0000000000051973
AUTHOR
Dariush Hinderberger
Pulse EPR methods for studying chemical and biological samples containing transition metals
This review discusses the application of pulse EPR to the characterization of disordered systems, with an emphasis on samples containing transition metals. Electron nuclear double-resonance (ENDOR), electron-spin-echo envelope-modulation (ESEEM), and double electron-electron resonance (DEER) methodologies are outlined. The theory of field modulation is outlined, and its application is illustrated with DEER experiments. The simulation of powder spectra in EPR is discussed, and strategies for optimization are given. The implementation of this armory of techniques is demonstrated on a rich variety of chemical systems: several porphyrin derivatives that are found in proteins and used as model s…
EPR spectroscopic characterization of persistent germyl-substituted Pb(III)- and Sn(III)-radicals.
In this report we present the synthesis and the detailed electron paramagnetic resonance (EPR) spectroscopic characterization of novel trivalent lead- and tin-based radicals comprising sterically demanding germyl substituents. The investigated radicals are derived from the recently reported trihypersilyl-substituted tetryl radicals *PbHyp3 and *SnHyp3. The tetryl radicals *Pb(Ge(SiMe3)3)3 (8), *Pb(Ge(SiMe3)3)2Si(SiMe3)3 (9), *PbGe(SiMe3)3(Si(SiMe3)3)2 (10), and *Sn(Ge(SiMe3)3)3 (11) show substitution patterns derived from stepwise (9, 10) or complete (8, 11) substitution of hypersilyl groups (Hyp = Si(SiMe3)3) in *PbHyp3 and *SnHyp3 by homologous hypergermyl groups (Hge = Ge(SiMe3)3). They …
Nitronyl Nitroxide Radicals Linked to Exchange‐Coupled Metal Dimers – Studies Using X‐ray Crystallography, Magnetic Susceptibility Measurements, EPR Spectroscopy, and DFT Calculations
To study long-range magnetic interactions between exchange-coupled metal centers and a radical moiety coordinated through a peripheral group, three new homodimetallic complexes with MnII, CoII, and ZnII bridged by a nitronyl nitroxide (NIT) substituted benzoate ligand with the structure [(NIT-C6H4-COO)M2(LR)](ClO4)2 {M = MnII, CoII, and ZnII; NIT = nitronyl nitroxide and LR = N,N,N′,N′-tetrakis(2-benzimidazolylalkyl)-2-hydroxy-1,3-diaminopropane} have been prepared and studied by X-ray crystallography, magnetic susceptibility measurements, EPR spectroscopy, and density functional theory calculations. For comparison, related complexes with MnII and CoII bridged by a diamagnetic nitrobenzoate…
Highly Defined, Colloid‐Like Ionic Clusters in Solution
Many societal challenges at the beginning of the 21st century lead to an apparent and growing need for functional materials and novel ways of materials synthesis and assembly. Rising to the challenge, the utilization of small, self-assembling building blocks for the bottom-up construction of new types of polymers and nanostructures has enjoyed increasing popularity among materials researchers in the recent past. Supramolecular materials like foldamers, surface films, nanoparticles, etc. are created by exploiting noncovalent forces [1] leading to an ordered arrangement of nanoscale building blocks. [2] In the search for new polymers based on noncovalent molecular forces, we are motivated by …
An L2 SUMO interacting motif is important for PML localization and infection of human papillomavirus type 16
Summary Human papillomaviruses (HPV) induce warts and cancers on skin and mucosa. The HPV16 capsid is composed of the proteins L1 and L2. After cell entry and virus disassembly, the L2 protein accompanies the viral DNA to promyelocytic leukaemia nuclear bodies (PML-NBs) within the host nuclei enabling viral transcription and replication. Multiple components of PML-NBs are regulated by small ubiquitin-like modifiers (SUMOs) either based on covalent SUMO modification (SUMOylation), or based on non-covalent SUMO interaction via SUMO interacting motifs (SIMs). We show here that the HPV16 L2 comprises at least one SIM, which is crucial for the L2 interaction with SUMO2 in immunoprecipitation and…
Aggregation behavior of amphiphilic p(HPMA)-co-p(LMA) copolymers studied by FCS and EPR spectroscopy.
