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Operation Mechanism of a Molecular Machine Revealed Using Time-Resolved Vibrational Spectroscopy

2010; American Association for the Advancement of Science; Volume: 328; Issue: 5983 Linguagem: Inglês

10.1126/science.1187967

1095-9203

Matthijs R. Panman, Pavol Bodis, Daniel J. Shaw, Bert H. Bakker, Arthur C. Newton, Euan R. Kay, Albert M. Brouwer, Wybren Jan Buma, David A. Leigh, Sander Woutersen,

Advanced NMR Techniques and Applications

Rotaxanes comprise macrocycles that can shuttle between docking stations along an axle. We explored the nanosecond shuttling mechanism by reversing the relative binding affinities of two stations through ultraviolet-induced transient reduction. We monitored the ensuing changes in the CO-stretching bands of the two stations and the shuttling macrocycle by means of an infrared probing pulse. Because hydrogen-bond scission and formation at the initial and final stations led to well-resolved changes in the respective CO-stretch frequencies, the departure and arrival of the macrocycle could be observed separately. We found that the shuttling involves two steps: thermally driven escape from the initial station, followed by rapid motion along the track ending either at the initial or final station. By varying the track's length, we found that the rapid motion approximates a biased one-dimensional random walk. However, surprisingly, the direction of the overall motion is opposite that of the bias.