circular dichroism, or CD, is defined as the differential absorption of left and right hand circularly polarized light:
CD = (CL - CR).
In general, this kind of phenomenon will be exhibited by any optically active molecule. As a consequence, circular dichroism is exhibited by biological molecules, because of the dextrorotary (sugars) and levulorotary (amino acids) molecules they contain. Noteworthy as well is that secondary structure will also impart a distinct CD absorption to their respective molecules. Therefore, the alpha helix of proteins and the double helix of nucleic acids will have a CD signature representative of their structure.