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-?-cyclodextrin (M-?-CD), hydroxypropyl-?-cyclodextrin (H-?-CD) and ?-cyclodextrin (?-CD) were investigated by EPR spectroscopy. In the presence of cyclodextrins (i.e., ?-CD, M-?-CD and H-?-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 the 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 different binding affinities with CT02-VT in the order of ?-CD (95 M(-1)) > M-?-CD (70 M(-1)) > H-?-CD (32 M(-1)). In addition, the effect of the linkers in TN biradicals on the host-guest interactions was also investigated. Among the 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 those of CT02-VT (1.6 G) due to the shorter linkers than those of CT02-VT. Furthermore, the stability of TN biradicals towards ascorbate was significantly enhanced after the complexation with CDs, with an almost 2-fold 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.

SUBMITTER: Tan X 

PROVIDER: S-EPMC4823007 | biostudies-literature | 2016 Feb

REPOSITORIES: biostudies-literature