Circular Dichroism Can Be Deceiving

A new study serves as a reminder that when it comes to circular dichroism (CD) spectra, things aren't always what they seem (Org. Lett., DOI: 10.1021/ol901181b). CD spectroscopy detects the degree to which molecules absorb left- and right-handed circularly polarized light, and it's a common tool for evaluating biomolecules' conformations. David P. Fairlie and coworkers at the University of Queensland, in Australia, used CD spectra to examine a cyclic tripeptide that they designed to adopt an α-helical structure. The peptide's CD spectrum looked like that of an α-helix, but further characterization by proton NMR revealed that the peptide wasn't helical at all. Instead, it adopted a novel β-strand structure (above). "This example highlights that the common practice of characterizing α-helices by CD spectra alone can be misleading," the authors write. "The power of CD lies in its ability to establish conformational changes, and not so much in defining the absolute conformation of either state involved in the transition," adds Paramjit Arora, who studies helix mimics at New York University.