Electron Flow through Proteins

ATP- and iron-protein-independent activation of nitrogenase catalysis by light

We report here the light-driven activation of the molybdenum-iron-protein (MoFeP) of nitrogenase for substrate reduction independent of ATP hydrolysis and the iron-protein (FeP), which have been believed to be essential for catalytic turnover. A MoFeP variant labeled on its surface with a Ru-photosensitizer is shown to photocatalytically reduce protons and acetylene, most likely at its active site, FeMoco. The uncoupling of nitrogenase catalysis from ATP hydrolysis should enable the study of redox dynamics within MoFeP and the population of discrete reaction intermediates for structural investigations.

How donor-bridge-acceptor energetics influence electron tunneling dynamics and their distance dependences

Long-range electron transfer may occur via two fundamentally different mechanisms depending on the combination of electron donor, acceptor, and the bridging medium between the two redox partners. Activating the so-called hopping mechanism requires matching the energy levels of the donor and the bridge. If electrons from the donor can thermodynamically access bridge-localized redox states, the bridge may be temporarily reduced before the electron is forwarded to the acceptor. As a result, electron transfer rates may demonstrate an extremely shallow dependence on distance. When transient reduction of the bridging medium is thermodynamically impossible, a tunneling mechanism that exponentially depends on distance becomes important for electron transport. Fifty years ago, superexchange theory had already predicted that electron transfer rates should be affected by donor-bridge-acceptor energetics even in the tunneling regime, in which the energy gap (Δε) is too large for electrons to hop from the donor onto the bridge. However, because electron tunneling rates depend on many parameters and the influence of donor-bridge energy gaps is difficult to distinguish from other influences, direct experimental support for the theoretical prediction has been difficult to find. Because of remarkable progress, particularly in the past couple of years, researchers have finally found direct evidence for the long-sought but elusive tunneling-energy gap effect. After a brief introduction to the theory of the tunneling mechanism, this Account discusses recent experimental results describing the importance of the tunneling-energy gap. Experimental studies in this area usually combine synthetic chemistry with electrochemical investigations and time-resolved (optical) spectroscopy. For example, we present a case study of hole tunneling through synthetic DNA hairpins, in which different donor-acceptor couples attached to the same hairpins resulted in tunneling rates with significantly different dependences on distance. Recent systematic studies of conjugated molecular bridges have demonstrated the same result: The distance decay constant (β), which describes the steepness of the exponential decrease of charge tunneling rates with increasing donor-acceptor distance, is not a property of the bridge alone; rather it is a sensitive function of the entire donor-bridge-acceptor (D-b-A) combination. In selected cases, researchers have found a quantitative relationship between the experimentally determined distance decay constant (β) and the magnitude of the tunneling-energy gap (Δε). The rates and efficiencies of charge transfer reactions occurring over long distances are of pivotal importance in light-to-chemical energy conversion and molecular electronics. Tunneling-energy gap effects play an intriguing role in the formation of long-lived charge-separated states after photoexcitation: The kinetic stabilization of these charge-separated states frequently exploits the inverted driving-force effect. Recent studies indicate that tunneling-energy gap effects can differentiate the distance dependences of energy-storing charge-separation reactions from those of energy-wasting charge-recombination processes. Thus, the exploitation of tunneling-energy gap effects may provide an additional way to obtain long-lived charge-separated states.

Hyperfine-shifted 13C resonance assignments in an iron-sulfur protein with quantum chemical verification: aliphatic C-H···S 3-center-4-electron interactions

Although the majority of noncovalent interactions associated with hydrogen and heavy atoms in proteins and other biomolecules are classical hydrogen bonds between polar N−H or O−H moieties and O atoms or aromatic π electrons, high-resolution X-ray crystallographic models deposited in the Protein Data Bank show evidence for weaker C−H···O hydrogen bonds, including ones involving sp³-hybridized carbon atoms. Little evidence is available in proteins for the (even) weaker C−H···S interactions described in the crystallographic literature on small molecules. Here, we report experimental evidence and theoretical verification for the existence of nine aliphatic (sp³-hybridized) C−H···S 3-center−4-electron interactions in the protein Clostridium pasteurianum rubredoxin. Our evidence comes from the analysis of carbon-13 NMR chemical shifts assigned to atoms near the iron at the active site of this protein. We detected anomalous chemical shifts for these carbon-13 nuclei and explained their origin in terms of unpaired spin density from the iron atom being delocalized through interactions of the type: C−H···S−Fe, where S is the sulfur of one of the four cysteine side chains covalently bonded to the iron. These results suggest that polarized sulfur atoms in proteins can engage in multiple weak interactions with surrounding aliphatic groups. We analyze the strength and angular dependence of these interactions and conclude that they may contribute small, but significant, stabilization to the molecule.

n-->pi* interactions in proteins

Hydrogen bonds between backbone amides are common in folded proteins. Here, we show that an intimate interaction between backbone amides also arises from the delocalization of a lone pair of electrons (n) from an oxygen atom to the antibonding orbital (pi*) of the subsequent carbonyl group. Natural bond orbital analysis predicted significant n-->pi* interactions in certain regions of the Ramachandran plot. These predictions were validated by a statistical analysis of a large, non-redundant subset of protein structures determined to high resolution. The correlation between these two independent studies is striking. Moreover, the n-->pi* interactions are abundant and especially prevalent in common secondary structures such as alpha-, 3(10)- and polyproline II helices and twisted beta-sheets. In addition to their evident effects on protein structure and stability, n-->pi* interactions could have important roles in protein folding and function, and merit inclusion in computational force fields.

Guidelines for tunneling in enzymes

Here we extend the engineering descriptions of simple, single-electron-tunneling chains common in oxidoreductases to quantify sequential oxidation-reduction rates of two-or-more electron cofactors and substrates. We identify when nicotinamides may be vulnerable to radical mediated oxidation-reduction and merge electron-tunneling expressions with the chemical rate expressions of Eyring. The work provides guidelines for the construction of new artificial oxidoreductases inspired by Nature but adopting independent design and redox engineering.

Electron flow through metalloproteins

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.