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

Abstract

Supramolecular host-guest interactions of trityl-nitroxide (TN) biradicals CT02-VT, CT02-AT and CT02-GT with methyl-β-cyclodextrin (M-β-CD), hydroxypropyl-β-cyclodextrin (H-β-CD) and γ-cyclodextrin (γ-CD) were investigated by EPR spectroscopy. In the presence of cyclodextrins (i.e., γ-CD, M-β-CD and H-β-CD), host-guest complexes of CT02-VT are formed where the nitroxide and linker parts possibly interact with the cyclodextrins’ cavities. Complexation with cyclodextrins leads to suppression of the intramolecular through-space spin-spin exchange coupling in CT02-VT, thus allowing determination of the through-bond spin-spin exchange coupling which was calculated to be 1.6 G using EPR simulations. Different types of cyclodextrins have variable binding affinity with CT02-VT with γ-CD (95 M−1) > M-β-CD (70 M−1) > H-β-CD (32 M−1). In addition, the effect of the linkers in TN biradicals on the host-guest interactions was also investigated. Among three TN biradicals studied, CT02-VT has the highest association constant with one designated cyclodextrin derivative. On the other hand, the complexes of CT02-GT (~ 22 G) and CT02-AT (7.7–9.0 G) with cyclodextrins have much higher through-bond spin-spin exchange couplings than that of CT02-VT (1.6 G) due to the shorter linkers than that of CT02-VT. Furthermore, the stability of TN biradicals towards ascorbate was significantly enhanced after the complexation with CDs, with an almost 2-time attenuation of the second-order rate constants for all the biradicals. Therefore, the supramolecular host-guest interactions with cyclodextrins will be an alternative method to modulate the magnitude of the spin-spin interactions and redox sensitivity of TN biradicals and the resulting complexes are promising as highly efficient DNP polarizing agents as well as EPR redox probes.

Supramolecular Approach to Electron Paramagnetic Resonance Distance Measurement of Spin-Labeled Proteins

“…CW EPR spectra of mono spin labeled DZD-T4L 135 at room temperature are clearly affected by the addition of β-CD, revealing the host–guest molecular recognition of DZD-nitroxide by β-CD; that is, τ rot of 1.1 ns increases to 2.5 ns, and isotropic hyperfine coupling constant A ( 14 N) decreases slightly (Figures S18–S20, Tables S9 and S10). , The decrease in the isotropic hyperfine, attributed to a decrease in the tensor component, A zz , based on simulations of the echo-detected spectra at 80 K, suggests a more hydrophobic environment for nitroxide inside β-CD. The increase in τ rot suggests restriction of the range of motion that is accessible to the protein-bound spin label complexed with β-CD.…”

“…Addition of β-CD to doubly spin labeled DZD-T4L 65/80 in 2:1 or 1:1 water/glycerol (w/g) in fluid solution caused broadening of CW EPR spectra, which suggests decreased mobility of the nitroxides. Because small-molecule nitroxides are known to form inclusion complexes with β-CD, we carried out similar experiments using MTSL. Addition of β-CD to MTSL-T4L 65/135 or 65/80 in 2:1 or 1:1 w/g leads to spectral broadening due to decreased mobility of MTSL, presumably as a result of complexation with β-CD.…”

Novel Biradicals for Direct Excitation Highfield Dynamic Nuclear Polarization

“…Finally, trityl-type radicals have been synthesized and used in dynamic nuclear polarization research. , As a prominent example, OX063 has been used as an efficient polarization source in DNP of quadrupolar nuclei. , Moreover, trityl radicals have been utilized in various applications such as to measure the extracellular and intracellular , oxygen level, to detect superoxide radical anions, or as spin labels for room-temperature interatomic distance measurements in biomolecules. , Compared to nitroxyl radicals, trityl radical derivatives are mostly inert to biological reductants such as glutathione and ascorbate. This restricts the application of trityl radical derivatives as redox probes. , The combination of nitroxyl and trityl functional groups in one biradical changes the electronic properties of the system and thus enables the specific application of these biradicals as redox probes in biological systems. − Furthermore, trityl-nitroxyl radicals have also been successfully applied as controllers in the nitroxide-mediated polymerization of styrene . Given these advantages, trityl-nitroxyl biradicals have recently been explored as a polarization source for ultrahigh field DNP. , These cited studies were motivated by the hypothesis that the combination of trityl and nitroxy moieties should yield a highly efficient polarization source.…”

Diastereoisomers of l-proline-linked trityl-nitroxide biradicals: synthesis and effect of chiral configurations on exchange interactions

“…We have fine-tuned the exchange interactions of TN biradicals by structural modification of the linkers 37 as well as by supramolecular interaction with cyclodextrins. 38 The largest DNP enhancement at a high magnetic field (18.8 T) has been achieved by using these TN biradicals as polarizing agents, 35 , 36 in part due to the lack of nuclear depolarization. 39 In the present work, we utilize the chiral effect of the linker and two radical parts to modulate the spin–spin exchange interaction of TN biradicals.…”