Evaluation of Copper(II) Transfer between Amyloid-beta Peptides by Relaxation-Induced Dipolar Modulation Enhancement (RIDME).

In the brains of Alzheimer's disease patients, fibrillar aggregates containing amyloid-beta (Aß) peptides are found, along with elevated concentrations of Cu(II) ions. The aggregation pathways of Aß peptides can be modulated by Cu(II) ions and is determined by the formation and nature of the Cu(II)-Aß complex. If spin-labeled, the Cu(II)-Aß complex contains two dipolar coupled paramagnetic centers, the spin label and the Cu(II) ion. Measurement of the dipolar coupling between these paramagnetic centers by relaxation-induced dipolar modulation enhancement (RIDME) allows to monitor the complex formation and thus opens a way to follow the Cu(II) transfer between peptides if a mixture of wild-type and spin-labeled ones is used. We evaluate this approach for a specific Cu(II)-Aß complex, the aggregation-inert Component II. The kinetics of the Cu(II) transfer can be resolved by performing RIDME in a time-dependent manner. A temporal resolution of seconds has been achieved, with the potential to reach milliseconds, using a rapid-freeze quench device to stop the Cu(II) transfer in solution after defined incubation times.

Equipping 1,4,7-Triazacyclononane with Substituents via Solid-Phase Synthesis.

Metal ion complexes frequently show substituted 1,4,7-triazacyclononane (tacn) as the ligand. Besides providing donor atoms for complex formation, tacn serves as a scaffold for equipping the complex with further functional units that are needed for the complementation and electronic tuning of the metal ion coordination sphere and/or add other features, e.g., light-absorbing antennas and groups for bioconjugation. To exploit the full potential of substituted tacn, strategies for directed syntheses of NO(R1,R1,R2) and NO(R1,R2,R3), i.e., tacn with two and even three different substituents R, are needed. Herein, we report a strategy that takes advantage of solid-phase synthesis in the assembly of the precursors NO(R1,R1,H) and NO(R1,R2,H). The assembly of NO(R1,R2,H) is based on a highly selective formation of NO(Cbz,tfAc,H), with Cbz being the link between tacn and solid phase. For this, tacn was loaded onto (4-nitrophenyl carbonate)-resin, thereby forming resin-bound (rb)-tacn, which corresponds to NO(Cbz,H,H) bound to the solid phase. Treatment of rb-tacn with ethyl trifluoroacetate gave rb-NO(tfAc,H), which corresponds to NO(Cbz,tfAc,H). With rb-tacn and rb-NO(tfAc,H) in hand, a variety of NO(R1,R1,H) and NO(R1,R2,H) were prepared, showing the broad applicability of the strategy with respect to the type of substituents and of reactions (nucleophilic substitution, reductive amination, aza-Michael addition, addition to epoxides, acylation). The study also identified limitations and points for improvement.

A GdIII-Based Spin Label at the Limits for Linewidth Reduction through Zero-Field Splitting Optimization.

The remarkably narrow central line in the electron paramagnetic resonance spectrum and the very weak zero-field splitting (ZFS) make [GdIII(NO3Pic)] ([GdIII(TPATCN)]) an attractive starting point for the development of spin labels. For retaining the narrow line of this parent complex when modifying it with a substituent enabling bioconjugation, alkyl with a somehow remote functional group as a substituent at the picolinate moiety was found to be highly suitable because ZFS stayed weak, even if the threefold axial symmetry was broken. The ZFS is so weak that hyperfine coupling and/or g-value variations noticeably determine the linewidth in Q band and higher fields when the biomolecule is protonated, which is the standard situation, and in W band and higher fields for the protonated complex in a fully deuterated surrounding. Clearly, [NDSE-{GdIII(NO3Pic)}], a spin label targeting the cysteines in a peptide, is at a limit of linewidth narrowing through ZFS minimization. The labeling reaction is highly chemoselective and, applied to a polyproline with two cysteine units, it took no more than a minute at 7 °C and pH 7.8. Subsequent disulfide scrambling is very slow and can therefore be prevented. Double electron-electron resonance and relaxation-induced dipolar modulation enhancement applied to the spin-labeled polyproline proved the spin label useful for distance determination in peptides.

Short-range ENDOR distance measurements between Gd(III) and trifluoromethyl labels in proteins.

