Fluorescent pyrimidopyrimidoindole nucleosides: Control of photophysical characterizations by substituent effects

10-(2-Deoxy-beta-D-ribofuranosyl)pyrimido[4',5':4,5]pyrimido[1,6-a]indole-6,9(7H)-dione (dC(PPI)) and its derivatives were synthesized via the Suzuki-Miyaura coupling reaction of 5-iododeoxycytidine with 5-substituted N-Boc-indole-2-borates and characterized by UV-vis and fluorescence spectroscopy. The new fluorescent nucleosides showed rather large Stokes shifts (116-139 nm) in an aqueous buffer. The fluorescent intensities were dependent on the nature of the substituents on the indole rings. The electron-withdrawing groups increased the fluorescent intensity while the electron-donating groups having lone pairs decreased it. Among the substituted dC(PPI) derivatives tested, the trimethylammonium derivative of dC(PPI) was found to emit the brightest fluorescent light. The solvatochromism of dC(PPI) and its derivatives was also studied. Some of the dC(PPI) derivatives showed interesting solvent-dependent fluorescence enhancement and could be useful as new fluorescent structural probes for nucleic acids. The Lippert-Mataga analyses of the Stokes shift were also carried out to obtain estimated values of the dipole moment of the excited states of some of the derivatives.