Absorption, fluorescence and circular dichroism measurements on 33458 Hoechst deoxyribonucleic acid (DNA) complexes are consistent with the existence of two types of dye binding interactions. One type, which persists at elevated solution ionic strength, is highly specific for adenine thymine rich DNA. Dye bound under this condition exhibits efficient fluorescence and strong optical activity. A less specific, largely electrostatic interaction is associated with less intense fluorescence and weaker optical activity. The fluorescence of 33258 Hoechst and several other bisbenzimidazole dyes is less when bound to poly(deoxyadenylate 5 bromodeoxyuridylate) than when bound to poly(deoxyadenylate deoxythymidylate). Quenching of 33258 Hoechst fluorescence can also be used to detect biosynthetic incorporation of 5 bromodeoxyuridine into the DNA of living cells. This property of 33258 Hoechst should allow fluorescence activated cell and chromosome sorting according to the extent of DNA synthesis, providing a bridge between biochemical and cytologic analyses of processes related to DNA replication.
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