A combined study of fluorescence correlation spectroscopy and electron paramagnetic resonance spectroscopy gave a unique picture of p(HPMA)-co-p(LMA) copolymers in aqueous solutions, ranging from the size of micelles and aggregates to the composition of the interior of these self-assembled systems. P(HPMA)-co-p(LMA) copolymers have shown high potential as brain drug delivery systems, and a detailed study of their physicochemical properties can help to elucidate their mechanism of action. Applying two complementary techniques, we found that the self-assembly behavior as well as the strength of hydrophobic attraction of the amphiphilic copolymers can be tuned by the hydrophobic LMA content or…
A Polyphenylene Dendrimer Drug Transporter with Precisely Positioned Amphiphilic Surface Patches
The design and synthesis of a polyphenylene dendrimer (PPD 3) with discrete binding sites for lipophilic guest molecules and characteristic surface patterns is presented. Its semi-rigidity in combination with a precise positioning of hydrophilic and hydrophobic groups at the periphery yields a refined architecture with lipophilic binding pockets that accommodate defined numbers of biologically relevant guest molecules such as fatty acids or the drug doxorubicin. The size, architecture, and surface textures allow to even penetrate brain endothelial cells that are a major component of the extremely tight blood-brain barrier. In addition, low to no toxicity is observed in in vivo studies using…
Effect of chelate ring expansion on Jahn-Teller distortion and Jahn-Teller dynamics in copper(II) complexes.
The expanded ligand N,N'-dimethyl-N,N'-dipyridin-2-yl-pyridin-2,6-diamine (ddpd) coordinates to copper(II) ions in a meridional fashion giving the dicationic complex mer-[Cu(ddpd)(2)](BF(4))(2) (1). In the solid state at temperatures below 100 K the cations of 1 localize in Jahn-Teller elongated CuN(6) polyhedra with the longest Cu-N bond pointing in the molecular x or y directions while the z axis is constrained by the tridentate ddpd ligand. The elongated polyhedra are ordered in an antiferrodistortive way giving an idealized zincblende structure. At higher temperature dynamically averaged (fluxional) polyhedra in the molecular x/y directions are observed by multifrequency variable temper…
Heparin–polynitroxides: Synthesis and preliminary evaluation as cardiovascular EPR/MR imaging probes and extracellular space-targeted antioxidants
We report here the synthesis of heparin-polynitroxide derivatives (HPNs) in which nitroxide moieties are linked either to uronic acid or glycosamine residues of the heparin macromolecule. HPNs have low anticoagulant activity, possess superoxide scavenging properties, bind to the vascular endothelium/extra-cellular matrix and can be detected by EPR and MRI techniques. As the vascular wall-targeted redox-active paramagnetic compounds, HPNs may have both diagnostic (molecular MRI) and therapeutic (ecSOD mimics) applications.
Long-Distance Magnetic Interaction of Exchange-Coupled Copper Dimers with Nitronyl Nitroxide and tert-Butyl Nitroxide Radicals
To study long-distance magnetic interactions between exchange-coupled metal centers and coordinated radical moieties, two new homodimetallic Cu(II) complexes held together by the chelating ligand L(nPr) = N,N,N',N'-tetrakis(N-propyl-2-benzimidazolyl)-2-hydroxy-1,3-diaminopropane and additionally bridged by either a nitronyl nitroxide (NIT) or a tert-butyl nitroxide (NOA) radical-substituted benzoate have been prepared. The complexes have been investigated by X-ray crystallography, magnetic susceptibility measurements, electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) calculations. For comparison additionally the related copper dimer with the bridging di…
How Structure-Related Collapse Mechanisms Determine Nanoscale Inhomogeneities in Thermoresponsive Polymers
Continuous wave electron paramagnetic resonance (CW EPR) spectroscopy on the amphiphilic spin probe TEMPO in solutions of selectively chosen functional, thermoresponsive poly(propylene oxide) (PPO)- and poly(ethylene oxide) (PEO)-based copolymers of both linear and branched structure is used to elucidate their host–guest interactions and inverse phase transitions. Three different fundamental types of host–guest interactions between probes and polymers could be correlated to the phase transition mechanisms (supported by MD simulations), evidencing that these proceed via nanoscale inhomogeneities of the polymers. Because of their ability to host small amphiphilic guest molecules, thermorespon…
Load-collapse-release cascades of amphiphilic guest molecules in charged dendronized polymers through spatial separation of noncovalent forces.
