Polynucleotides: VII. Spectrophotometric study of the kinetics of formation of the two-stranded helical complex resulting from the interaction of polyriboadenylate and polyribouridylate

Abstract

The kinetics of interaction of equimolar mixtures of poly A and poly U in low and moderate [Na+] at several temperatures has been followed at three wavelengths: 259·0 mμ, where the hypochromic change per U residue incorporated into helix is the same for poly (A + U) and poly (A + U + U); 280·0 mμ, where the hypochromic change is due only to the formation of poly (A + U + U); and 283·5 mμ. where the hyperchromic change is due only to the formation of poly (A + U). The results indicate that, under conditions that do not allow formation of poly (A + U + U) (0·01 M-Na+ at 25°C), poly (A + U) is formed directly and at a rate that can be described by a standard second-order kinetic expression. By contrast, under conditions that do allow poly (A + U + U) formation (0·01 M-Na+ at < 22°C or in 0·2 M-Na+ at 25°C), the complete formation of poly (A + U) does not proceed directly. In addition to rapid formation of poly (A + U), substantial amounts of poly (A + U + U) appear with an induction period. This formation of incidental poly (A + U + U) reaches a maximum shortly before all the poly U is consumed. Thereafter, the formation of poly (A + U) is notably slower and proceeds at a rate that is indistinguishable from the rate of disappearance of a half-equivalent of poly (A + U + U). Complete formation in 0·2 M-Na+ at 25°C is not approached until approximately 72 hours. Four rate expressions derived from standard kinetic laws usually applied to reactions between small molecules are proposed as a quantitative description of the entire sequence of events.