Biomedical subjects
Publications and source records attributed to F Zerbetto.
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Catenanes can undergo rotation of one ring through the cavity of the other. Since macroscopic and molecular properties must clearly vary with the relative positions and orientations of the interlocked components, a complete understanding of the way that the rings rotate is of considerable importance. Here we show that low-dimensional quantum-mechanical modeling can yield rate constants and barriers similar to those obtained by temperature-dependent nuclear magnetic resonance experiments. Data from both non-hydrogen bond disrupting (e.g. CDCl3) and hydrogen bond disrupting (e.g. [D6]DMSO) solvents are well reproduced demonstrating the validity of the model. The successful simulation of the rates of circumrotations by entirely harmonic transition state theory originates from the description of the anharmonic levels of the systems through an effective harmonic frequency, not very different from twice the zero point energy. The nature of the model makes it extendable, in principle, to the calculation of properties dependent upon circumrotational activity.
Analogues of mechanical devices that operate on the molecular level, such as shuttles, brakes, ratchets, turnstiles and unidirectional spinning motors, are current targets of both synthetic chemistry and nanotechnology. These structures are designed to restrict the degrees of freedom of submolecular components such that they can only move with respect to each other in a predetermined manner, ideally under the influence of some external stimuli. Alternating-current (a.c.) electric fields are commonly used to probe electronic structure, but can also change the orientation of molecules (a phenomenon exploited in liquid crystal displays), or interact with large-scale molecular motions, such as the backbone fluctuations of semi-rigid polymers. Here we show that modest a.c. fields can be used to monitor and influence the relative motion within certain rotaxanes, molecules comprising a ring that rotates around a linear 'thread' carrying bulky 'stoppers' at each end. We observe strong birefringence at frequencies that correspond to the rate at which the molecular ring pirouettes about the thread, with the frequency of maximum birefringence, and by inference also the rate of ring pirouetting giving rise to it, changing as the electric field strength is varied. Computer simulations and nuclear magnetic resonance spectroscopy show the ring rotation to be the only dynamic process occurring on a timescale corresponding to the frequency of maximum birefringence, thus confirming that mechanical motion within the rotaxanes can be addressed, and to some extent controlled, by oscillating electric fields.
An analogy with a cart on a roller coaster partially explains the shuttling motion of macrocycles in peptide rotaxanes. Just above the barrier to shuttling, the macrocycle statistically populates the "track" rather than the low-energy "stations" (see the potential energy curve). The dynamics of the movement is described in terms of a simplified one-dimensional model based on the solution of the relevant quantum-mechanical equation. x=position.