Crystallographic order and decomposition of [MnIII6CrIII]3+ single-molecule magnets deposited in submonolayers and monolayers on HOPG studied by means of molecular resolved Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy in UHV

Gryzia A, Volkmann T, Brechling A, et al. Crystallographic order and decomposition of [MnIII6CrIII]3+ single-molecule magnets deposited in submonolayers and monolayers on HOPG studied by means of molecular resolved Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy in UHV. Nanoscale Research Letters. 2014;9(1): 60.Monolayers and submonolayers of [MnIII6CrIII]3+ single-molecule magnets (SMMs) adsorbed on highly oriented pyrolytic graphite (HOPG) using the droplet technique characterized by non-contact atomic force microscopy (nc-AFM) as well as by Kelvin probe force microscopy (KPFM) show island-like structures with heights resembling the height of the molecule. Furthermore, islands were found which revealed ordered 1D as well as 2D structures with periods close to the width of the SMMs. Along this, islands which show half the heights of intact SMMs were observed which are evidences for a decomposing process of the molecules during the preparation. Finally, models for the structure of the ordered SMM adsorbates are proposed to explain the observations

Across the tree of life, radiation resistance is governed by antioxidant Mn2+, gauged by paramagnetic resonance

Despite concerted functional genomic efforts to understand the complex phenotype of ionizing radiation (IR) resistance, a genome sequence cannot predict whether a cell is IR-resistant or not. Instead, we report that absorption-display electron paramagnetic resonance (EPR) spectroscopy of nonirradiated cells is highly diagnostic of IR survival and repair efficiency of DNA double-strand breaks (DSBs) caused by exposure to gamma radiation across archaea, bacteria, and eukaryotes, including fungi and human cells. IR-resistant cells, which are efficient at DSB repair, contain a high cellular content of manganous ions (Mn2+) in high-symmetry (H) antioxidant complexes with small metabolites (e.g., orthophosphate, peptides), which exhibit narrow EPR signals (small zero-field splitting). In contrast, Mn2+ ions in IR-sensitive cells, which are inefficient at DSB repair, exist largely as low-symmetry (L) complexes with substantially broadened spectra seen with enzymes and strongly chelating ligands. The fraction of cellular Mn2+ present as H-complexes (H-Mn2+), as measured by EPR of live, nonirradiated Mn-replete cells, is now the strongest known gauge of biological IR resistance between and within organisms representing all three domains of life: Antioxidant H-Mn2+ complexes, not antioxidant enzymes (e.g., Mn superoxide dismutase), govern IR survival. As the pool of intracellular metabolites needed to form H-Mn2+ complexes depends on the nutritional status of the cell, we conclude that IR resistance is predominantly a metabolic phenomenon. In a cross-kingdom analysis, the vast differences in taxonomic classification, genome size, and radioresistance between cell types studied here support that IR resistance is not controlled by the repertoire of DNA repair and antioxidant enzymes

High-Resolution ENDOR Spectroscopy Combined with Quantum Chemical Calculations Reveals the Structure of Nitrogenase Janus Intermediate E\u3csub\u3e4\u3c/sub\u3e(4H)

