The interaction of polyxanthylic acid with polyadenylic acid

The stoichiometry of interaction between polyxanthylic acid (poly(X)) and polyadenylic acid (poly(A)) was investigated by construction of mixing curves as a function of wavelength. Wavelengths were found which exhibited a break at 48-50 mol % poly(A) or two breaks, one at 48-50 mol % poly(A) and the other at 33-30 mol % poly(A). The melting profile (in 0.1 M salt) of the 50 mol % poly(A) mixture was monophasic (Tm=83 degrees C) at all wavelengths but that of the 33 mol % poly(A) mixture was biphasic showing a transition at Tm=40 degrees C and another at Tm=83 degrees C. Construction of a mixing curve between poly(A) and poly(X) at a temperature of 52 degrees C gave rise to plots which showed only one break (at 49--50 mol % poly(A)) at all wavelengths. Thus, while (at 20 degrees C in 0.1 M salt, pH 7) both poly(A)-poly(X) and poly(A)-2 poly(X) form at their respective stoichiometric end-points, the triplex poly(A)-2 poly(X), upon melting undergoes a disproportionation reaction resulting in the loss of one poly(X) strand and the formation of the poly(A)-poly(X) duplex.