The measurement of distances in proteins can be challenging in the 5-20 Å range, which is outside those accessible through conventional NMR and EPR methods. Recently it was demonstrated that distances in this range could be measured between a nitroxide as a paramagnetic spin label and a nearby fluorine atom (19F) as a nuclear spin label using high-field (W-band/3.4 T) ENDOR spectroscopy. Here we show that such measurements can also be performed using a gadolinium ion (Gd3+) as the paramagnetic tag. Gd3+ has two advantages. (i) A greater electronic spin (S = 7/2) and fast electronic spin-lattice (T1) relaxation, improving sensitivity by allowing data to be collected at lower temperatures. (ii) A narrow EPR signal for the -½ ↔ ½ transition, and therefore no orientation selection artefacts. Signal intensities can be further enhanced by using a trifluoromethyl (C19F3) group instead of a single 19F atom. Using the protein calbindin D9k with a Ca2+ ion replaced by a Gd3+ ion and a trifluoromethylphenylalanine in position 50, we show that distances up to about 10 Å can be readily measured. Longer distances proved more difficult to measure due to variable electronic TM relaxation rates, which lead to broader Lorentzian ENDOR lineshapes. Gd3+ complexes (Gd3+ tags), which reliably display longer TM times, allow longer distances to be measured (8-16 Å). We also provide preliminary evidence that the intensity of ENDOR signals follows the predicted 1/r6 dependence, indicating that distances r > 20 Å can be measured by this method.

Dynamical decoupling in water-glycerol glasses: a comparison of nitroxides, trityl radicals and gadolinium complexes.

Our previous study on nitroxides in o-terphenyl (OTP) revealed two separable decoherence processes at low temperatures, best captured by the sum of two stretched exponentials (SSE) model. Dynamical decoupling (DD) extends both associated dephasing times linearly for 1 to 5 refocusing pulses [Soetbeer et al., Phys. Chem. Chem. Phys., 2018, 20, 1615]. Here we demonstrate an analogous DD behavior of water-soluble nitroxides in water-glycerol glass by using nitroxide and/or solvent deuteration for component assignment. Compared to the conventional Hahn experiment, we show that Carr-Purcell and Uhrig DD schemes are superior in resolving and identifying active dephasing mechanisms. Thereby, we observe a partial coherence loss to intramolecular nitroxide and trityl nuclei that can be alleviated, while the zero field splitting-induced losses for gadolinium labels cannot be refocused and contribute even at the central transition of this spin-7/2 system. Independent of the studied spin system, Uhrig DD leads to a characteristic convex dephasing envelope in both protonated water-glycerol and OTP glass, thus outperforming the Carr-Purcell scheme.

A Chemiluminescent Metal-Organic Framework.

The synthesis and characterization of a chemiluminescent metal-organic framework with high porosity is reported. It consists of Zr6 O6 (OH)4 nodes connected by 4,4'-(anthracene-9,10-diyl)dibenzoate as the linker and luminophore. It shows the topology known for UiO-66 and is therefore denoted PAP-UiO. The MOF was not only obtained as bulk material but also as a thin film. Exposure of PAP-UiO as bulk or film to a mixture of bis-(2,4,6-trichlorophenyl) oxalate, hydrogen peroxide, and sodium salicylate in a mixture of dimethyl and dibutyl phthalate evoked strong and long lasting chemiluminescence of the PAP-UiO crystals. Time dependent fluorescence spectroscopy on bulk PAP-UiO and, for comparison, on dimethyl 4,4'-(anthracene-9,10-diyl)dibenzoate provided evidence that the chemiluminescence originates from luminophores being part of the PAP-UiO, including the luminophores inside the crystals.

Trityl Radicals with a Combination of the Orthogonal Functional Groups Ethyne and Carboxyl: Synthesis without a Statistical Step and EPR Characterization.

Finland trityl radical (FTR) shows very attractive EPR spectroscopic properties for a manifold of applications. For most of its applications only one chemically reactive functional group is needed. The presence of three equally reactive carboxyl groups leads to FTR modifications through reactions which give statistical mixtures of 1-fold-, 2-fold-, and 3-fold-modified and unmodified FTR. To avoid the side effects of such a statistical reaction-limited yields and separation challenges-we took a route to FTR-type trityl radicals with scaffold assembly by addition of an aryllithium with one type of substituent to a diarylketone with another type of substituent. This gave the two FTR-type trityl radicals 1 and 2 which carry a combination of the chemically orthogonal groups, carboxyl and triisopropylsilylethynyl. Standard column chromatography was sufficient for product isolation on all stages, whereby polar tagging helped. The EPR spectroscopic properties of the trityl radicals 1 and 2 in ethanol were determined in X and W bands. Their g anisotropy and T1 and T2 relaxation times make them spin labels as good as the benchmark FTR. This paper discloses also details on the synthesis of building blocks used for FTR preparation and improved access to the bare FTR scaffold.