The ability to pack guest molecules into charged dendronized polymers (denpols) and the possibility to release these guest molecules from subsequently densely aggregated denpols in a load-collapse-release cascade is described. Charged denpols, which constitute molecular objects with a persistent, well-defined envelope and interior, are capable of incorporating large amounts of amphiphilic guest molecules. Simultaneously, multivalent ions can coordinate to the surfaces of charged denpols, leading to counterion-induced aggregation of the already guest-loaded host structures. Thus, although the local guest concentration in denpol-based molecular transport might already be initially high due to…
Stereochemical Consequences of Oxygen Atom Transfer and Electron Transfer in Imido/Oxido Molybdenum(IV, V, VI) Complexes with Two Unsymmetric Bidentate Ligands
Two equivalents of the unsymmetrical Schiff base ligand (L(tBu))(-) (4-tert-butyl phenyl(pyrrolato-2-ylmethylene)amine) and MoCl(2)(NtBu)O(dme) (dme = 1,2-dimethoxyethane) gave a single stereoisomer of a mixed imido/oxido Mo(VI) complex 2(tBu). The stereochemistry of 2(tBu) was elucidated using X-ray diffraction, NMR spectroscopy, and DFT calculations. The complex is active in an oxygen atom transfer (OAT) reaction to trimethyl phosphane. The putative intermediate five-coordinate Mo(IV) imido complex coordinates a PMe(3) ligand, giving the six-coordinate imido phosphane Mo(IV) complex 5(tBu). The stereochemistry of 5(tBu) is different from that of 2(tBu) as shown by NMR spectroscopy, DFT ca…
Structure and reactivity of a mononuclear gold(II) complex.
Mononuclear gold(II) complexes are very rare labile species. Transient gold(II) species have been suggested in homogeneous catalysis and in medical applications, but their geometric and electronic structures have remained essentially unexplored: even fundamental data, such as the ionic radius of gold(II), are unknown. Now, an unprecedentedly stable neutral gold(II) complex of a porphyrin derivative has been isolated, and its structural and spectroscopic features determined. The gold atom adopts a 2+2 coordination mode in between those of gold(III) (four-coordinate square planar) and gold(I) (two-coordinate linear), owing to a second-order Jahn–Teller distortion enabled by the relativistical…
Stimuli-Responsive Tertiary Amine Functional PEGs Based on N,N-Dialkylglycidylamines
Amine-functional poly(ethylene glycol) (PEG) copolymers have been prepared that exhibit thermo- and pH- responsive behavior in aqueous solution. Three novel tertiary di(n-alkyl)glycidylamine monomers have been introduced for anionic ring-opening copolymerization (AROcP) with ethylene oxide (EO): N,N-di(n-butyl)glycidylamine (DButGA), N,N-di(n-hexyl)glycidylamine (DHexGA), and N,N-di(n-octyl)glycidylamine (DOctGA). Via controlled AROcP we synthesized well-defined (Mw/Mn = 1.05–1.14), water-soluble block- and gradient-type PEG copolymers, containing up to 25 mol % of the respective dialkylglycidylamine comonomer. Molecular weights ranged from 4900 to 12 000 g mol–1. Detailed in-situ 1H NMR ki…
Impact of Amino-Functionalization on the Response of Poly(ethylene glycol) (PEG) to External Stimuli
It is shown that amino-functionalization of poly(ethylene glycol) (PEG) with the comonomer N,N-diethyl glycidyl amine (DEGA) triggers the emergence of extraordinary stimuli responsiveness and phase behavior of PEG. In dependence of the solution pH, tapered PEG-co-PDEGA exhibits a highly cooperative two-step inverse phase transition with respect to temperature. The polymer forms dispersed metastable nanoglobules in the medically relevant temperature range around human body temperature. Independently, cloud points can be adjusted between 40 and 90 °C via the pH of the solution. Changing the polymer architecture to a block structure, in pronounced contrast, the polymer exhibits a gradual growt…
Tunable dynamic hydrophobic attachment of guest molecules in amphiphilic core–shell polymers
In this study, synthesis and dynamic properties of amphiphilic core–shell polymers are reported as monitored through their interaction with small amphiphilic molecules. Brush-like structures are formed with a hydrophobic core surrounded by a hydrophilic shell utilizing controlled radical addition–fragmentation chain transfer (RAFT) polymerization of macromonomers consisting of linear polyglycerol chains attached to alkylene methacrylate. Continuous wave electron paramagnetic resonance (CW EPR) spectroscopy is employed to study how the amphiphilic, paramagnetic spin probe 16-DSA (16-doxyl stearic acid) interacts with polymers of different alkylene chain lengths in their hydrophobic cores and…
