Raman spectroscopic study of the conformation and melting of poly(dG) center dot poly(dC) and poly(dG-C) center dot poly(dG-dC) in aqueous solution

Raman spectroscopic measurements on aqueous solutions of poly(dG).poly(dC) indicate that the conformation of the polynucleotides in this double helica l complex are distributed between the A and B types at room temperature, th e A form being predominant at -15 degreesC and decreasing progressively upo n raising the temperature to 65 degreesC. A reversible pretransition has be en found in this complex near 70 degreesC. Modifications in the spectra at this temperature indicate no major conformational changes, but rather sugge st altered base pairing and hydration of the carbonyl groups, accompanied b y a slight distortion of the double helix, resulting in a slightly reduced stacking of the cytosine bases. Measurements in self-pressurized solutions of the complex at high temperature show that it melts at 103 degreesC in 0. 1M NaCl solution (107 degreesC in 0.5M NaCl). These values are somewhat low er than those we have determined in the same manner for the complex poly(dG -dC).poly(dG-dC): 117 degreesC in 0.1M MgCl2 and 113 degreesC or higher in 0.1M NaCl solution.