Improving the accuracy of Cu(ii)-nitroxide RIDME in the presence of orientation correlation in water-soluble Cu(ii)-nitroxide rulers.

Orientation selection is a challenge in distance determination with double electron electron resonance (DEER) spectroscopy of rigid molecules. The problem is reduced when applying the Relaxation-Induced Dipolar Modulation Enhancement (RIDME) experiment. Here we present an in-depth study on nitroxide-detected RIDME in Cu(ii)-nitroxide spin pairs using two Cu(ii)-nitroxide rulers that are both water soluble and have comparable spin-spin distances. They differ in the type of the ligand (TAHA and PyMTA) for the Cu(ii) ion which results in different contributions of exchange coupling. Both rulers feature substantial orientation correlation between the molecular frames of the Cu(ii) complex and the nitroxide. We discuss how the spin-spin couplings can be accurately measured and how they can be correlated to the nitroxide resonance frequencies. In that, we pay particular attention to the suppression of nuclear modulation and of echo crossing artefacts, to background correction, and to orientation averaging. With a nitroxide observer sequence based on chirp pulses, we achieve wideband detection of all nitroxide orientations. Two-dimensional Fourier transformation of data obtained in this manner affords observer-EPR correlated RIDME spectra that enable visual understanding of the orientation correlation. The syntheses of the Cu(ii)-nitroxide rulers are presented. The synthetic route is considered to be of general use for the preparation of [metal ion complex]-nitroxide rulers, including water soluble ones.

Intermolecular background decay in RIDME experiments.

The relaxation-induced dipolar modulation enhancement (RIDME) technique allows the determination of distances and distance distributions in pairs containing two paramagnetic metal centers, a paramagnetic metal center and an organic radical, and, under some conditions, also in pairs of organic radicals. The strengths of the RIDME technique are its simple setup requirements, and the absence of bandwidth limitations for spin inversion which occurs through relaxation. A strong limitation of the RIDME technique is the background decay, which is often steeper than that in the double electron electron resonance experiment, and the absence of an appropriate description of the intermolecular background signal. Here we address the latter problem and present an analytical calculation of the RIDME background decay in the simple case of two types of randomly distributed spin centers each with total spin S = 1/2. The obtained equations allow the explaination of the key trends in RIDME experiments on frozen chelated metal ion solutions, and singly spin-labeled proteins. At low spin label concentrations, the RIDME background shape is determined by nuclear-driven spectral diffusion processes. This fact opens up a new path for structural characterization of soft matter and biomacromolecules through the determination of the local distribution of protons in the vicinity of the spin-labeled site.

Pulsed EPR Methods to Study Biomolecular Interactions.

Orthogonal site-directed spin labelling in combination with pulsed EPR spectroscopy is a powerful approach to study biomolecular interactions on a molecular level. Following a surge in pulse EPR method development, it is now possible to access distance distributions in the nanometre range in systems of complex composition. In this article we briefly outline the necessary considerations for measurements of distance distributions in macromolecular systems labelled with two or more different types of paramagnetic centres. We illustrate the approach with two examples: an application of the Double Electron-Electron Resonance (DEER) method on a triple spin-labelled protein dimer labelled with nitroxide and Gd(III), and an optimisation study of the Relaxation Induced Dipolar Modulation Enhancement (RIDME) experiment for the orthogonal spin pair Cu(II)-nitroxide.

On-Surface Synthesis of Highly Oriented Thin Metal-Organic Framework Films through Vapor-Assisted Conversion.

