Polarography of polynucleotides. IV. Effect of the molecular weight of poly(U) On its absorption at a charged surface.

Abstract

AbstractEffects of molecular size on the adsorption properties of poly(U) were studied using alternating current polarography as a technique and a dropping mercury electrode (d.m.e.) as a model interface. The measurements were carried out on fractions (by molecular weight) of poly(U) characterized by sedimentation and viscosity measurements. The data indicate that the rate of poly(U) adsorption is controlled by diffusion. The adsorption equilibrium can be established during the drop‐life at higher concentrations of poly(U) where the electrode is fully covered, while at low concentrations corresponding to the linearized isotherm, the time τ for the establishment of the adsorption equilibrium is much longer than can be obtained with d.m.e. The time τ increases with increasing chain length, n (in monomeric units). From the concentration dependence of the course of current time, I ∼t, curves, or of the current values at the end of the drop life, a constant K was calculated which depends on n. In the range of molecular weights studied (25 × 103 to 564 × 103) the data obeys the relationship log n = a + b log K (where a and b are constants). Area, A, occupied by the monomer of an adsorbed poly(U) molecule was calculated from experimental values of K and D (diffusion coefficient). The decrease of A with increasing n was explained in terms of the looping of longer poly(U) chains out from the electrode surface.