Nucleic Acid Circular Dichroism
Living reference work entry
Deoxyribonucleic acid, DNA, is composed of planar aromatic nucleic acid bases linked to a sugar and a phosphate as illustrated in Fig. 1. Ribonucleic acid, RNA, differs from DNA by an extra oxygen on the sugar, and also thymine is replaced by uracil which does not have the methyl substituent. Most of the DNA and RNA transitions we measure with UV spectroscopy are due to the π–π* transitions of the bases. These transitions have no intrinsic CD signal since the bases are planar and hence achiral. However, the isolated nucleosides acquire asymmetry in their electronic transitions by coupling with the chiral ribose sugar units of the backbone. The magnitude of Δ ε max (the molar difference in absorbance of left and right circularly polarized light) for each base is of the order of 2 mol −1 dm 3 cm −1at 270 nm; the purine bases have a negative signal, whereas the pyrimidine ones have a positive CD in that region. When the bases are linked by phosphodiester bonds to form...
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