We have shown that the key state in N2 reduction to two NH3 molecules by the enzyme nitrogenase is E4(4H), the Janus intermediate, which has accumulated four [e-/H+] and is poised to undergo reductive elimination of H2 coupled to N2 binding and activation. Initial 1H and 95Mo ENDOR studies of freeze-trapped E4(4H) revealed that the catalytic multimetallic cluster (FeMo-co) binds two Fe-bridging hydrides, [Fe-H-Fe]. However, the analysis failed to provide a satisfactory picture of the relative spatial relationships of the two [Fe-H-Fe]. Our recent density functional theory (DFT) study yielded a lowest-energy form, denoted as E4(4H)(a), with two parallel Fe-H-Fe planes bridging pairs of anchor Fe on the Fe2,3,6,7 face of FeMo-co. However, the relative energies of structures E4(4H)(b), with one bridging and one terminal hydride, and E4(4H)(c), with one pair of anchor Fe supporting two bridging hydrides, were not beyond the uncertainties in the calculation. Moreover, a structure of V-dependent nitrogenase resulted in a proposed structure analogous to E4(4H)(c), and additional structures have been proposed in the DFT studies of others. To resolve the nature of hydride binding to the Janus intermediate, we performed exhaustive, high-resolution CW-stochastic 1H-ENDOR experiments using improved instrumentation, Mims 2H ENDOR, and a recently developed pulsed-ENDOR protocol ( PESTRE ) to obtain absolute hyperfine interaction signs. These measurements are coupled to DFT structural models through an analytical point-dipole Hamiltonian for the hydride electron-nuclear dipolar coupling to its anchoring Fe ions, an approach that overcomes limitations inherent in both experimental interpretation and computational accuracy. The result is the freeze-trapped, lowest-energy Janus intermediate structure, E4(4H)(a)

Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [Mn^III₆Os^III]³⁺: Promoting SMM Behavior through 3d–5d Transition Metal Substitution

The reaction of the <i>in situ</i> generated trinuclear triplesalen complex [(talen^{<i>t</i>‑Bu₂})­Mn^III₃(solv)_<i>n</i>]³⁺ with (Ph₄P)₃[Os^III(CN)₆] and NaClO₄·H₂O affords <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>(ClO₄)₃ (= [{(talen^{<i>t</i>‑Bu₂})­Mn^III₃}₂­{Os^III(CN)₆}]­(ClO₄)₃) in the presence of the oxidizing agent [(tacn)₂Ni^III]­(ClO₄)₃ (tacn =1,4,7-triazacyclononane), while the reaction of [(talen^{<i>t</i>‑Bu₂})­Mn^III₃(solv)_<i>n</i>]³⁺ with K₄[Os^II(CN)₆] and NaClO₄·H₂O yields <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>(ClO₄)₂ under an argon atmosphere. The molecular structure of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> as determined by single-crystal X-ray diffraction is closely related to the already published <b>[Mn</b>^<b>III</b>_<b>6</b><b>M</b>^<b>c</b><b>]</b>^<b>3+</b> complexes (M^c = Cr^III, Fe^III, Co^III, Mn^III). The half-wave potential of the Os^III/Os^II couple is <i>E</i>_1/2 = 0.07 V vs Fc⁺/Fc. The FT-IR and electronic absorption spectra of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>^<b>2+</b> and <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> exhibit distinct features of dicationic and tricationic <b>[Mn</b>^<b>III</b>_<b>6</b><b>M</b>^<b>c</b><b>]</b>^{<b><i>n</i>+</b>} complexes, respectively. The dc magnetic data (μ_eff vs <i>T</i>, <i>M</i> vs <i>B</i>, and VTVH) of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>^<b>2+</b> are successfully simulated by a full-matrix diagonalization of a spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the <b>D</b>-tensors, and Zeeman interaction, indicating antiferromagnetic Mn^III–Mn^III interactions within the trinuclear triplesalen subunits (<i>J</i>_Mn–Mn⁽¹⁾ = −(0.53 ± 0.01) cm^–1, <i>Ĥ</i>_ex = −2∑_{<i>i</i><<i>j</i>} <i>J</i>_<i>ij</i><b>Ŝ</b>_<i>i</i>·<b>Ŝ</b>_<i>j</i>) as well as across the central Os^II ion (<i>J</i>_Mn–Mn^(2,cis) = −(0.06 ± 0.01) cm^–1, <i>J</i>_Mn–Mn^(2,trans) = −(0.15 ± 0.01) cm^–1), while <i>D</i>_Mn = −(3.9 ± 0.1) cm^–1. The μ_eff vs <i>T</i> data of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> are excellently reproduced assuming an anisotropic Ising-like Os^III–Mn^III superexchange with a nonzero component <i>J</i>_Os–Mn^(aniso) = −(11.0 ± 1.0) cm^–1 along the Os–Mn direction, while <i>J</i>_Mn–Mn = −(0.9 ± 0.1) cm^–1 and <i>D</i>_Mn = −(3.0 ± 1.0) cm^–1. Alternating current measurements indicate a slower relaxation of the magnetization in the SMM <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> compared to the 3d analogue <b>[Mn</b>^<b>III</b>_<b>6</b><b>Fe</b>^<b>III</b><b>]</b>^<b>3+</b> due to the stronger and anisotropic M^c–Mn^III exchange interaction