Controlled on-surface film growth of porous and crystalline frameworks is a central prerequisite for incorporating these materials into functional platforms and operational devices. Here, we present the synthesis of thin zirconium-based metal-organic framework (MOF) films by vapor-assisted conversion (VAC). We established protocols adequate for the growth of UiO-66, UiO-66(NH2), UiO-67, and UiO-68(NH2) as well as the porous interpenetrated Zr-organic framework, PPPP-PIZOF-1, as highly oriented thin films. Through the VAC approach, precursors in a cast solution layer on a bare gold surface are reacting to form a porous continuous MOF film, oriented along the [111] crystal axis, by exposure to a solvent vapor at elevated temperature of 100 °C and 3 h reaction time. It was found that the concentration of dicarboxylic acid, the modulator, the droplet volume, and the reaction time are vital parameters to be controlled for obtaining oriented MOF films. Using VAC for the MOF film growth on gold surfaces modified with thiol SAMs and on a bare silicon surface yielded oriented MOF films, rendering the VAC process robust toward chemical surface variations. Ethanol sorption experiments show that a substantial part of the material pores is accessible. Thereby, the practical VAC method is an important addition to the toolbox of synthesis methods for thin MOF films. We expect that the VAC approach will open new horizons in the formation of highly defined functional thin MOF films for numerous applications.

Postsynthetic Modification of Metal-Organic Frameworks through Nitrile Oxide-Alkyne Cycloaddition.

Postsynthetic modification of metal-organic frameworks is an important method to tailor their properties. We report on the nitrile oxide-alkyne cycloaddition (NOAC) as a modification tool, a reaction requiring neither strained alkynes nor a catalyst. This is demonstrated with the reaction of nitrile oxides with PEPEP-PIZOF-15 and -19 at room temperature. PIZOF-15 and -19 are porous Zr-based MOFs (BET surface areas 1740 and 960 m2 g-1, respectively) consisting of two mutually interpenetrating UiO-type frameworks with linkers of the type -O2C[PE-P(R1,R2)-EP]CO2- (P, phenylene; E, ethynylene; R1 and R2, side chains at the central benzene ring with R1 = R2 = OCH2C≡CH or R1 = OCH2C≡CH and R2 = O(CH2CH2O)3Me). Their syntheses, using benzoic acid as a modulator, and their characterization are reported herein. The propargyloxy (OCH2C≡CH) side chains contain the ethyne moieties needed for NOAC. Formation of nitrile oxides through oxidation of oximes in aqueous ethanolic solution in the presence of PEPEP-PIZOF-15 and -19 resulted in the reaction of 96-100% of the ethyne moieties to give isoxazoles. Thereby the framework was preserved. The type of nitrile oxide RCNO was greatly varied with R being isopentyl, tolyl, 2-pyridyl, and pentafluorophenyl. A detailed NMR spectroscopic investigation showed the formation of the 3,5-disubstituted isoxazole to be clearly favored (≥96%) over that of the constitutional isomeric 3,4-disubstituted isoxazole, except for one example.

Dynamical decoupling of nitroxides in o-terphenyl: a study of temperature, deuteration and concentration effects.

We have characterized the temperature dependent transverse relaxation for 100 µM protonated and deuterated nitroxides in both protonated and deuterated o-terphenyl (OTP and dOTP) in distinct temperature regimes between 10 K and room temperature (RT). The choice of sample compositions allowed for a clear separation into slow and fast relaxation contributions based on a sum of two stretched exponential (SSE) parameterization between 10 and 60 K, and likewise at RT. The slow contribution is purely matrix dependent, while the fast process is determined by an interplay between a molecule and a matrix. Our systematic study of dynamical decoupling (DD) as a function of temperature (at 40, 80 K and RT), spin concentration, deuteration of nitroxide and/or OTP matrix and DD scheme for 1 to 5 refocusing pulses reveals that DD significantly prolongs phase memory times with respect to Hahn echo relaxation at 40 K, which we discuss in an SSE framework. At 80 K and RT, where (intra)molecular motions dominate relaxation, DD does not preserve electron spin coherence independent of the sample composition. Instead, we report a matrix nuclei dependent performance of the applied DD scheme at 40 K with Uhrig outperforming Carr-Purcell DD in OTP, and vice versa for a dOTP matrix.

Quantitative analysis of zero-field splitting parameter distributions in Gd(iii) complexes.

