PULSED ELECTRIC LINEAR DICHROISM OF DOUBLE-STRANDED ANTIPARALLEL POLY(RA)CENTER-DOT-POLY(RU) AND POLY(DA)CENTER-DOT-POLY(DT) HELICES IN SOLUTION

Reduced electric linear dichroism (ELD), Delta epsilon/epsilon, Of the title duplexes in ionic solutions was measured at 7 degrees C in the 1.5-24 kV/cm field strength and 300-190 nm wavelength regions. The rod like (rA)(n) .(rU)(n) and (dA)(n) .(dT)(n) samples, whose counterion w as Mg2+ in most cases, were prepared by sonication and subsequent frac tionation. The weight-average molecular weight and molecular weight di stribution of each fraction, determined by the GPC/LALLS method, were taken into account in the analysis of the field strength dependence of Delta epsilon/epsilon values. These values were fitted to theoretical orientation functions to evaluate the saturated reduced dichroism, (D elta epsilon/epsilon)(s), at absorption peaks. The wavelength dependen ce of (Delta epsilon/epsilon),, the ELD spectrum, was undulatory in th e 300-190 nm region. The isotropic absorption and second-derivative sp ectra of each duplex were decomposed into eight component bands, which were assigned to constituent bases. The analysis of ELD spectrum reve aled that the individual bases are both tilted and rolled in the rA rU and dA dT base pair and that the planes are bent. From transient dich roism signals, the weight-average rotational relaxation time, tau(w), was obtained, from which the axial translation per base pair, h, was e valuated. The limiting h values were found to be ca. 2.6 Angstrom for (rA)(n) .(rU)(n) and ca. 3.0 Angstrom for (dA)(n) .(dT)(n). Thus, both duplexes were concluded to belong to the A-form family.