Strong and Anisotropic Superexchange in the Single-Molecule Magnet (SMM) [Mn^III₆Os^III]³⁺: Promoting SMM Behavior through 3d–5d Transition Metal Substitution

The reaction of the <i>in situ</i> generated trinuclear triplesalen complex [(talen^{<i>t</i>‑Bu₂})­Mn^III₃(solv)_<i>n</i>]³⁺ with (Ph₄P)₃[Os^III(CN)₆] and NaClO₄·H₂O affords <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>(ClO₄)₃ (= [{(talen^{<i>t</i>‑Bu₂})­Mn^III₃}₂­{Os^III(CN)₆}]­(ClO₄)₃) in the presence of the oxidizing agent [(tacn)₂Ni^III]­(ClO₄)₃ (tacn =1,4,7-triazacyclononane), while the reaction of [(talen^{<i>t</i>‑Bu₂})­Mn^III₃(solv)_<i>n</i>]³⁺ with K₄[Os^II(CN)₆] and NaClO₄·H₂O yields <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>(ClO₄)₂ under an argon atmosphere. The molecular structure of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> as determined by single-crystal X-ray diffraction is closely related to the already published <b>[Mn</b>^<b>III</b>_<b>6</b><b>M</b>^<b>c</b><b>]</b>^<b>3+</b> complexes (M^c = Cr^III, Fe^III, Co^III, Mn^III). The half-wave potential of the Os^III/Os^II couple is <i>E</i>_1/2 = 0.07 V vs Fc⁺/Fc. The FT-IR and electronic absorption spectra of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>^<b>2+</b> and <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> exhibit distinct features of dicationic and tricationic <b>[Mn</b>^<b>III</b>_<b>6</b><b>M</b>^<b>c</b><b>]</b>^{<b><i>n</i>+</b>} complexes, respectively. The dc magnetic data (μ_eff vs <i>T</i>, <i>M</i> vs <i>B</i>, and VTVH) of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>II</b><b>]</b>^<b>2+</b> are successfully simulated by a full-matrix diagonalization of a spin-Hamiltonian including isotropic exchange, zero-field splitting with full consideration of the relative orientation of the <b>D</b>-tensors, and Zeeman interaction, indicating antiferromagnetic Mn^III–Mn^III interactions within the trinuclear triplesalen subunits (<i>J</i>_Mn–Mn⁽¹⁾ = −(0.53 ± 0.01) cm^–1, <i>Ĥ</i>_ex = −2∑_{<i>i</i><<i>j</i>} <i>J</i>_<i>ij</i><b>Ŝ</b>_<i>i</i>·<b>Ŝ</b>_<i>j</i>) as well as across the central Os^II ion (<i>J</i>_Mn–Mn^(2,cis) = −(0.06 ± 0.01) cm^–1, <i>J</i>_Mn–Mn^(2,trans) = −(0.15 ± 0.01) cm^–1), while <i>D</i>_Mn = −(3.9 ± 0.1) cm^–1. The μ_eff vs <i>T</i> data of <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> are excellently reproduced assuming an anisotropic Ising-like Os^III–Mn^III superexchange with a nonzero component <i>J</i>_Os–Mn^(aniso) = −(11.0 ± 1.0) cm^–1 along the Os–Mn direction, while <i>J</i>_Mn–Mn = −(0.9 ± 0.1) cm^–1 and <i>D</i>_Mn = −(3.0 ± 1.0) cm^–1. Alternating current measurements indicate a slower relaxation of the magnetization in the SMM <b>[Mn</b>^<b>III</b>_<b>6</b><b>Os</b>^<b>III</b><b>]</b>^<b>3+</b> compared to the 3d analogue <b>[Mn</b>^<b>III</b>_<b>6</b><b>Fe</b>^<b>III</b><b>]</b>^<b>3+</b> due to the stronger and anisotropic M^c–Mn^III exchange interaction

