A Folding Ruler

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Science  18 Feb 2005:
Vol. 307, Issue 5712, pp. 1017
DOI: 10.1126/science.307.5712.1017c

The development of a suite of fluorescent probes that can be introduced into cells has made it feasible to estimate intracellular distances (static and dynamic) via the technique of Förster resonance energy transfer (FRET). To a first approximation, the efficiency of energy transfer from a donor to an acceptor fluorophore depends on the inverse sixth power of the distance separating them. Converting efficiency into units of distance requires a calibrating ruler with a donor fixed at one end and an acceptor at the other. Schuler et al. use contemporary single-molecule technology to reexamine the classical ruler, a rigid rod of 12 proline residues, introduced originally by Stryer and Haugland almost four decades ago. A number of factors conspire to make the observed efficiencies deviate from the values predicted by Förster theory, especially for much longer rulers that, surprisingly, turn out to be much less rigid than the canonical polyproline helix. — GJC

Proc. Natl. Acad. Sci. U.S.A. 102, 2754 (2005).

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