Copper Uptake Induces Self-Assembly of 18.5 kDa Myelin Basic Protein (MBP)
Myelin basic protein (MBP) is predominantly found in the membranes of the myelin sheath of the central nervous system and is involved in important protein-protein and protein-lipid interactions in vivo and in vitro. Furthermore, divalent transition metal ions, especially Zn(2+) and Cu(2+), seem to directly affect the MBP-mediated formation and stabilization of the myelin sheath of the central nervous system. MBP belongs to the realm of intrinsically disordered proteins, and only fragmentary information is available regarding its partial structure(s) or supramolecular arrangements. Here, using standard continuous wave and modern pulse electron paramagnetic resonance methods, as well as dynam…
Dinitrogen complexation with main group radicals
In this report we present data from hyperfine sublevel correlation spectroscopy (HYSCORE), an electron paramagnetic resonance (EPR) spectroscopic technique, that reveals weak but distinct interactions between several main group element radicals and physically dissolved dinitrogen in solution. These interactions are the basis for a well-defined coordination of N2 to the paramagnetic centers. The complexes formed are primarily of the van der Waals-type but also feature a non-negligible orbital overlap between dinitrogen and the radicals' SOMO. Our spectroscopic findings are strongly supported by experiments with isotope-labeled 15N2, spectral simulations and quantum chemical and density funct…
DNA condensation with spermine dendrimers: interactions in solution, charge inversion, and morphology control
In this report, we characterize the formation of structured condensates of 884 base pair, double stranded DNA and spin-labeled, second generation dendrons (SL-G2) that are spermine-based and cationic using continuous wave electron paramagnetic resonance spectroscopy (CW EPR) and transmission electron microscopy (TEM). The electrostatic interaction between DNA and SL-G2 in solution leads to condensation of DNA into densely packed structures. At a particular charge ratio of 2.3 (cationic charges/anionic charges), the structures appear as thick rod-like condensates of parallelly ordered, stretched DNA and SL-G2. Depending on the concentration of DNA, the charge ratio and the ionic strength, wh…
Heme Binding Constricts the Conformational Dynamics of the Cytochrome b559′ Heme Binding Cavity
Cytochrome b(559)' is a transmembrane protein formed by homodimerization of the 44-residue PsbF polypeptide and noncovalent binding of a heme cofactor. The PsbF polypeptide can dimerize in the absence and presence of heme. To monitor structural alterations associated with binding of heme to the apo-cytochrome, we analyzed the apo- and holo-cytochrome structure by electron paramagnetic resonance spectroscopy. Spin labeling of amino acids located close to the heme binding domain of the cytochrome revealed that the structure of the heme binding domain is unconstrained in the absence of heme. Heme binding restricts the conformational dynamics of the heme binding domain, resulting in the structu…
Development of a Polarizer and Biocompatible Polarizing Agents for Use in Dynamic Nuclear Polarization DNP-Enhanced NMR and MRI
The application of 13C (or other low γ nuclei) NMR spectroscopy and imaging for clinical diagnosis has been constrained by the extremely long imaging and spectroscopy acquisition times that are required to obtain high SNR under physiological conditions (low natural abundance of 13C, low concentration of 13C-compounds, physiological temperature etc.). However, this obstacle could be overcome by in vitro hyperpolarization of a 13C-containing molecule with long spin lattice relaxation time via dynamic nuclear polarization (DNP) and subsequent injection into the animal or patient of investigation [1, 2]. DNP is achieved by resonant excitation of electron spins of radicals (electron paramagnetic…
Ligand-Binding Cooperativity Effects in Polymer-Protein Conjugation.
