Protein Unfolding: 1H‐NMR Studies of Paramagnetic Ferricytochrome c‐550 from Horse Heart

Electronic transfer protein cytochrome c‐550 from horse heart is studied in the unfolded state by means of paramagnetic ¹H NMR. The protein contains 104 aminoacid residues and a heme group with low spin Fe^III ion in the oxidized form of protein. The global secondary structure is of the α‐helix type as occurs in the case of very other cytochromes c investigated such as cyt c‐550 from Thiobacillus versutus or cyt c‐551 from Pseudomonas aeruginosa. We have studied the coordination characteristic and electronic properties of heme iron horse heart ferricytochrome c‐550 at increasing denaturing conditions (up to 3.1 M GuHCl and 288‐323 K). The ¹H T₁ values of the signals were measured and some resonance assignments made based on EXSY experiments. The electronic structure of the iron(III) is discussed on the basis of the temperature dependence of the isotropic shifts and relaxation times. These results show that it is produced a change of spin, from low‐spin iron(III) (²T₂, S=1/2) in the folded state to high‐spin iron(III) (⁶A₁, S=5/2) in the unfolded state. It seems to be possible that in the opened structure the ferricyt c‐550 loses one axial ligand (His/‐) appearing the spin transition.     

Identification of the Chromophore in the Apatite Pigment [Sr10(PO4)6(CuxOH1−x−y)2]: Linear OCuO− Featuring a Resonance Raman Effect,an Extreme Magnetic Anisotropy,and Slow Spin Relaxation

A new chromophore has been identified in copper‐doped apatite pigments having the general composition [Sr₁₀(PO₄)₆(Cu_xOH_1?x?y)₂], in which x=0.1, 0.3 and y=0.01–0.42. By using X‐ray absorption spectroscopy, low‐temperature magnetization measurements, and synchrotron X‐ray powder structure refinement, it has been shown that the oxygenated compounds contain simultaneously diamagnetic Cu¹⁺ and paramagnetic Cu³⁺ with S=1. Cu³⁺ is located at the same crystallographic position as Cu¹⁺, being linearly coordinated by two oxygen atoms and forming the OCuO^? anion. The Raman spectroscopy study of [A₁₀(PO₄)₆(Cu_xOH_1?x?y)₂,], in which A=Ca, Sr, Ba, reveals resonance bands at 651–656 cm^?1 assigned to the symmetric stretching vibration (ν₁) of OCuO^?. The strontium apatite pigment exhibits a strong paramagnetic anisotropy with an unprecedentedly large negative zero‐field splitting parameter (D) of ≈?400 cm^?1. The extreme magnetic anisotropy causes slow magnetization relaxation with relaxation times (τ) up to 0.3 s at T=2 K, which relates the compounds to single‐ion magnets. At low temperature, τ is limited by a spin quantum‐tunneling, whereas at high temperature a thermally activated relaxation prevails with U_eff≈48 cm^?1. Strong dependence of τ on the paramagnetic center concentration at low temperature suggests that the spin‐spin relaxation dominates in the spin quantum‐tunneling process. The compound is the first example of a d‐metal‐based single‐ion magnet with S=1, the smallest spin at which an energy barrier arises for the spin flipping.     

Spin Crossover in Nitrito‐Myoglobin as Revealed by Resonance Raman Spectroscopy

The myoglobin (Mb) heme Fe‐O‐N=O and heme Fe‐O‐N=O/2‐nitrovinyl species have been characterized by resonance Raman spectroscopy. In the heme Fe‐O‐N=O species, the bound nitrite ligand is removed by solvent exchange, thus reforming metmyoglobin (metMb). The high‐spin heme Fe‐O‐N=O unit is converted into a low‐spin heme Fe‐O‐N=O/2‐nitrovinyl species that can be reversibly switched between a low‐ and a high‐spin state without removing the bound nitrite ligand, as observed in the case of the heme Fe‐O‐N=O species. This spin‐state change is likely to be accompanied by a general structural rearrangement in the protein‐binding pocket. This example is the first of a globin protein that can reversibly change its metal spin state through an internal perturbation. These findings provide a basis for understanding the structure–function relationship of the spin cross found in other metalloenzymes and Fe^III–porphyrin complexes.     

