and alkane bridges, the distance dependence is not strongly dependent on the properties of the donor and acceptor. In contrast, we show here that the distance decay of DNA ET rates varies markedly with the energetics of the donor and acceptor relative to the bridge. Specifically, we find that an increase in the energy of the bridge states by 0.25 eV (1 eV = 1.602 × 10−19 J) relative to the donor and acceptor energies for photochemical oxidation of nucleotides, without changing the reaction free energy, results in an increase in the characteristic exponential distance decay constant for the ET rates from 0.71 to 1.1 Å−1. These results show that, in the small tunneling energy gap regime of DNA ET, the distance dependence is not universal; it varies strongly with the tunneling energy gap. These DNA ET reactions fill a “missing link” or transition regime between the large barrier (rapidly decaying) tunneling regime and the (slowly decaying) hopping regime in the general theory of bridge-mediated ET processes.