Laser-induced fluorescence of acridine–DNA complexes

A description is given of a laser microfluorometer suitable for the measurement of the decay of the fluorescence emitted from microscopic biological samples. The apparatus consists of a dye laser pumped by a nitrogen laser, a microscope system, and a digital signal averager. Its principal parameters are a temporal resolution of ~0.3 nsec and a spatial resolution of ~0.3 μ. This microfluorometer was used to study the fluorescence of complexes formed by acridine dyes with deoxyribonucleic acid (DNA) in synthetic polynucleotides, purified native DNA, and cytological preparations. The experiments were performed on smears of several bacteria with known adenine–thymine (AT) pair percentages and they indicated that, after a transient, each fluorescence curve could be described by an exponential decay law with a single time constant depending linearly on the square of the AT content. The results were interpreted on the basis of a mechanism of energy transfer between dye molecules forming different complexes with DNA.