Perils in Circular Dichroism

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Science  12 Jun 2009:
Vol. 324, Issue 5933, pp. 1366
DOI: 10.1126/science.324_1366c

Circular dichroism (CD) spectra nominally measure the extent to which a particular chiral molecule preferentially absorbs one sense of circularly polarized light over the other sense. However, the spectra have traditionally been acquired using samples that comprise millions of trillions of molecules, and they have largely been treated as empirical data. In this context, a study that probed the interaction of circularly polarized light with one molecule at a time (Hassey et al., Reports, p. 1437, 1 December 2006) potentially offered deep mechanistic insight. The authors found that on an individual basis, single stereoisomers of a helicene molecule differed across a huge range in their CD response—some, in fact, appeared to favor light polarized opposite to the sense preferentially absorbed overall in an ensemble of like enantiomers. The method relied on fluorescence microscopy to achieve single-molecule resolution, and Tang et al. have now suggested that the data might be more easily explained by an artifact associated with the apparatus than by an inherent molecular property. Specifically, the second group of authors attempted to reproduce the findings, but observed that the circular polarization of incoming light was distorted into an ellipse upon reflection at a dichroic mirror, thus becoming more sensitive to molecular orientation. After correcting for this effect, they failed to detect distinct signals from the different single stereoisomers. They therefore propose that the previously observed wide-ranging CD response was actually a manifestation of linear dichroism (a function of orientation rather than chirality), and they argue for careful analysis of mirror-induced distortions in future related studies.

J. Phys. Chem. A 113, 6213 (2009).

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