Synthesis
of novel trityl-nitroxyl biradicals and their performance
as polarization agents in DNP-enhanced solid-state MAS NMR spectroscopy
is presented. Signal enhancements in 1H, 1H
→ 13C CP MAS, and 13C MAS experiments
obtained with these radicals dissolved in 1,1,2,2-tetrachloroethane
(TCE) solution are compared with the enhancements obtained from TCE
solutions of binitroxyl radicals. The signal enhancements are correlated
with the distance between the radical centers of the biradicals, as
determined by theoretical structure calculations. Some of the biradical
TCE solutions display direct-channel resonances in 13C
MAS experiments as well as indirect channel resonances induced via
the proton spin reservoir. Differential scanning calorimetry reveals
that only these solutions do not form any solid crystalline phases
upon rapid cooling, suggesting that molecular motions needed for polarization
transfer from radicals to 13C via the proton spin reservoir
remain active at the experimental low temperatures of nominal 120
K. DNP magnetic field sweep enhancement profiles for selected new
biradicals are presented as well. These indicate that the DNP transfer
is dominated by the cross-effect mechanism.
Sterically shielded nitroxides, which demonstrate high resistance to bioreduction, are the spin labels of choice for structural studies inside living cells using pulsed EPR and functional MRI and EPRI in vivo. To prepare new sterically shielded nitroxides, a reaction of cyclic nitrones, including various 1-pyrroline-1-oxides, 2,5-dihydroimidazole-3-oxide and 4H-imidazole-3-oxide with alkynylmagnesium bromide wereused. The reaction gave corresponding nitroxides with an alkynyl group adjacent to the N-O moiety. The hydrogenation of resulting 2-ethynyl-substituted nitroxides with subsequent re-oxidation of the N-OH group produced the corresponding sterically shielded tetraalkylnitroxides of pyrrolidine, imidazolidine and 2,5-dihydroimidazole series. EPR studies revealed large additional couplings up to 4 G in the spectra of pyrrolidine and imidazolidine nitroxides with substituents in 3- and/or 4-positions of the ring.
Pyrrolidine nitroxides with four bulky alkyl substituents adjacent to N–O group are known for their high resistance to bioreduction. The 3,4-unsubstituted 2-tert-butyl-2-ethylpyrrolidine-1-oxyls were prepared from the corresponding 2-tert-butyl-1-pyrroline-1-oxides via either the addition of ethinylmagnesium bromide with subsequent hydrogenation or via treatment with ethyllithium. The new nitroxides showed excellent stability to reduction with ascorbate with no evidence for additional large hyperfine couplings in the EPR spectra.
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