Contribution of the Resonance Raman Spectroscopy to the Identification of Z DNA

Abstract

Poly(dG-dC)•poly(dG-dC) at low salt concentration (0.1 M NaCl) and at high salt concentration (4.5 M NaCl) has been studied by Raman resonance spectroscopy using two excitation wavelengths: 257 nm and 295 nm. As resonance enhances the intensity of the lines in a proportion corresponding to the square of the molar absorption coefficient, the intensities of the lines with 295 nm wavelength excitation are enhanced about sevenfold during the B to Z transition. With 257 nm excitation wavelength the 1580 cm⁻¹ line of guanosine is greatly enhanced in the Z form whereas with 295 nm excitation several lines are sensitive to the modifications of the conformation: the guanine band around 650 cm⁻¹ and at 1193 cm⁻¹ and the bands of the cytosines at 780 cm⁻¹, 1242 cm⁻¹ and 1268 cm⁻¹. By comparison with the U.V. resonance Raman spectra of DNA, we conclude that resonance Raman spectroscopy allows one to characterize the B to Z transition from one line with 257 nm excitation wavelength and from three lines with 295 nm excitation. The conjoined study of these four lines should permit to observe a few base pairs being in Z form in a DNA.