A series of rate constant expressions for nonadiabatic proton-coupled electron transfer (PCET) reactions are analyzed and compared. The approximations underlying each expression are enumerated, and the regimes of validity for each expression are illustrated by calculations on model systems. In addition, the kinetic isotope effects (KIEs) for a series of model PCET reactions are analyzed to elucidate the fundamental physical principles dictating the magnitude of the KIE and the dependence of the KIE on the physical properties of the system, including temperature, reorganization energy, driving force, equilibrium proton donor-acceptor distance, and effective frequency of the proton donor-acceptor mode. These calculations lead to three physical insights that are directly relevant to experimental data. First, these calculations provide an explanation for a decrease in the KIE as the proton donor-acceptor distance increases, even though typically the KIE will increase with increasing equilibrium proton donor-acceptor distance if all other parameters remain fixed. Often the proton donor-acceptor frequency decreases as the proton donor-acceptor distance increases, and these two effects impact the KIE in opposite directions, so either trend could be observed. Second, these calculations provide an explanation for an increase in the KIE as the temperature increases, even though typically the KIE will decrease with increasing temperature if all other parameters remain fixed. The combination of a rigid hydrogen bond, which corresponds to a high proton donor-acceptor frequency, and low solvent polarity, which corresponds to small solvent reorganization energy, allows the KIE to either increase or decrease with temperature, depending on the other properties of the system. Third, these calculations provide insight into the dependence of the rate constant and KIE on the driving force, which has been studied experimentally for a wide range of PCET systems. The rate constant increases as the driving force becomes more negative because excited vibronic product states associated with low free energy barriers and relatively large vibronic couplings become accessible. The ln[KIE] has a maximum near zero driving force and decreases significantly as the driving force becomes more positive or negative because the contributions from excited vibronic states increase as the reaction becomes more asymmetric, and contributions from excited vibronic states decrease the KIE. These calculations and analyses lead to experimentally testable predictions of trends in the KIEs for PCET systems.

The impact of distal mutation on the hydrogen transfer interface properties and on the substrate mobility, conformation, and orientation in soybean lipoxygenase-1 (SLO) is examined. SLO catalyzes a hydrogen abstraction reaction that occurs by a proton-coupled electron transfer mechanism. Mutation of isoleucine 553 to less bulky residues has been found experimentally to increase the magnitude and temperature dependence of the kinetic isotope effect for this reaction. This residue borders the linoleic acid substrate but is ~15 Å from the active site iron. In the present study, we model these experimental data with a vibronically nonadiabatic theory and perform allatom molecular dynamics simulations on the complete solvated wild-type and mutant enzymes. Our calculations indicate that the proton transfer equilibrium distance increases and the associated frequency decreases as residue 553 becomes less bulky. The molecular dynamics simulations illustrate that this mutation impacts the mobility, geometrical conformation, and orientation of the linoleic acid within the active site. In turn, these effects alter the proton donor-acceptor equilibrium distance and frequency, leading to the experimentally observed changes in the magnitude and temperature dependence of the kinetic isotope effect. This study provides insight into how the effects of distal mutations may be transmitted in enzymes to ultimately impact the catalytic rates.

Magnetic reconnection, a change of magnetic field connectivity, is a fundamental physical process in which magnetic energy is released explosively, and it is responsible for various eruptive phenomena in the universe. However, this process is difficult to observe directly. Here, the magnetic topology associated with a solar reconnection event is studied in three dimensions using the combined perspectives of two spacecraft. The sequence of extreme ultraviolet images clearly shows that two groups of oppositely directed and non-coplanar magnetic loops gradually approach each other, forming a separator or quasi-separator and then reconnecting. The plasma near the reconnection site is subsequently heated from ∼1 to ≥5 MK. Shortly afterwards, warm flare loops (∼3 MK) appear underneath the hot plasma. Other observational signatures of reconnection, including plasma inflows and downflows, are unambiguously revealed and quantitatively measured. These observations provide direct evidence of magnetic reconnection in a three-dimensional configuration and reveal its origin.

