| dc.description.abstract | The effects of the univalent ions like Li?, Na?, K?, Cs?, etc. and their counter ions on the CD spectra of various DNA samples have been investigated in some detail. With the increase in the concentration of the univalent electrolytes the CD spectra of DNA, which resemble closely the B?form spectra, undergo changes finally approaching close to the spectra of the C?form of DNA. The structural changes brought about by these electrolytes are thus transitions from one form to the other maintaining the double helical nature and the UV absorption is not affected appreciably. These transitions occur at considerably high concentrations of these ions. Similar types of transitions from one form to the other also occur in non?aqueous solvents like methanol, ethanol, etc.
On the other hand, the heavy metals like Hg, Cu, U, Ag etc. which bind mainly to the bases of the nucleic acids cause large distortion of their CD or ORD spectra even at very low concentration. The distorted spectra do not coincide with the spectra of any known form of the nucleic acids and are characterized by the red?shift of the positive band along with the reduction of the mean residue ellipticity.
According to the theoretical calculation of Tinoco, the nature and the magnitude of ellipticity of CD spectra of polynucleotides in the UV?region is determined by the base chromophores mainly due to their n ? ?* transitions. The CD spectra are strongly affected by the base?base interactions of the polymers and consequently the stacking interaction of the bases plays an important role in characterizing the CD spectra. The electronic configuration of the bases determines the strength of the transition dipole. Thus any chemical change of the bases like protonation, hydrogen bonding or interaction with metal ions would cause an alteration in the electronic configuration and the CD spectra will be distorted. It has been observed that the unstacking of bases causes the decrease in the ellipticity. The disruption of hydrogen bonds between the bases of the double helical nucleic acids is accompanied by bathochromic shifts of the CD extrema.
Interaction of Au(III) with the nucleic acids causes the positive CD band to shift to the longer wavelength side for all the native and denatured DNA and RNA. This indicates that the bases are chemically modified by the interaction with Au(III) resulting in the alteration of electronic configuration and disruption of hydrogen bonds of the double?stranded nucleic acids. The mean residue ellipticity near the positive band is largely decreased with the increased amount of Au(III) bound to nucleic acids. The interaction with Au(III) as well as thermal denaturation of DNA proceeds with the increase in the UV?absorption. But the positive CD band exhibits stronger decrease in complex formation with Au(III) while thermal denaturation causes very little decrease in the ellipticity of the positive band. (In the present investigation it is observed that the mean residue ellipticity at 275 nm is a little more for denatured DNA than that for native DNA. This is thought to be due to the difference in the medium. In the present investigation pH is very low (5.0) compared to the usual medium of pH = 7.0. At low pH protonation of denatured DNA may cause this increase.)
The basic difference between the interaction of Au(III) with the thermal denaturation of DNA is that the thermal denaturation causes the separation of the complementary strands of DNA which assume random configuration whereas Au(III)?binding crosslinks the two strands and the bases are properly arranged to form chemical bonds with Au(III). Thus in the Au(III) complexes there exist locally symmetric regions of the chromophoric groups. This arrangement may be such that the rotatory powers of the groups are cancelled by each other giving lower ellipticity.
It should be noted that with the increase in r?value the [?] values decrease regularly for all the systems. The decrease in mean residue ellipticities of denatured DNA and RNA are more compared to that of native DNA. This may arise due to greater flexibility of the single?stranded nucleic acids compared to rigid double?helical DNA.
The greater distortion of the CD spectra of GC?rich calf?DNA in complexing with Au(III) compared to the other two AT?rich DNAs (CT?DNA and CP?DNA) at the same value of r may be due to the stronger binding of Au(III) to GC?rich DNA. But the equilibrium investigations indicate that Au(III) binds preferentially to the AT?rich DNA, though the kinetic studies in the present investigation have shown that the GC?rich DNAs react relatively faster. The GC?specificity in the equilibrium state cannot be predicted unambiguously from the foregoing CD observations because the more distortion in the CD spectra of GC?rich DNA may also arise from the difference in the arrangement of the GC?base pairs with that of the AT?base pairs in the Au(III)?complex.
It can be concluded from the circular dichroism studies on the complexes of Au(III) with nucleic acids that the structures of nucleic acids are largely altered from their uncomplexed structure in solution. This supports the earlier observations regarding large decrease in the viscosity of the solutions of nucleic acids after adding Au(III). CD study confirms that Au(III) interacts with the bases of the nucleic acids though the participation of the phosphate group is not ruled out. The binding of Au(III) causes the disruption of a considerable amount of hydrogen bonding in the double?stranded nucleic acids. The binding of Au(III) gives rise to structures which have local symmetry regarding the bases of the nucleic acids. | |