The magnetic properties of paramagnetic species with spin S > 1/2 are parameterized by the familiar g tensor as well as "zero-field splitting" (ZFS) terms that break the degeneracy between spin states even in the absence of a magnetic field. In this work, we determine the mean values and distributions of the ZFS parameters D and E for six Gd(iii) complexes (S = 7/2) and critically discuss the accuracy of such determination. EPR spectra of the Gd(iii) complexes were recorded in glassy frozen solutions at 10 K or below at Q-band (∼34 GHz), W-band (∼94 GHz) and G-band (240 GHz) frequencies, and simulated with two widely used models for the form of the distributions of the ZFS parameters D and E. We find that the form of the distribution of the ZFS parameter D is bimodal, consisting roughly of two Gaussians centered at D and -D with unequal amplitudes. The extracted values of D (σD) for the six complexes are, in MHz: Gd-NO3Pic, 485 ± 20 (155 ± 37); Gd-DOTA/Gd-maleimide-DOTA, -714 ± 43 (328 ± 99); iodo-(Gd-PyMTA)/MOMethynyl-(Gd-PyMTA), 1213 ± 60 (418 ± 141); Gd-TAHA, 1361 ± 69 (457 ± 178); iodo-Gd-PCTA-[12], 1861 ± 135 (467 ± 292); and Gd-PyDTTA, 1830 ± 105 (390 ± 242). The sign of D was adjusted based on the Gaussian component with larger amplitude. We relate the extracted P(D) distributions to the structure of the individual Gd(iii) complexes by fitting them to a model that superposes the contribution to the D tensor from each coordinating atom of the ligand. Using this model, we predict D, σD, and E values for several additional Gd(iii) complexes that were not measured in this work. The results of this paper may be useful as benchmarks for the verification of quantum chemical calculations of ZFS parameters, and point the way to designing Gd(iii) complexes for particular applications and estimating their magnetic properties a priori.

Expanding the Group of Porous Interpenetrated Zr-Organic Frameworks (PIZOFs) with Linkers of Different Lengths.

A Zr-based MOF of the PIZOF type, which consists of two independent and mutually interpenetrating UiO-type frameworks with [Zr6O4(OH)4(O2C)12] nodes, does not only form with a PEPEP dicarboxylic acid (P = phenylene, E = ethynylene). Also dicarboxylic acids with the shorter PPPP and PEPP spacers were found to give PIZOFs, denoted PPPP-PIZOF and PEPP-PIZOF, respectively. Reducing the spacer length even further to a PEEP segment caused a switchover to the formation of a UiO framework. The hysteresis in the Ar sorption curve of PEPP-PIZOF-1 and the slightly too large amount of combustion residue from PPPP-PIZOF-1 suggest structural defects. These hint at a mismatch between the requirement of the optimal linker length for PIZOF formation and the lengths of the PEPP and PPPP dicarboxylates. Nevertheless, these dicarboxylates prefer the formation of a PIZOF over the formation of a UiO structure. PEPEP-PIZOF-2, PPPP-PIZOF-1, and PEPP-PIZOF-1 are stable in air up to 325, 350, and 300 °C, respectively, and have BET surface areas of 2350, 2020, and 1650 m2 g-1, respectively. PEPEP-PIZOFs, even those with very hydrophilic oligo(ethylene glycol) side chains on the linkers, are very stable in water and also during drying from a water-wetted state. On the contrary, PEPP-PIZOF-1 and PPPP-PIZOF-1 that had been exposed to water required exchange of water for ethanol before drying to mostly preserve the framework. The results emphasize the importance of differentiating between framework damage caused through hydrolysis in water and through drying from a water-wetted state. The sensitivity of PEPP-PIZOF-1 and PPPP-PIZOF-1 against drying from a water-wetted state may be the consequence of defects. The drying stability of water-wetted PEPEP-PIZOFs lets us suggest that reversible bending of the linkers contributes to the stability of the PEPEP-PIZOFs.

Gd3+-Gd3+ distances exceeding 3 nm determined by very high frequency continuous wave electron paramagnetic resonance.

Electron paramagnetic resonance spectroscopy in combination with site-directed spin labeling is a very powerful tool for elucidating the structure and organization of biomolecules. Gd3+ complexes have recently emerged as a new class of spin labels for distance determination by pulsed EPR spectroscopy at Q- and W-band. We present CW EPR measurements at 240 GHz (8.6 Tesla) on a series of Gd-rulers of the type Gd-PyMTA-spacer-Gd-PyMTA, with Gd-Gd distances ranging from 1.2 nm to 4.3 nm. CW EPR measurements of these Gd-rulers show that significant dipolar broadening of the central |-1/2〉 → |1/2〉 transition occurs at 30 K for Gd-Gd distances up to ∼3.4 nm with Gd-PyMTA as the spin label. This represents a significant extension for distances accessible by CW EPR, as nitroxide-based spin labels at X-band frequencies can typically only access distances up to ∼2 nm. We show that this broadening persists at biologically relevant temperatures above 200 K, and that this method is further extendable up to room temperature by immobilizing the sample in glassy trehalose. We show that the peak-to-peak broadening of the central transition follows the expected 1/r3 dependence for the electron-electron dipolar interaction, from cryogenic temperatures up to room temperature. A simple procedure for simulating the dependence of the lineshape on interspin distance is presented, in which the broadening of the central transition is modeled as an S = 1/2 spin whose CW EPR lineshape is broadened through electron-electron dipolar interactions with a neighboring S = 7/2 spin.

