A new twist on mirror-image molecules.

Like hands or ears, many molecules come in two similar but distinct forms that are mirror images. This property, known as chirality, leads to a curious optical effect. Polarized light shone on a solution of one molecular form rotates clockwise; shone on the mirror image, the light will rotate counterclockwise.

Even though scientists have known about this phenomenon for 150 years, only recently have they been able to predict the angle and direction of the rotation for a particular molecule. Now, Rama K. Kondru, Peter Wipf, and David N. Beratan of the University of Pittsburgh have further advanced the understanding of chirality by figuring out a way to calculate how much individual atoms and their electrons within the molecule contribute to the optical rotation. They describe their method in the Dec. 18, 1998 SCIENCE.

The calculations require a lot of computing power, so the researchers limited the molecules they tested to 10 or 12 atoms, says Beratan. Based on electron distributions, they predicted optical rotation for three classes of compounds: oxiranes, simple chiral molecules that are small and rigid; more complex cyclic esters; and linear molecules, specifically fluorohexane. The "most provocative" finding, Beratan says, is that the presence of a twist in a fluorohexane chain, as well as the arrangement of its atoms, appears to influence the light rotation.