EFFECT OF ETHIDIUM ON THE FORMATION OF POLY(DA)CENTER-DOT[POLY(DT)](2) TRIPLEX - A KINETIC-STUDY BY OPTICAL SPECTROSCOPIC METHODS

The kinetics of the formation of tripler poly(dA).[poly(dT)](2) from p oly(dA).poly(dT) and poly(dT) is examined by various optical spectrosc opic methods, including absorption, circular and linear dichroism (LD) spectroscopy. In the pseudo first order condition, where the poly(dT) concentration is kept lower than that of duplex, the association of t he poly(dT) is enhanced by the presence of ethidium; the rate constant is proportional to the amount of ethidium in the mixture. When the co ncentration of the duplex and the single strand is the same, a spectra l change is explained by double exponential curves, indicating that at least two steps are involved, the fast association and slow rearrange ment steps. In contrast to the pseudo first order kinetics, the associ ation step in an equimolar condition is not affected by the presence o f ethidium. In the rearrangement step, the magnitude of LD decreases w ith an increase in ethidium concentration, suggesting that the bending of polynucleotide around the intercalation site occurs in the rearran gement step.