SINGLE-STRANDED POLYADENYLIC ACID-POLY-L-LYSINE COMPLEX . A CONFORMATIONAL STUDY AND CHARACTERIZATION

The effect of poly-L-lysine on the conformation of single-stranded polyadenylic acid has been studied by optical rotatory dispersion and ultraviolet spectroscopy. The ultraviolet and optical rotatory dispersion spectra of polyadenylic acid are altered on addition of poly-L-lysine at pH 7.0. The ultraviolet maximum of 256 mμ for polyadenylic acid shifts to 262 mμ for the complex. The optical rotatory dispersion spectrum is also red shifted (4 mμ), and decreased in molar rotation ([m]256poly A = -78,000, [m]260complex = -23,000) and complexity as compared with polyadenylic acid. Complex formation occurs in two steps. The primary interaction involves single-chain association, which is followed by a secondary aggregative interaction. The primary association occurs with a 1:1 residue stoichiometry, a large association constant, relatively little insolubility, and an apparent insensitivity to both the degree of charge on the poly-L-lysine and to ionic strength (100-40% charge, H2O to 0.2 M NaF). The secondary (aggregative) interaction, which occurs near residue equivalence, is marked by further red shifts of the optical rotatory spectra and increased insolubility. Aggregation occurs maximally when the poly-L-lysine is 50% helical. There is no complex formation in 0.2 m NaF when the poly-L-lysine has zero charge and is 100% helical. It is concluded that poly-L-lysine forms a well-defined complex with polyadenylic acid, altering the conformation of the latter. © 1969, American Chemical Society. All rights reserved.