Terbium(III)-induced conformational transition in poly(dG-dC) fluorescence and circular dichroic studies

Poly(dG-dC) in 60% aqueous alcohol exhibits the characteristic inversion of the circular dichroism spectrum associated with the formation of left-handed helix. Upon complexation with Tb3+, poly(dG-dC) in this medium induces marked enhancement of the Tb(III) fluorescence emission at 488 and 545 nm, when excited at 290 nm. The degree of fluorescence enhancement is dependent on the concentration of Tb(III) at a fixed poly(dG-dC) concentration. Neither poly(dG-dC) in water nor poly(dA-dT) in water or 60% alcohol, causes any significant fluorescence spectral changes of Tb3+. Tb(III)-poly(dG-dC) in 60% alcohol shows circular dichroic spectra associated with a broad positive molar ellipticity ranging from 6000 to 10 000 degree X cm2 X dmol-1 between 270 and 280 nm, and a small negative band around 240 nm.