Preparation of monolayers of [Mn^III ₆Cr^III]³⁺single-molecule magnets on HOPG, mica and silicon surfaces and characterization by means of non-contact AFM

Abstract We report on the characterization of various salts of [MnIII 6CrIII]3+ complexes prepared on substrates such as highly oriented pyrolytic graphite (HOPG), mica, SiO2, and Si3N4. [MnIII 6CrIII]3+ is a single-molecule magnet, i.e., a superparamagnetic molecule, with a blocking temperature around 2 K. The three positive charges of [MnIII 6CrIII]3+ were electrically neutralized by use of various anions such as tetraphenylborate (BPh4 -), lactate (C3H5O3 -), or perchlorate (ClO4 -). The molecule was prepared on the substrates out of solution using the droplet technique. The main subject of investigation was how the anions and substrates influence the emerging surface topology during and after the preparation. Regarding HOPG and SiO2, flat island-like and hemispheric-shaped structures were created. We observed a strong correlation between the electronic properties of the substrate and the analyzed structures, especially in the case of mica where we observed a gradient in the analyzed structures across the surface.</p

Hysteresis in the ground and excited spin state up to 10 T of a [(Mn6MnIII)-Mn-III](3+) triplesalen single-molecule magnet

10.1039/c2sc20649hCHEMICAL SCIENCE392868-288

13C ENDOR Spectroscopy of Lipoxygenase-Substrate Complexes Reveals the Structural Basis for C-H Activation by Tunneling.

In enzymatic C-H activation by hydrogen tunneling, reduced barrier width is important for efficient hydrogen wave function overlap during catalysis. For native enzymes displaying nonadiabatic tunneling, the dominant reactive hydrogen donor-acceptor distance (DAD) is typically ca. 2.7 Å, considerably shorter than normal van der Waals distances. Without a ground state substrate-bound structure for the prototypical nonadiabatic tunneling system, soybean lipoxygenase (SLO), it has remained unclear whether the requisite close tunneling distance occurs through an unusual ground state active site arrangement or by thermally sampling conformational substates. Herein, we introduce Mn2+ as a spin-probe surrogate for the SLO Fe ion; X-ray diffraction shows Mn-SLO is structurally faithful to the native enzyme. 13C ENDOR then reveals the locations of 13C10 and reactive 13C11 of linoleic acid relative to the metal; 1H ENDOR and molecular dynamics simulations of the fully solvated SLO model using ENDOR-derived restraints give additional metrical information. The resulting three-dimensional representation of the SLO active site ground state contains a reactive (a) conformer with hydrogen DAD of ∼3.1 Å, approximately van der Waals contact, plus an inactive (b) conformer with even longer DAD, establishing that stochastic conformational sampling is required to achieve reactive tunneling geometries. Tunneling-impaired SLO variants show increased DADs and variations in substrate positioning and rigidity, confirming previous kinetic and theoretical predictions of such behavior. Overall, this investigation highlights the (i) predictive power of nonadiabatic quantum treatments of proton-coupled electron transfer in SLO and (ii) sensitivity of ENDOR probes to test, detect, and corroborate kinetically predicted trends in active site reactivity and to reveal unexpected features of active site architecture