ELD STUDIES OF NUCLEIC-ACID STRUCTURE .4. PULSED ELECTRIC LINEAR DICHROISM OF POLY(DA)CENTER-DOT-POLY(DT)CENTER-DOT-POLY(DT) AND POLY(RA)CENTER-DOT-POLY(RU)CENTER-DOT-POLY(RU) IN SOLUTIONS

The dependence of reduced dichroism (Delta epsilon/epsilon) of sonicat ed rodlike title triplexes with the counterion Mg2+, (dA)(n).(dT)(n) ( dT')(n) and (rA)(n).(rU)(n).(rU')(n), on the wavelength (190-300 nm) a nd on the applied-field strength (3.8-23.1 kV cm(-1)) was studied in a queous ionic solutions at 7 degrees C. The sonicated samples were frac tionated and well-characterized by the gel-permeation chromatography/l ow-angle laser light-scattering method. The reduced dichroism saturate d at infinitely high electric fields ((Delta epsilon/epsilon)s) was es timated to be -1.22 for (dA)(n).(dT)(n).(dT')(n) and -1.09 for (rA)(n) .(rU)(n).(rU')(n), by fitting the observed Delta epsilon/epsilon value s at 260 nm to a theoretical orientation function. The electric linear dichroism (ELD) spectra, i.e., the dependence of (Delta epsilon/epsil on)(s) on the wavelength, of (dA)(n).(dT)(n).(dT')(n) and (rA)(n).(rU) (n).(rU')(n) were determined. The measured ELD spectra were simulated by assigning the appropriate roll (Theta(R)) and tilt (Theta(T)) angle s to the constituent bases. The most probable Theta(R) and Theta(T) va lues, determined for each tripler, indicate that the structures of tri ple helices both belong to an ''A-form'' family. The constituent bases in base-triple do not lie on a common plane. From an analysis of tran sient decay signals, the translation per base-triple (h) was estimated for each tripler. The structures of the present high molecular-weight triple-stranded helices in aqueous solution were determined for the f irst time and the plausible structural models were constructed from th e experimentally evaluated sets of Theta(R) and Theta(T).