[19] Circular dichroism spectroscopy of DNA

Circular dichroism (CD) measurements are used to study the conformations of nucleic acids in solution. The reliance on CD spectroscopy to study DNA conformations has stemmed from the sensitivity and ease of CD measurements, the nondestructive nature of such measurements, the fact that conformations can be studied in solution, and the requirement for relatively small amounts of material. Although detailed structural information, such as from X-ray crystallography or NMR spectroscopy, is not available from CD spectra, the CD spectrum of DNA in solution can provide a reliable determination of its overall conformational state when compared with the CD spectra of reference samples. Moreover, CD spectroscopy is applicable to a wide range of samples, including those that are difficult to crystallize or to obtain at high concentrations. The chief practical application of CD spectroscopy to the study of DNA structures has been by making empirical comparisons with the CD spectra of known structures. The chapter provides examples from laboratory of CD spectral changes in the near ultraviolet range that illustrate the CD characteristics of structural transitions in natural and synthetic DNAs.