Tetrazine Chromophore‐Based Metal–Organic Frameworks with Unusual Configurations: Synthetic,Structural, Theoretical,Fluorescent, and Nonlinear Optical Studies

Three unusual three‐dimensional (3D) tetrazine chromophore‐based metal–organic frameworks (MOFs) {(Et₄N)[WS₄Cu₃(CN)₂(4,4′‐pytz)_0.5]}_n ( 1 ), {[MoS₄Cu₄(CN)₂(4,4′‐pytz)₂] ? CH₂Cl₂}_n ( 2 ), and {[WS₄Cu₃(4,4′‐pytz)₃] ? [N(CN)₂]}_n ( 3 ; 4,4′‐pytz=3,6‐bis(4‐pyridyl)tetrazine) have been synthesized and characterized by using FTIR and UV/Vis spectroscopy, elemental analysis, powder X‐ray diffraction, gel permeation chromatography, steady‐state fluorescence, and thermogravimetric analysis; their identities were confirmed by single‐crystal X‐ray diffraction studies. MOF 1 possesses the first five‐connected M/S/Cu (M=Mo, W) framework with an unusual 3D (4⁴?6⁶) topology constructed from T‐shaped [WS₄Cu₃]⁺ clusters as nodes and single CN^?/4,4′‐pytz bridges as linkers. MOF 2 features a novel 3D MOF structure with (4²⁰?6⁸) topology, in which the bridging 4,4′‐pytz ligands exhibit unique distorted arch structures. MOF 3 displays the first 3D MOF structure based on flywheel‐shaped [WS₄Cu₃]⁺ clusters with a non‐interpenetrating honeycomb‐like framework and a heavily distorted “ACS” topology. Steady‐state fluorescence studies of 1 – 3 reveal significant fluorescence emissions. The nonlinear optical (NLO) properties of 1 – 3 were investigated by using a Z‐scan technique with 5 ns pulses at λ=532 nm. The Z‐scan experimental results show that the π‐delocalizable tetrazine‐based 4,4′‐pytz ligands contribute to the strong third‐order NLO properties exhibited by 1 – 3 . Time‐dependent density functional theory studies afforded insight into the electronic transitions and spectral characterization of these functionalized NLO molecular materials.     

Distribution of the methyl radical adduct of N‐benzylidene‐1‐diethoxyphosphoryl‐1‐methylethylamine N‐oxide‐type nitrones in water–sodium dodecyl sulfate micelles: an electron paramagnetic resonance spin trapping study

The behavior of the methyl radical adduct of six β‐phosphorylated nitrones in the N‐benzylidene‐1‐diethoxyphosphoryl‐1‐methylethylamine N‐oxide series in the presence of sodium dodecyl sulfate (SDS) micelles was followed by electron paramagnetic resonance spectroscopy. Except when the highly hydrophilic trap 4‐PyOPN (2) was used, all the adducts were found to partition significantly between micelles and the bulk aqueous phase. The average correlation time τ of the exchange of spin adducts between SDS micelles and water was found to be in the range 5 × 10^?8—4 × 10^?7 s, which is in the region of the life time of an SDS monomer in the micelle structure. In each case, the adduct affinity for the micelles has been quantified by evaluating its micelle–water distribution coefficient K_d. Copyright © 2002 John Wiley & Sons, Ltd.     

Two Extended 5‐Methyltetrazolate‐Based Magnetic Complexes: Synthesis,Structure, and Magnetic Properties

Two 5‐methyl‐tetrazolate (mtz^–)‐based paramagnetic metal coordination polymers, {[Cu₂(H₂O)₂(mtz)(μ₃‐OH)(nip)] · H₂O}_n ( 1 ) and [Cu(H₂O)(mtz)₂]_n ( 2 ), were obtained in the presence and absence of aromatic 5‐nitroisophathalate (nip^2–) coligand by varying the preparation methods. Structural determinations reveal that 1 is a three‐dimensional (3D) framework with corner‐sharing triangular ribbons infinitely extended by ditopic nip^2– connectors. In contrast, 2 is a coplanar (4,4) layer constructed from square‐pyramidal Cu^II ions and μ‐N1,N4‐mtz^– linkers, which is further assembled into a 3D supramolecular network by interlayer hydrogen‐bonding interactions. Magnetically, spin‐frustrated antiferromagnetic ordering is observed in the 3D framework of 1 and canted antiferromagnetic behavior with a slight spin‐flop transition is presented in 2 , which are structurally resulting from the locally Δ‐ribbon in 1 and asymmetric magnetic superexchange mediated by the μ‐N1,N4‐mtz^– bridge of 2 .     

