Conformation- and phosphorylation-dependent electron tunnelling across self-assembled monolayers of tau peptides.

We report on charge transport across self-assembled monolayers (SAMs) of short tau peptides by probing the electron tunneling rates and quantum mechanical simulation. We measured the electron tunneling rates across SAMs of carboxyl-terminated linker molecules (C6H12O2S) and short cis-tau (CT) and trans-tau (TT) peptides, supported on template-stripped gold (AuTS) bottom electrode, with Eutectic Gallium-Indium (EGaIn)(EGaIn) top electrode. Measurements of the current density across thousands of AuTS/linker/tau//Ga2O3/EGaIn single-molecule junctions show that the tunneling current across CT peptide is one order of magnitude lower than that of TT peptide. Quantum mechanical simulation demonstrated a wider energy bandgap of the CT peptide, as compared to the TT peptide, which causes a reduction in its electron tunneling current. Our findings also revealed the critical role of phosphorylation in altering the charge transport characteristics of short peptides; more specifically, we found that the presence of phosphate groups can reduce the energy band gap in tau peptides and alter their electrical properties. Our results suggest that conformational and phosphorylation of short peptides (e.g., tau) can significantly change their charge transport characteristics and energy levels.

Synthesis, Structure, and Characterization of Tris(1-ethyl-4-isopropyl-imidazolyl-κN)phosphine Nickel(II) Complexes.

In this work we report the synthesis of five new nickel(II) complexes all coordinated to the tripodal ligand tris(1-ethyl-4- i Pr-imidazolyl)phosphine (TlEt4iPrIP). They are [Ni(T1Et4iPrIP)(CH3CN)2(OTf)](OTf) (1), [Ni(T1Et4iPrIP)(OTf)2] (2), [Ni(T1Et4iPrIP)(H2O)(OTf)](OTf) (3), [Ni(T1Et4iPrIP)Cl](OTf) (4), and [Ni(T1Et4iPrIP)Cl2] (5). The complexes serve as bioinorganic structural model complexes for histidine-coordinated nickel proteins. The X-ray structures have been determine for all complexes which feature coordination numbers 4-6. We investigated the spectroscopic interconversions for these compound in dichloromethane solution and demonstrate interconversion between 1-3 and conversion of 2 to 4. Complex 5 can be spectroscopically converted to the cation of 4 by dissolving it in dichloromethane. Fits of variable temperature magnetic susceptibility data yielded the following parameters: g = 1.944, D = -0.327 cm-1, E/D = 3.706 for 1; g = 2.280, D = -0.365 cm-1, E/D = 22.178 for 2; g = 2.000, D = -7.402 cm-1, E/D = -0.272 for 3; g = 2.176, D = -0.128 cm-1, E/D = -0.783 for 4; g = 2.258, D = 14.288 cm-1, E/D = 0.095 for 5. DFT structure optimizations afforded HOMO and LUMO energies indicating that complex 1 is the most stable.

Structural, Spectroscopic, Electrochemical, and Magnetic Properties for Manganese(II) Triazamacrocyclic Complexes.

We report the synthesis of [Mn(tacud)2](OTf)2 (1) (tacud = 1,4,8-triazacycloundecane), [Mn(tacd)2](OTf)2 (2) (tacd = 1,4,7-triazacyclodecane), and [Mn(tacn)2](OTf)2 (3) (tacn = 1,4,7-triazacyclononane). Electrochemical measurements on the MnIII/II redox couple show that complex 1 has the largest anodic potential of the set (E 1/2 = 1.16 V vs NHE, ΔE p = 106 mV) compared to 2 (E 1/2 = 0.95 V, ΔE p = 108 mV) and 3 (E 1/2 = 0.93 V, ΔE p = 96 mV). This is due to the fact that 1 has the fewest 5-membered chelate rings and thus is least stabilized. Magnetic studies of 1-3 revealed that all complexes remain high spin throughout the temperature range investigated (2 - 300 K). X-band EPR investigations in methanol glass indicated that the manganese(II) centers for 2 and 3 resided in a more distorted octahedral geometric configuration compared to 1. To ease spectral interpretation and extract ZFS parameters, we performed high-frequency high-field EPR (HFEPR) at frequencies above 200 GHz and a field of 7.5 T. Simulation of the spectral data yielded g = 2.0013 and D = -0.031 cm-1 for 1, g = 2.0008, D = -0.0824 cm-1, |E/D| = 0.12 for 2, and g = 2.00028, D = -0.0884 cm-1 for 3. These results are consistent with 3 possessing the most distorted geometry. Calculations (PBE0/6-31G(d)) were performed on 1-3. Results show that 1 has the largest HOMO-LUMO gap energy (6.37 eV) compared to 2 (6.12 eV) and 3 (6.26 eV). Complex 1 also has the lowest HOMO energies indicating higher stability.

Phase control in a spin-triplet SQUID.

It is now well established that a Josephson junction made from conventional spin-singlet superconductors containing ferromagnetic layers can carry spin-triplet supercurrent under certain conditions. The first experimental signature of that fact is the propagation of such supercurrent over long distances through strong ferromagnetic materials. Surprisingly, one of the most salient predictions of the theory has yet to be verified experimentally-namely, that a Josephson junction containing three magnetic layers with coplanar magnetizations should exhibit a ground-state phase shift of either zero or π depending on the relative orientations of those magnetizations. We demonstrate this property using Josephson junctions containing three different types of magnetic layers, chosen so that the magnetization of one layer can be switched by 180° without disturbing the other two. Phase-sensitive detection is accomplished using a superconducting quantum interference device, or SQUID. Such a phase-controllable junction could be used as the memory element in a fully superconducting computer.

