Louis Sullivan had architecture in mind when he declared that "form ever follows function." Had his interest been biomolecules instead of buildings, he might have inverted the famous credo—when it comes to biological processes, a molecule's effect nearly always depends on its shape. An improper protein fold, for example, can spell the difference between a vital organism and a fatally diseased one.
Understanding biological systems, then, rests in part on understanding how biomolecules shift shape. One tool at biologists' disposal is an effect known as fluorescence (or Förster) resonance energy transfer. In FRET, a donor dye, on being excited by a laser, may transfer energy to an acceptor dye through dipole–dipole interactions. As a result, irradiation of the donor causes both donor and acceptor to fluoresce—each with a distinct color—at relative intensities that depend on the distance between them.
A biomolecule tagged with donor and acceptor dyes thus emits a...