High binding yet accelerated guest rotation within a cucurbit[7]uril complex. Toward paramagnetic gyroscopes and rolling nanomachines

Abstract

The (15-oxo-3,7,11-triazadispiro[5.1.5.3]hexadec-7-yl)oxidanyl, a bis-spiropiperidinium nitroxide derived from TEMPONE, can be included in cucurbit[7]uril to form a strong (K(a)∼ 2 × 10(5) M(-1)) CB[7]@bPTO complex. EPR and MS spectra, DFT calculations, and unparalleled increased resistance (a factor of ∼10(3)) toward ascorbic acid reduction show evidence of deep inclusion of bPTO inside CB[7]. The unusual shape of the CB[7]@bPTO EPR spectrum can be explained by an anisotropic Brownian rotational diffusion, the global tumbling of the complex being slower than rotation of bPTO around its "long molecular axis" inside CB[7]. The CB[7] (stator) with the encapsulated bPTO (rotator) behaves as a supramolecular paramagnetic rotor with increased rotational speed of the rotator that has great potential for advanced nanoscale machines requiring wheels such as cucurbiturils with virtually no friction between the wheel and the axle for optimum wheel rotation (i.e. nanopulleys and nanocars).

Nitroxide Radicals with Cucurbit[n]urils and Other Cavitands

“…[32] However, the rotational dynamics of a bis-appended ammonium nitroxide ( Figure 5) showed unexpected EPR features with a high-field line sharpened upon CB [7] and CB [8] inclusion complexation. [33] Simulations of the EPR spectra using several models agreed with a decrease in the rotational correlation time of the nitroxide inside CB[n], assigned to an acceleration of the molecular motion of the spin probe in the cavities. Such an effect was previously scarcely observed [34] and could have relevance for low friction (in the sense of steric hindrance) molecular machines [35] although the rotation is likely random.…”

“…[42] In 2009, TEMPO was reported to be dramatically protected against one of the more reducing agents of nitroxides, ascorbic acid, when complexed with CB [7]. [33] Parallel to that work, a derivatization approach, directly on the nitroxide skeleton has been introduced, that revealed to be very efficient for lengthening the nitroxide lifetime in vivo without macrocycle. [33] Parallel to that work, a derivatization approach, directly on the nitroxide skeleton has been introduced, that revealed to be very efficient for lengthening the nitroxide lifetime in vivo without macrocycle.…”

“…a N ) giving 3-line spectra as for isolated nitroxides and (ii) 5-line spectra when the radicals are very close (J @ a N ). Time monitoring for the intensity of the EPR spectra of bPTO and TEMPO in cyclodextrins or in cucurbiturils in highly reducing conditions (reproduced from reference [33] with permission from The Royal Society of Chemistry). Similarly, for closely related nitroxides in a trinitroxide, 7-line spectra can be observed with respective intensity ratio of the lines 1 : 3 : 6 : 7 : 6 : 3 : 1 and for tetranitroxides, 9-line spectra with intensity ratios of 1 : 4 : 10 : 16 : 19 : 16 : 10 : 4 : 1 (Figure 9).…”

Kinetics Inside, Outside and Through Cucurbiturils

“…[55] Those rotational correlation times would return reasonable, albeit large hydrodynamic radii (5.3 and 11 Å , respectively), assuming that the viscosity inside CB [7] is similar to bulk water, a very questionable hypothesis! [56] Rotational correlation times along the minor axis are not affected by the macrocycle (550 vs. 537 ps). [56] Rotational correlation times along the minor axis are not affected by the macrocycle (550 vs. 537 ps).…”

“…CB [8] also increased rotation rates by a factor of 2, while CB [7] encapsulation of radical 23 b resulted in a 34-fold decrease in the rotational correlation time. [56] The two studies described above represent the only cases were CB[n] complex rotation in the bulk and guest rotation inside CB[n] may have been successfully isolated. [56] The two studies described above represent the only cases were CB[n] complex rotation in the bulk and guest rotation inside CB[n] may have been successfully isolated.…”