DNA dynamics observed with long lifetime metal‐ligand complexes

Measurements of DNA dynamics are typically limited to the nanosecond timescale because of the nanosecond decay times of commonly used fluorophores such as ethidium bromide and acridine derivatives. We show that the time‐resolved anisotropy measurements can be extended to the submicrosecond timescale using metal‐ligand complexes. The ruthenium complex [Ru(2,2′,‐bipyridine)₂(dipyrido[3,2‐a : 2′,3′‐c]phenazine)]²⁺, or [Ru(bpy)₂(dppz)]²⁺, was found to display high anisotropy near 0.2, at −60°C in 100% glycerol, when excited in its long‐wavelength absorption bands. The decay time of [Ru(bpy)₂(dppz)]²⁺ when bound to DNA is over 100 ns, allowing intensity to be measured to over 400 ns. Other Ru complexes display still longer lifetimes to 750 ns when bound to DNA. This class of fluorophores should enable measurement of DNA dynamics over a wide range of times (from 1 ns to several μs), which were previously inaccessible using time‐resolved fluorescence. © 1995 John Wiley & Sons, Inc.