New cationic fluorochromes from diaryloxazole scintillators: fluorescence of chromatin DNA induced by N-quaternary POPOP derivatives.

N-quaternary derivatives of the diaryloxazole scintillators POPOP and dimethyl-POPOP (dmPOPOP) in chloroform solution were obtained by methylation with dimethylsulfate. After drying, aqueous solutions of the corresponding oxazolium compounds (Q-POPOP and Q-dmPOPOP) revealed strong fluorescence (peaks at 485 and 493 nm, respectively). Under 365 nm excitation, both N-quaternary derivatives induced a bright greenish blue fluorescence in nuclei of chicken erythrocytes and human buccal cells, as well as in the kinetoplast DNA of Trypanosoma cruzi epimastigotes; mouse mast cell granules showed a green-yellow metachromatic emission. Chromatin fluorescence was dependent on the presence of DNA; it was abolished by washing with a 10 mM solution of the bisguanidine compound Phenformin, whereas 1 M NaCl or MgCl2 had no effect. The oxazolium derivatives were hydrophilic (log P: -6.409 and -5.373 for Q-POPOP and Q-dmPOPOP, respectively). Molecular modelling studies revealed that these cationic and non-rigid (cis) scintillator derivatives are well suited to locate along the convex floor of the narrow DNA minor groove from adenine-thymine regions.