Spin-orbital probes of biomolecular structure. A model DNA-acridine system.

Heavy atoms, such as bromine or iodine, perturb the excited-state properties of aromatic chromophores through a spin-orbital coupling mechanism. In the present work the use of specifically directed spin-orbital probes to study subtle structural relationships in biopolymers is described. Heavy atoms are introduced into defined sites in biochemical systems and the emission spectrum of a ligand or intrinsic chromophore is monitored for perturbation by the bound heavy atom. This technique is illustrated by a study of acridine dye binding to the copolymer poly(dA-BrdU). The results are interpreted in terms of an "externally" bound dye fraction whose emission is perturbed by the heavy atom in the polymer and an intercalated dye component unperturbed by bromine.