Overcoming artificial broadening in Gd³⁺–Gd³⁺ distance distributions arising from dipolar pseudo-secular terms in DEER experiments

By providing accurate distance measurements between spin labels site-specifically attached to bio-macromolecules, double electron–electron resonance (DEER) spectroscopy provides a unique tool to probe the structural and conformational changes in these molecules. Gd3+-tags present an important family of spin-labels for such purposes, as they feature high chemical stability and high sensitivity in high-field DEER measurements. The high sensitivity of the Gd3+ ion is associated with its high spin (S = 7/2) and small zero field splitting (ZFS), resulting in a narrow spectral width of its central transition at high fields. However, under the conditions of short distances and exceptionally small ZFS, the weak coupling approximation, which is essential for straightforward DEER data analysis, becomes invalid and the pseudo-secular terms of the dipolar Hamiltonian can no longer be ignored. This work further explores the effects of pseudo-secular terms on Gd3+–Gd3+ DEER measurements using a specifically designed ruler molecule; a rigid bis-Gd3+-DOTA model compound with an expected Gd3+–Gd3+ distance of 2.35 nm and a very narrow central transition at the W-band (95 GHz). We show that the DEER dipolar modulations are damped under the standard W-band DEER measurement conditions with a frequency separation, Δν, of 100 MHz between the pump and observe pulses. Consequently, the DEER spectrum deviates considerably from the expected Pake pattern. We show that the Pake pattern and the associated dipolar modulations can be restored with the aid of a dual mode cavity by increasing Δν from 100 MHz to 1.09 GHz, allowing for a straightforward measurement of a Gd3+–Gd3+ distance of 2.35 nm. The increase in Δν increases the contribution of the |−5/2〉 → |−3/2〉 and |−7/2〉 → |−5/2〉 transitions to the signal at the expense of the |−3/2 〉 → |−1/2〉 transition, thus minimizing the effect of dipolar pseudo-secular terms and restoring the validity of the weak coupling approximation. We apply this approach to the A93C/N140C mutant of T4 lysozyme labeled with two different Gd3+ tags that have narrow central transitions and show that even for a distance of 4 nm there is still a significant (about two-fold) broadening that is removed by increasing Δν to 636 MHz and 898 MHz.This research was supported by the Israeli Science Foundation (grant 334/14) and made possible in part by the historic generosity of the Harold Perlman Family. D. G. holds the Erich Klieger professorial chair in Chemical Physic

CCDC 1858108: Experimental Crystal Structure Determination

PIHQAB : tris{4-methyl-4'-[2-(pyridin-4-yl)ethenyl]-2,2'-bipyridine}-ruthenium bis(hexafluorophosphate) unknown solvat

Generation of Mono- and Bimetallic Palladium Complexes and Mechanistic Insight into an Operative Metal Ring-Walking Process

This combined experimental and computational study demonstrates how a metal center can drastically influence the reactivity of a coordinated ligand. We found that the consecutive formation of zerovalent bimetallic complexes proceeds by fast η²-CC coordination of only one Pd­(PEt₃)₂ moiety, which then significantly slows down the subsequent reaction. Electronic effects induced by complexation of the first metal center have a major effect on the subsequent formation of the bimetallic complexes. These effects are reduced by partial fluorination of the bis­(vinylpyridine)-arene ligand. The monometallic complexes display migration of the Pd­(PEt₃)₂ moiety between the two olefinic bonds of the corresponding ligand, as indicated by various solution NMR experiments, including variable-temperature NMR spectroscopy, 2D ¹H–¹H exchange spectroscopy, and spin saturation transfer. Density functional theory studies were performed at the SMD­(toluene)–PBE0+d­(v3)/B2//B97D/B1 level of theory

CCDC 1858110: Experimental Crystal Structure Determination

PIHQIJ : mer-tris{4-methyl-4'-[2-(pyridin-4-yl)ethenyl]-2,2'-bipyridine}-osmium bis(hexafluorophosphate) unknown solvat

Sorting of Molecular Building Blocks from Solution to Surface

We demonstrate that molecular gradients on an organic monolayer is formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometer-thick assembly of 1,3,5-tris(4-pyridylethenyl)benzene (TPEB) covalently attached to a silicon or metal-oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nm. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences cause the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer significantly contributes to the observed differences in chemical composition on the surface.</p

Authorizing Multiple Chemical Passwords by a Combinatorial Molecular Keypad Lock

A combinatorial fluorescent molecular sensor operates as a highly efficient molecular security system. The ability of a pattern-generating molecule to process diverse sets of chemical inputs, discriminate among their concentrations, and form multivalent and kinetically stable complexes is demonstrated as a powerful tool for processing a wide range of chemical “passwords” of different lengths. This system thus indicates the potential for obtaining unbreakable combination locks at the molecular scale

Large effect of a small substitution: competition of dehydration with charge retention and Coulomb explosion in gaseous [(bipy^R)Au(μ-O)₂Au(bipy^R)]²⁺ dications

Dinuclear gold(III) clusters with a rhombic Au2O2 core and 2,2′-bipyridyl ligands substituted in the 6-position (bipyR) are examined by tandem mass spectrometry. Electrospray ionization of the hexafluorophosphate salts affords the complexes [(bipyR)Au(μ-O)2Au(bipyR)]2+ as free dications in the gas phase. The fragmentation behavior of the mass-selected dications is probed by means of collision-induced dissociation experiments which reveal an exceptionally pronounced effect of substitution. Thus, for the parent compound with R = H, i.e., [(bipy)Au(μ-O)2Au(bipy)]2+, fragmentation at the dicationic stage prevails to result in a loss of neutral H2O concomitant with an assumed rollover cyclometalation of the bipyridine ligands. In marked contrast, all complexes with alkyl substituents in the 6-position of the ligands (bipyR with R = CH3, CH(CH3)2, CH2C(CH3)3, and 2,6-C6H3(CH3)2) as well as the corresponding complex with 6,6′-dimethyl-2,2′-dipyridyl as a ligand exclusively undergo Coulomb explosion to produce two monocationic fragments. It is proposed that the additional steric strain introduced to the central Au2O2 core by the substituents on the bipyridine ligand, in conjunction with the presence of oxidizable C−H bonds in the substituents, crucially affects the subtle balance between dication dissociation under maintenance of the 2-fold charge and Coulomb explosion into two singly charged fragments

Sorting of Molecular Building Blocks from Solution to Surface

We demonstrate that molecular gradients on an organic monolayer is formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometer-thick assembly of 1,3,5-tris­(4-pyridylethenyl)­benzene (TPEB) covalently attached to a silicon or metal-oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nm. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences cause the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer significantly contributes to the observed differences in chemical composition on the surface