Nitroxide–nitroxide and nitroxide–metal distance measurements in transition metal complexes with two or three paramagnetic centres give access to thermodynamic and kinetic stabilities

Abstract

Fundamentally, the stability of coordination complexes and of templated (bio)macromolecular assemblies depends on the thermodynamic and kinetic properties of the intermediates and final complexes formed. Here, we used pulse EPR (electron paramagnetic resonance) spectroscopy to determine the stabilities of nanoscopic assemblies formed between one or two nitroxide spin-labelled tridentate 2,2':6',2''-terpyridine (tpy) ligands and divalent metal ions (FeII, ZnII, CoII and CuII). In three distinct approaches we exploited (a) the modulation depth of pulsed electron-electron double resonance (PELDOR) experiments in samples with increasing metal-to-ligand ratios, (b) the frequencies of PELDOR under broadband excitation using shaped pulses and (c) the distances recovered from well-resolved PELDOR data in fully deuterated solvents measured at 34 GHz. The results demonstrate that PELDOR is highly sensitive to resolving the stability of templated dimers and allows to readily distinguish anti-cooperative binding (for CuII ions) from cooperative binding (for CoII or FeII ions). In the case of paramagnetic ions (CoII and CuII) the use of broadband PELDOR allowed to identify the cooperativity of binding from the time domain and distance data. By using a second labelled tpy ligand and by mixing two homoleptic complexes of the same metal centre we could probe the kinetic stability on a timescale of tens of seconds. Here, tpy complexes of CuII and ZnII were found to be substitutionally labile, CoII showed very slow exchange and FeII was inert under our conditions. Not only do our chemical models allow studying metal-ligand interactions via PELDOR spectroscopy, the design of our study is directly transferable to (bio)macromolecular systems for determining the kinetic and thermodynamic stabilities underpinning (templated) multimerisation. Considering the limited methods available to obtain direct information on the composition and stability of complex assemblies we believe our approach to be a valuable addition to the armoury of methods currently used to study these systems.

Coordinating Ability of Anions, Solvents, Amino Acids, and Gases towards Alkaline and Alkaline‐Earth Elements, Transition Metals, and Lanthanides

“…Measurements in less coordinating polar solvents such as acetone give the mostp ronouncedC u II spectroscopic features, whereas more coordinating polar solvents such as MeCN attenuate the Cu II signals. [68] Finally,i nP ELDOR (pulsed electronelectron double resonance) experiments, [69] the Co II relaxation was found to improve upon substitution of the BPh 4 À anion by the weakerc oordinating BAr F 4 À .…”

Pulsed EPR Dipolar Spectroscopy on Spin Pairs with one Highly Anisotropic Spin Center: The Low‐Spin FeIII Case

“…[8][9][10][11][12] The most common spin labels are nitroxides, [9][10][11][12] althoughanumber of alternatives based on Gd 3 + , [13,14] Cu 2 + , [15,16] trityl [17][18][19][20] and photoexcited porphyrins [21] have been reported. In addition to the spin labels, there is a keen interest of using naturally occurring paramagneticc ofactors, such as Cu 2 + , [22][23][24][25][26][27][28] low-spin (LS) Fe 3 + , [6,[29][30][31][32] high-spin( HS) Fe 3 + , [33] HS Mn 2 + , [34][35][36][37][38] Mo 5 + , [30] Co 2 + , [39,40] iron-sulfur clusters, [27,41,42] manganese clusters, [43] tyrosins, [44,45] semiquinones [46] or flavins, [29,47,…”

Pulsed EPR Dipolar Spectroscopy under the Breakdown of the High‐Field Approximation: The High‐Spin Iron(III) Case

“…The latter case has severala dvantages and applications: 1) the use of metal centers forP DS allows to reduce the number of spin labels and, consequently,t he number of protein mutations, 2) it enables orthogonal spin labeling [8] as the metal center has different spectroscopic properties than the majority of spin labels, 3) the distance constraintso btained from such PDS measurementse nable the localization of metal ions within the protein fold by trilateration [9,10] and the docking of different parts of protein complexes using metal ions as anchor points. [11] To date, PDS measurements have been applied to av ariety of different intrinsic metal centers, such as Cu 2 + , [12][13][14][15][16][17][18][19][20] Mn 2 + , [21][22][23][24][25] Co 2 + , [26,27] and low-spin Fe 3 + , [6,[28][29][30][31] as well as ironsulfur [17,[32][33][34] and manganese [35,36] clusters. In all these cases, the protocols for PSD measurements are well-established and the extractiono fd istance constraints from the corresponding PDS data can be readily achieved under the high-field approximation.…”