Absolute Minimal Sampling of Homonuclear 2D NMR TOCSY Spectra for High‐Throughput Applications of Complex Mixtures

Modern applications of 2D NMR spectroscopy to diagnostic screening, metabolomics, quality control, and other high‐throughput applications are often limited by the time‐consuming sampling requirements along the indirect time domain t ₁. 2D total correlation spectroscopy (TOCSY) provides unique spin connectivity information for the analysis of a large number of compounds in complex mixtures, but standard methods typically require >100 t ₁ increments for an accurate spectral reconstruction, rendering these experiments ineffective for high‐throughput applications. For a complex metabolite mixture it is demonstrated that absolute minimal sampling (AMS), based on direct fitting of resonance frequencies and amplitudes in the time domain, yields an accurate spectral reconstruction of TOCSY spectra using as few as 16 t ₁ points. This permits the rapid collection of homonuclear 2D NMR experiments at high resolution with measurement times that previously were only the realm of 1D experiments.     

Stacking Structure of Confined 1‐Butanol in SBA‐15 Investigated by Solid‐State NMR Spectroscopy

Understanding the complex thermodynamic behavior of confined amphiphilic molecules in biological or mesoporous hosts requires detailed knowledge of the stacking structures. Here, we present detailed solid‐state NMR spectroscopic investigations on 1‐butanol molecules confined in the hydrophilic mesoporous SBA‐15 host. A range of NMR spectroscopic measurements comprising of ¹H spin–lattice (T₁), spin–spin (T₂) relaxation, ¹³C cross‐polarization (CP), and ¹H,¹H two‐dimensional nuclear Overhauser enhancement spectroscopy (¹H,¹H 2D NOESY) with the magic angle spinning (MAS) technique as well as static wide‐line ²H NMR spectra have been used to investigate the dynamics and to observe the stacking structure of confined 1‐butanol in SBA‐15. The results suggest that not only the molecular reorientation but also the exchange motions of confined molecules of 1‐butanol are extremely restricted in the confined space of the SBA‐15 pores. The dynamics of the confined molecules of 1‐butanol imply that the ¹H,¹H 2D NOESY should be an appropriate technique to observe the stacking structure of confined amphiphilc molecules. This study is the first to observe that a significant part of confined 1‐butanol molecules are orientated as tilted bilayered structures on the surface of the host SBA‐15 pores in a time‐average state by solid‐state NMR spectroscopy with the ¹H,¹H 2D NOESY technique.     

Coordination‐Induced Spin‐State Switching of an Aminyl‐Radical‐Bridged Nickel(II) Porphyrin Dimer between Doublet and Sextet States

A bis(Ni^II‐porphyrinyl)aminyl radical with meso‐C₆F₅ groups was prepared as a spin‐delocalized stable aminyl radical with a doublet spin state. Upon addition of pyridine, both Ni^II centers became hexacoordinated by accepting two axial pyridines, which triggered a spin‐state change of the Ni^II centers from diamagnetic (S=0) to paramagnetic (S=1). The resulting high‐spin Ni^II centers interact with the aminyl radical ferromagnetically to give rise to an overall sextet state (S=5/2). Importantly, this coordination‐induced spin‐state switching can be conducted in a reversible manner, in that washing of the high‐spin radical with aqueous hydrochloric acid regenerates the original doublet radical in good yield.     