We present an electron paramagnetic resonance (EPR) spectroscopic characterization of structural and dynamic effects that stem from post-translational modifications of bovine serum albumin (BSA), an established model system for polymer-protein conjugation. Beyond the typical drug delivery and biocompatibility aspect of such systems, we illustrate the causes that alter internal dynamics and therefore functionality in terms of ligand-binding to the BSA protein core. Uptake of the paramagnetic fatty acid derivative 16-doxyl stearic acid by several BSA-based squaric acid macroinitiators and polymer-protein conjugates was studied by EPR spectroscopy, aided by dynamic light scattering (DLS) and z…
Evidence for Water-Tuned Structural Differences in Proteins: An Approach Emphasizing Variations in Local Hydrophilicity
We present experimental evidence for the significant effect that water can have on the functional structure of proteins in solution. Human (HSA) and Bovine Serum Albumin (BSA) have an amino acid sequence identity of 75.52% and are chosen as model proteins. We employ EPR-based nanoscale distance measurements using double electron-electron resonance (DEER) spectroscopy and both albumins loaded with long chain fatty acids (FAs) in solution to globally (yet indirectly) characterize the tertiary protein structures from the bound ligands' points of view. The complete primary structures and crystal structures of HSA and as of recently also BSA are available. We complement the picture as we have re…
Spin‐Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization
A potentially biocompatible class of spin-labeled macromolecules, spin-labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser-type dynamic nuclear polarization (DNP). All presented SL-heparins show high 1 H DNP enhancement factors up to E=-110, which validates that effectively more than one hyperfine line can be saturated even for spin-labeled polarizing agents. The parameters for the Overhauser-type DNP are determined and discussed. A striking result is that for spin-labeled heparins, the off-resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non-negligible part…
Persistent radicals of trivalent tin and lead.
In this report we present synthetic, crystallographic, and new electron paramagnetic resonance (EPR) spectroscopic work that shows that the synthetic route leading to the recently reported, first persistent plumbyl radical *PbEbt3 (Ebt = ethylbis(trimethylsilyl)silyl), that is, the oxidation of the related PbEbt3-anion, was easily extended to the synthesis of other persistent molecular mononuclear radicals of lead and tin. At first, various novel solvates of homoleptic potassium metallates KSnHyp3 (4a), KPbHyp3 (3a), KSnEbt3 (4b), KPbIbt3 (3c), and KSnIbt3 (4c) (Hyp = tris(trimethylsilyl)silyl, Ibt = isopropylbis(trimethylsilyl)silyl), as well as some heteroleptic metallates, such as [Li(OE…
CCDC 1520924: Experimental Crystal Structure Determination
Related Article: Sebastian Preiß, Christoph Förster, Sven Otto, Matthias Bauer, Patrick Müller, Dariush Hinderberger, Haleh Hashemi Haeri, Luca Carrella, Katja Heinze|2017|Nature Chemistry|9|1249|doi:10.1038/nchem.2836