Structural Factors Controlling the Spin–Spin Exchange Coupling: EPR Spectroscopic Studies of Highly Asymmetric Trityl–Nitroxide Biradicals

Abstract

Strongly asymmetric exchange-coupled biradicals, like the trityl-nitroxides (TN), possess particular magnetic properties opening new possibilities for their application in biophysical, physicochemical and biological studies. In the present work, we investigated the effect of the linker length on the spin-spin coupling interaction in TN biradicals using the newly synthesized biradicals CT02-GT, CT02-AT, CT02-VT and CT02-PPT as well as the previously reported biradicals TNN14 and TN1. Results show that the magnitude of the spin-spin interaction (J) can be easily tuned from ~ 4 G (conformer 1 in CT02-PPT) to over 1200 G (in TNN14) using various linkers separating the two radical moieties and with varying temperature. Computer simulation of EPR spectra was carried out to directly estimate J values of the TN biradicals. In addition to the spin-spin coupling interaction of TN biradicals, their g, hyperfine splitting and zero-field splitting interactions were explored at low temperature (220 K). Our present study clearly shows that the spin-spin interaction variation as a function of linker distance and temperature provides an effective strategy to develop new TN biradicals which can find wide applications in relevant fields.

Spin Exchange Monitoring of the Strong Positive Homotropic Allosteric Binding of a Tetraradical by a Synthetic Receptor in Water

“…As a consequence, the interspin distance is now comprised between 14.2 and 17.2 Å, precluding any strong spin exchange interaction to occur (Figure 4a). 42 In this geometry, the two cationic charges are stabilized by ion-dipole interactions and there are numerous host-guest hydrogen bonds (21) plus 3 intramolecular hydrogen bonds and 4 host-host C-H•••O=C hydrogen bonds further stabilizing the architecture and reflecting the very strong binding between the guest and the two hosts (Figure 4b and 4c). The observed host host multiple weak interactions (Figure 4c) also support the asymmetric binding model deduced from NMR experiments (Scheme 2 right and Figure 3c).…”

Supramolecular host–guest interaction of trityl-nitroxide biradicals with cyclodextrins: modulation of spin–spin interaction and redox sensitivity

“…1, in the absence of CDs, the TN biradical CT02-VT has a strongly asymmetric EPR triplet signal consisting of a broad and almost invisible low-field peak, strong center-field peak and moderate high-field peak. 55 Addition of γ-CD (10 mM) to the aqueous solution of CT02-VT (50 µM) resulted in the host-guest interaction between γ-CD and CT02-VT and induced the appearance of a new EPR component with three lines separating by 15.7 G (Fig. 1, #) plus a single line in the center field (Fig.…”

“…55 γ-CD, M-β-CD and H-β-CD were purchased from Sigma-Aldrich. All other chemicals and solvents were of the highest grade commercially available.…”

“…56, 57 The fitting routine used to determine the J values was similar to the method described in our previous studies. 51–53, 55 The following parameters were optimized: g1 and g2; the hyperfine splitting constant of the nitrogen atom, α N ; the relaxation (or linewidth variation) parameters α, β and γ which can be used to indicate the tumbling rates of free or associated biradicals; and J and its standard deviation ΔJ. The ratio of free and associated biradicals in the presence of cyclodextrins was used to calculate association constants ( K ).…”

Isotropic exchange interaction between Mo and the proximal FeS center in the xanthine oxidase family member aldehyde oxidoreductase from Desulfovibrio gigas on native and polyalcohol inhibited samples: an EPR and QM/MM study

“…The study of spin systems coupled with isotropic exchange interaction (H ex = -J S 1 ÁS 2 ), also known as superexchange interaction when a bridging ligand is involved, is a research field that constitutes a cross point between biology and physical chemistry [1][2][3][4][5][6][7][8][9][10]. The sign and magnitude of the isotropic exchange parameter J is related to the nature and structural topology of the bridging chemical path and the properties of the magnetic orbitals of the interacting spins.…”