Generally for any normal DNA (Salmon Sperm DNA) we got the UV-absorption peak at 260 nm, But in Circular Dichroism we got two peak one positive peak at 275nm and one negative peak at 245 nm. Why? If this is due to cotton effect then what is actually mean by cotton effect (any reference )?
The exciton coupling of pi-pi* transitions in the plane of stacked bases gives rise to CD bands of equal magnitude and opposite sign that are slightly shifted from the absorption maximum. For a right-handed helix, the positive band is shifted to longer wavelengths and the negative band to shorter wavelengths. These bands cancel at the absorption maximum, so the CD vanishes near 260 nm. They give rise to a positive band with a maximum at 275 nm and a negative band with a maximum at 245 nm.
The CD spectrum of DNA shows characteristic differences between B- and A-forms. In addition, tertiary structure affect the CD spectrum so that CD can be used to monitor supercoiling, triplex or quadruplex formation. In addition to the references mentioned previously, I recommend two reviews by Don Gray and coworkers in Methods in Enzymology (vol. 211, p. 389 (1992) and 246, p. 19 (1995)).
Dear Debasish,
CD gives rise to different signals when compared to UV-Vis absorption techniques because the basis of each technique is different. In CD, as you may know, the lamp emits circularly polarized light, very different from that one used in UV-Vis spectroscopy. Therefore the resulting spectra is different. Then, the explanation about what you're seeing was already given in perfection by Robert Woody on the previous answer. Try to see in your spectra acquisition software if you have the option to choose more than one channel in which you'll be able to choose CD as well as UV absorption as two different outputs and you'll see the differences.
Best regards.
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