2-PHOTON EXCITATION OF ETHIDIUM-BROMIDE LABELED DNA

We examined the steady state and time-resolved emission of DNA stained with ethidium bromide (EB) when excited with 90 fs pulses from a mode -locked titanium sapphire laser. Over the wavelength range from 840 to 880 nm EB-DNA was found to display two-photon excitation, with a cros s-section near 7 X 10(-50) cm(4)s/photon. Frequency-domain intensity d ecay measurements revealed similar multi-exponential intensity decays for one-and two-photon excitation, Time-resolved anisotropy decay meas urements revealed similar correlation times, but different amplitudes as has been observed previously for two-versus one-photon excitation. These results indicate that two-photon excitation of EB-DNA can be acc omplished with the fundamental output of a Ti:sapphire laser without o bvious heating or perturbation of the DNA. (C) 1997 Elsevier Science B .V.