The adaptor protein ankyrin-R interacts via its membrane binding domain with the cytoplasmic domain of the anion exchange protein (AE1) and via its spectrin binding domain with the spectrin-based membrane skeleton in human erythrocytes. This set of interactions provides a bridge between the lipid bilayer and the membrane skeleton, thereby stabilizing the membrane. Crystal structures for the dimeric cytoplasmic domain of AE1 (cdb3) and for a 12-ankyrin repeat segment (repeats 13-24) from the membrane binding domain of ankyrin-R (AnkD34) have been reported. However, structural data on how these proteins assemble to form a stable complex have not been reported. In the current studies, site-directed spin labeling, in combination with electron paramagnetic resonance (EPR) and double electron-electron resonance, has been utilized to map the binding interfaces of the two proteins in the complex and to obtain inter-protein distance constraints. These data have been utilized to construct a family of structural models that are consistent with the full range of experimental data. These models indicate that an extensive area on the peripheral domain of cdb3 binds to ankyrin repeats 18 -20 on the top loop surface of AnkD34 primarily through hydrophobic interactions. This is a previously uncharacterized surface for binding of cdb3 to AnkD34. Because a second dimer of cdb3 is known to bind to ankyrin repeats 7-12 of the membrane binding domain of ankyrin-R, the current models have significant implications regarding the structural nature of a tetrameric form of AE1 that is hypothesized to be involved in binding to full-length ankyrin-R in the erythrocyte membrane.Human erythrocytes exhibit an unusual biconcave disc shape and remarkable plasma membrane mechanical stability and deformability, all of which are necessary for their survival in the circulatory system. It is now well established that the unusual cell shape and membrane mechanical properties are due in large part to the presence of an extensive membrane skeleton, composed primarily of the proteins spectrin and actin, that lines the inner membrane surface and to specific bridging interactions between this membrane skeleton and intrinsic membrane proteins in the lipid bilayer. The spectrin-actin skeleton associates with the membrane bilayer via two types of contacts, one involving short actin protofilaments and protein 4.1, which interact with the cytoplasmic domain of glycophorin C, and the second involving the bridging protein ankyrin-R and protein 4.2, which interact with the cytoplasmic domain of the anion exchange protein (AE1) 3 also known as band 3. Alterations in this second class of interactions often result in spherical erythrocytes with decreased cell size and increased fragility, a condition known clinically as hereditary spherocytosis. Hereditary spherocytosis is a spectrum of inherited diseases, occurring in one family out of 2,000 -3,000, which present clinically as varying degrees of hemolytic anemia resulting from hemolysis of the spherical erythrocytes ...

The driving force dependence of the rate constants for nonadiabatic electron transfer (ET), proton transfer (PT), and proton-coupled electron transfer (PCET) reactions are examined. Inverted region behavior, where the rate constant decreases as the reaction becomes more exoergic (i.e., as ΔG 0 becomes more negative), has been observed experimentally for ET and PT. This behavior was predicted theoretically for ET but is not well understood for PT and PCET. The objective of this Letter is to predict the experimental conditions that could lead to observation of inverted region behavior for PT and PCET. The driving force dependence of the rate constant is qualitatively different for PT and PCET than for ET because of the high proton vibrational frequency and substantial shift between the reactant and product proton vibrational wavefunctions. As a result, inverted region behavior is predicted to be experimentally inaccessible for PT and PCET if only the driving force is varied. This behavior may be observed for PT over a limited range of rates and driving forces if the solvent reorganization energy is low enough to cause observable oscillations. Moreover, this behavior may be observed for PT or PCET if the proton donor-acceptor distance increases as ΔG 0 becomes more negative. Thus, a plausible explanation for experimentally observed inverted region behavior for PT or PCET is that varying the driving force also impacts other properties of the system, such as the proton donor-acceptor distance.According to standard Marcus theory of nonadiabatic electron transfer (ET), 1 the dependence of the logarithm of the ET rate constant on the driving force ΔG 0 is represented by an inverted parabola. The maximum rate corresponds to the activationless regime with -ΔG 0 = λ, and the inverted region is defined as -ΔG 0 = λ, where λ is the reorganization energy of the reaction. The existence of the inverted region, where the ET rate constant decreases as the reaction becomes more exoergic (i.e., as ΔG 0 becomes more negative), has been confirmed experimentally. 1,2 The inverted region behavior has also been experimentally observed for proton transfer (PT) reactions, 3-5 although the theoretical basis for these experimental observations has not been well understood. 6 To our knowledge, the inverted region behavior has not yet been experimentally observed for proton-coupled electron transfer (PCET) reactions, in which an electron and proton transfer simultaneously with no stable intermediate. [7][8][9] The objectives of this Letter are to elucidate the fundamental differences in the driving shs@chem.psu.edu. Supporting Information Available: Driving force dependence of the nonadiabatic rate constant with λ = 20 kcal/mol and (a) ω = 400 cm −1 and δx = 0.5 Å, (b) ω = 3000 cm −1 and δx = 0.1 Å; driving force dependence of the nonadiabatic rate constant with two uncoupled modes, whereλ = 20 kcal/mol, ω = 3000 cm −1 and δx = 0.5 Å for the first mode, and ω = 400 cm −1 and δx = 0.1 Å for the second mode; illustration of proton vibra...