Computing distance distributions from dipolar evolution data with overtones: RIDME spectroscopy with Gd(iii)-based spin labels.

Extraction of distance distributions between high-spin paramagnetic centers from relaxation induced dipolar modulation enhancement (RIDME) data is affected by the presence of overtones of dipolar frequencies. As previously proposed, we account for these overtones by using a modified kernel function in Tikhonov regularization analysis. This paper analyzes the performance of such an approach on a series of model compounds with the Gd(iii)-PyMTA complex serving as paramagnetic high-spin label. We describe the calibration of the overtone coefficients for the RIDME kernel, demonstrate the accuracy of distance distributions obtained with this approach, and show that for our series of Gd-rulers RIDME technique provides more accurate distance distributions than Gd(iii)-Gd(iii) double electron-electron resonance (DEER). The analysis of RIDME data including harmonic overtones can be performed using the MATLAB-based program OvertoneAnalysis, which is available as open-source software from the web page of ETH Zurich. This approach opens a perspective for the routine use of the RIDME technique with high-spin labels in structural biology and structural studies of other soft matter.

Bis-Gadolinium Complexes for Solid Effect and Cross Effect Dynamic Nuclear Polarization.

High-spin complexes act as polarizing agents (PAs) for dynamic nuclear polarization (DNP) in solid-state NMR spectroscopy and feature promising aspects towards biomolecular DNP. We present a study on bis(Gd-chelate)s which enable cross effect (CE) DNP owing to spatial confinement of two dipolar-coupled electron spins. Their well-defined Gd⋅⋅⋅Gd distances in the range of 1.2-3.4 nm allowed us to elucidate the Gd⋅⋅⋅Gd distance dependence of the DNP mechanism and NMR signal enhancement. We found that Gd⋅⋅⋅Gd distances above 2.1 nm result in solid effect DNP while distances between 1.2 and 2.1 nm enable CE for 1 H, 13 C, and 15 N nuclear spins. We compare 263 GHz electron paramagnetic resonance (EPR) spectra with the obtained DNP field profiles and discuss possible CE matching conditions within the high-spin system and the influence of dipolar broadening of the EPR signal. Our findings foster the understanding of the CE mechanism and the design of high-spin PAs for specific applications of DNP.

Spacers for Geometrically Well-Defined Water-Soluble Molecular Rulers and Their Application.

The synthesis and application of monodisperse oligo(para-phenyleneethynylene)s (oligoPPEs) with side chains that are adjustable to specific needs, such as water solubility, on a very late stage of the multistep synthesis are described. The adjustable side chains allow for circumventing problems associated with the isolation of highly hydrophilic compounds during early stages of a synthesis. Furthermore, these oligoPPEs can be stocked as adaptable advanced building blocks for rapid assembly of tailor-made spacers and rulers. A rapid growth synthesis provides oligoPPEs with alkyne termini protected with the orthogonal groups trimethylsilyl and 1-hydroxy-1-methylethyl (HOP) and with propargyloxy side chains protected with the triisopropylsilyl group. The three protecting groups allow independent modification of the two termini and the side chains. The HOP group not only acts as a protecting group but also as a polar tag for simple product isolation. We demonstrate one application of these oligoPPEs as spacers for the water-soluble Gd rulers of the type Gd-PyMTA-spacer-Gd-PyMTA with Gd-Gd distances of 2.1-10.9 nm. For this purpose, the terminal alkyne units were used for backbone assembly and attachment of the ligand PyMTA, and the porpargyloxy side chains were used to attach water solubilizing poly(ethylene glycol) chains through a click reaction after spacer assembly.

Correction: Gd(iii)-Gd(iii) EPR distance measurements - the range of accessible distances and the impact of zero field splitting.

Correction for 'Gd(iii)-Gd(iii) EPR distance measurements - the range of accessible distances and the impact of zero field splitting' by Arina Dalaloyan et al., Phys. Chem. Chem. Phys., 2015, 17, 18464-18476.