Characterization of Mononuclear Non‐heme Iron(III)‐Superoxo Complex with a Five‐Azole Ligand Set

Reaction of O₂ with a high‐spin mononuclear iron(II) complex supported by a five‐azole donor set yields the corresponding mononuclear non‐heme iron(III)–superoxo species, which was characterized by UV/Vis spectroscopy and resonance Raman spectroscopy. ¹H NMR analysis reveals diamagnetic nature of the superoxo complex arising from antiferromagnetic coupling between the spins on the low‐spin iron(III) and superoxide. This superoxo species reacts with H‐atom donating reagents to give a low‐spin iron(III)–hydroperoxo species showing characteristic UV/Vis, resonance Raman, and EPR spectra.     

Structure Identification of Aporphine Alkaloids by On‐Line Coupling of HPLC‐NMR with Loop Storage

The potential of coupling HPLC separation methodology to on‐line high resolution nuclear magnetic resonance (NMR) spectroscopy has been demonstrated with a mixture of nine aporphine alkaloids. A loop storage procedure after separation has facilitated the identification of closely eluted peaks (α = k₂/k₁ = 1.01 and Δv = 0.37 min = 367 μL). These were collected off‐line and identified by a 1D ¹H NMR spectrum. For this purpose an automated procedure has been implemented which includes the use of shaped pulses, multiple solvents suppression and ¹³C satellite suppression of acetonitrile.     

Polarized Neutron Diffraction to Probe Local Magnetic Anisotropy of a Low‐Spin Fe(III) Complex

We have determined by polarized neutron diffraction (PND) the low‐temperature molecular magnetic susceptibility tensor of the anisotropic low‐spin complex PPh₄[Fe^III(Tp)(CN)₃]?H₂O. We found the existence of a pronounced molecular easy magnetization axis, almost parallel to the C₃ pseudo‐axis of the molecule, which also corresponds to a trigonal elongation direction of the octahedral coordination sphere of the Fe^III ion. The PND results are coherent with electron paramagnetic resonance (EPR) spectroscopy, magnetometry, and ab initio investigations. Through this particular example, we demonstrate the capabilities of PND to provide a unique, direct, and straightforward picture of the magnetic anisotropy and susceptibility tensors, offering a clear‐cut way to establish magneto‐structural correlations in paramagnetic molecular complexes.     

Large Easy‐Plane Magnetic Anisotropy in a Three‐Coordinate Cobalt(II) Complex [Li(THF)4][Co(NPh2)3]

Magnetic anisotropy is the key element in the construction of single‐ion magnets, a kind of nanomagnets for high‐density information storage. This works describes an unusual large easy‐plane magnetic anisotropy (with a zero‐field splitting parameter D of +40.2 cm^?1), mainly arising from the second‐order spin‐orbit coupling effect in a trigonal‐planar Co^II complex [Li(THF)₄][Co(NPh₂)₃], revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations. Meanwhile, the field‐induced slow magnetic relaxation in this complex was mainly attributed to the Raman process.     

Breaking the Semi‐Quinoid Structure: Spin‐Switching from Strongly Coupled Singlet to Polarized Triplet State

2,7‐TMPNO (4,5,9,10‐tetramethoxypyrene‐2,7‐bis(tert‐butylnitroxide)) was found to exist in semi‐quinoid form with unprecedented strong intramolecular magnetic exchange interaction of 2 J/k_B=1185 K operating over a distance of 10 Å. Structural transformations with the activation energy of ΔE_eq=949 K were observed by varying the temperature, from more quinoid structure at low temperature to more biradicaloid structure at higher temperature. Moreover, this molecule undergoes a transient spin transition from singlet to polarized triplet state upon photoexcitation revealed by TREPR spectroscopy. The spin Hamiltonian parameters were determined to be S=1, g=2.0065, D=?0.0112 cm^?1, and E=?0.0014 cm^?1 by spectral simulation with the hybrid Eigenfield/exact diagonalization method.     

