Molecular field theory of hysteresis in helix-coil transitions of polynucleotides.

The influence of cooperativity on conformational changes in biopolymers has been extensively studied both experimentally and theoretically.' Nevertheless relatively small attention has been paid to the existence of hysteresis cycles in some of these transitions. This might be due to the fact that non-reversible transitions are rather suspicious to many scientists. It is the merit of Aharon Katchalsky to have recognized the importance of hysteresis phenomena, especially as a mechanism for molecular memory recording. The aim of this paper is to present a theory of hysteresis in polyelectrolytes and to apply it to the helix-coil transition of the three-stranded complex poly(A) .2 poly(U) into the constituent polymers polyriboadenylate (poly(A)), and polyribouridylate (poly(U)). This conformational change preceeds the formation of the helix poly(A) .poly(A) in the acid titration of poly(A) -2 poly(U) according to Eq. (1) :