Synthesis and reactivity of a 4His enzyme model complex.

A new iron(II) complex has been prepared and characterized. [Fe(TrIm)4(OTf)2] (1, TrIm = 1-Tritylimidazole). The solid state structure of 1 has been determined by X-ray crystallography. Compound 1 crystallizes in triclinic space group P1̄, with a = 13.342(7) Å, b = 13.5131(7) Å and c = 13.7025(7) Å. The iron center resides in distorted octahedral geometry coordinated to four equatorial imidazole groups and two axial triflate oxygens groups. The complex is high spin between 20 K and 300 K as indicated by variable field variable temperature magnetic measurements. A fit of the magnetic data yielded g = 2.24 and D = -0.80 cm-1. A large HOMO-LUMO gap energy (3.89 eV) exists for 1 indicating high stability. Addition of H2O2 or t BuOOH to 1 results in formation of an oxygenated intermediate which upon decomposition results in oxidation of the trityl substituent on the imidazole ligand.

Thermally Induced Oxidation of [FeII(tacn)2](OTf)2 (tacn = 1,4,7-triazacyclononane).

We previously reported the spin-crossover (SC) properties of [FeII(tacn)2](OTf)2 (1) (tacn = 1,4,7-triazacyclononane) [Eur. J. Inorg. Chem. 2013, 2115]. Upon heating under dynamic vacuum, 1 undergoes oxidation to generate a low spin iron(III) complex. The oxidation of the iron center was found to be facilitated by initial oxidation of the ligand via loss of an H atom. The resulting complex was hypothesized to have the formulation [FeIII(tacn)(tacn-H)](OTf)2 (2) where tacn-H is N-deprotonated tacn. The formulation was confirmed by ESI-MS. The powder EPR spectrum of the oxidized product at 77 K reveals the formation of a low-spin iron(III) species with rhombic spectrum (g = 1.98, 2.10, 2.19). We have indirectly detected H2 formation during the heating of 1 by reacting the headspace with HgO. Formation of water (1HNMR in anhydrous d6-DMSO) and elemental mercury were observed. To further support this claim, we independently synthesized [FeIII(tacn)2](OTf)3 (3) and treated it with one equiv base yielding 2. The structures of 3 was characterized by X-ray crystallography. Compound 2 also exhibits a low spin iron(III) rhombic signal (g = 1.97, 2.11, 2.23) in DMF at 77 K. Variable temperature magnetic susceptibility measurements indicate that 3 undergoes gradual spin increase from 2 to 400 K. DFT studies indicate that the deprotonated nitrogen in 2 forms a bond to iron(III) exhibiting double bond character (Fe-N, 1.807 Å).

New binuclear Mn(II) and Fe(II) complexes supported by 1,4,8-triazacycloundecane.

Two new binuclear metal complexes supported by 1,4,8-triazacycloundecane (tacud) are reported. [Fe(2)(tacud)(2)(µ-Cl)(2)Cl(2)] (1) and [Mn(2)(tacud)(2)(µ-Cl)(2)Cl(2)] (2) are isomorphs consisting of bis(µ-chloro) bridged metal centers along with terminal chloro groups and tacud ligands. Both compounds 1 and 2 crystallize in the P1 space group. For 1, a = 7.7321(12) Å, b = 7.8896(12) Å, c = 11.4945(17) Å, α = 107.832(2)°, ß = 107.827(2)°, γ = 92.642(2)°, V = 627.85(17) Å(3) and Z = 1. For 2, a = 7.7607(12) Å, b = 7.9068(12) Å, c = 11.6111(18) Å, α = 108.201(2)°, ß = 108.041(2)°, γ = 92.118(3)°, V = 636.47(17) Å(3) and Z = 1. Variable-temperature and variable-field magnetic susceptibility studies on 1 indicate the presence of weak ferromagnetic interactions between the high-spin iron(ii) centers in the dimer (J = + 1.6 cm(-1)) and the crystalline field anisotropy of the ferrous ion (D = - 2.8, E = - 0.1 cm(-1)). Variable temperature magnetic susceptometry studies on 2 indicate that weak antiferromagnetic coupling exists between the manganese(ii) centers (J = - 1.8 cm(-1)). Compounds 1 and 2 retain their dinuclearity in weakly coordinating or low polarity solvents, while both become mononuclear in solvents such as methanol.

Enhancing reactivity via structural distortion.

To examine how small structural changes influence the reactivity and magnetic properties of biologically relevant metal complexes, the reactivity and magnetic properties of two structurally related five-coordinate Fe(III) thiolate compounds are compared. (Et,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))]PF(6) (3) is synthesized via the abstraction of a sulfur from alkyl persulfide ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))-S(pers)]PF(6) (2) using PEt(3). (Et,Pr)-3 is structurally related to (Pr,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))]PF(6) (1), a nitrile hydratase model compound previously reported by our group, except it contains one fewer methylene unit in its ligand backbone. Removal of this methylene distorts the geometry, opens a S-Fe-N angle by approximately 10 degrees, alters the magnetic properties by stabilizing the S = 1/2 state relative to the S = 3/2 state, and increases reactivity. Reactivity differences between 3 and 1 were assessed by comparing the thermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOH solutions. The ambient temperature K(eq) describing the equilibrium between five-coordinate 1 or 3 and azide-bound 1-N(3) or 3-N(3) in MeOH is approximately 10 times larger for the (Et,Pr) system. In CH(2)Cl(2), azide binds approximately 3 times faster to 3 relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N(3) relative to 1-N(3). The increased on rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to an approximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e., spin-state differences). Dissociation rates from both 3-N(3) and 1-N(3) are much faster than one would expect for low-spin Fe(III). Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in these structures.