O2 Binding to Heme is Strongly Facilitated by Near‐Degeneracy of Electronic States

This paper reports the computed O₂ binding to heme, which for the first time explains experimental enthalpies for this process of central importance to bioinorganic chemistry. All four spin states along the relaxed Fe? O₂‐binding curves were optimized using the full heme system with dispersion, thermodynamic, and scalar‐relativistic corrections, applying several density functionals. When including all these physical terms, the experimental enthalpy of O₂ binding (?59 kJ mol^?1) is closely reproduced by TPSSh‐D3 (?66 kJ mol^?1). Dispersion changes the potential energy surfaces and leads to the correct electronic singlet and heptet states for bound and dissociated O₂. The experimental activation enthalpy of dissociation (～82 kJ mol^?1) was also accurately computed (～75 kJ mol^?1) with an actual barrier height of ～60 kJ mol^?1 plus a vibrational component of ～10 and ～5 kJ mol^?1 due to the spin‐forbidden nature of the process, explaining the experimentally observed difference of ～20 kJ mol^?1 in enthalpies of binding and activation. Most importantly, the work shows how the nearly degenerate singlet and triplet states increase crossover probability up to ～0.5 and accelerate binding by ～100 times, explaining why the spin‐forbidden binding of O₂ to heme, so fundamental to higher life forms, is fast and reversible.     

Hadamard Homonuclear Broadband Decoupled TOCSY NMR: Improved Efficacy in Detecting Long‐range Chemical Shift Correlations

The importance of Hadamard encoding pulses in one‐dimensional pure shift yielded by the chirp excitation version of selective total correlation spectroscopy (1D PSYCHE–TOCSY) experiments is discussed for chemical‐shift analysis of complex natural products at ultrahigh resolution. Herein, we adapted H_n Hadamard matrices to 1D PSYCHE–TOCSY and observed an overall circa square root of n‐fold enhancement in the signal‐to‐noise (S/N) ratio when compared to conventional 1D PSYCHE–TOCSY recorded by refocusing only one spin at a time. This enhancement in S/N facilitates the observation of very weak long‐range chemical‐shift correlations from Hadamard‐encoded PSYCHE–TOCSY (HE–PSYCHE–TOCSY). The proposed method will have a significant impact on structure determination of complex isolated/ synthetic natural products.     

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm^?1, E_T=0 cm^?1), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm^?1, E_Q=0.035 cm^?1), and septet trinitrene 8 (g=2.003, D_S=?0.0904 cm^?1, E_S=?0.0102 cm^?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ～1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations.     

Synthesis,Structure, and Magnetic Properties of Three Novel Sandwich‐type Tungstobismuthates with Triethanolamine

Three novel sandwich‐type polyoxotungstates ( 1 – 3 ) were synthesized in good yield using an in‐situ conventional solution synthesis method by reaction in aqueous media below 80 °C. Compounds 1 – 3 represent the first structurally characterized β‐B‐BiW₉ sandwich‐type polyoxometalates with triethanolamine cations. All three compounds have the same building unit [(X(H₂O)₃)₂(X_0.5W_0.5O)₂(β‐B‐BiW₉O₃₃)₂)]^10– [X = Mn^II ( 1 ), Co^II ( 2 ), Ni^II ( 3 )]. The adjacent units of 1 or 2 are joined by Na⁺ cations in different ways to construct 1D chains or 2D sheets. A 3D supramolecular structure is further formed by hydrogen bond interactions among water molecules and protonated triethanolamine cations. Meanwhile only compound 3 shows a 0D structure. The compounds were characterized by elemental analysis, IR spectroscopy, TG analysis, and single‐crystal X‐ray diffraction. Magnetic measurements on a sample of 1 show the presence of paramagnetic interactions.     

ESR Studies on a Friedel‐Crafts Alkylation Reaction in [bmim]Cl‐AlCl3 Ionic Liquid

The paramagnetic complexes formed in Friedel‐Crafts alkylation reaction systems are invistigated by electron spin resonance (ESR) spectroscopy, in room temperature ionic liquids system 1‐butyl‐3‐methyl‐limidazolium chloride‐aluminium chloride ([bmim]Cl‐AlCl₃). The results indicate that ESR spectra observed are due to polycyclic aromatic radical cations formed from their parent hydrocarbons. ESR spectrum of spin adduct is obtained in an ionic liquid system composed of [bmim]Cl‐AlCl₃. In acidic solution the ¹⁴N hyperfine coupling constant of 4‐oxo‐TEMPO, 2.15 mT, is appreciably larger due to an adduct formed with AlCl₃.