Chemistry

Pulled but not Distorted

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Science  17 Mar 2006:
Vol. 311, Issue 5767, pp. 1523-1525
DOI: 10.1126/science.311.5767.1523d

Single-molecule force spectroscopy can be used to examine the potential energy landscape of displacement reactions. Such analysis assumes that the reaction mechanism remains the same when the dissociation step is assisted by mechanical force. Kersey et al. explored this question by attaching substituted pyridines to a substrate and an atomic force microscope tip, and then bridging the tip and substrate using a molecule with square-planar Pd centers that could bind each pyridine. They then measured force-extension curves for various loading rates that captured bond rupture events in which dimethyl-sulfoxide (DMSO) solvent displaced the pyridine ligands from Pd. The thermal rates extrapolated from the data corresponded well with rates measured by nuclear magnetic resonance for the analogous displacement reaction in free solution. Thus, the same bimolecular mechanism appears to operate in both the thermal and nonequilibrium applied stress regimes, with the observed stress-induced acceleration (approximately a 10-fold rate increase for a 50-piconewton force) arising from a lower-energy transition state for Pd-pyridine bond scission and Pd-DMSO bond formation. — PDS

J. Am. Chem. Soc. 10.1021/ja058516b (2006).

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