A series of oligodeoxynucleotides labeled by it pyrimidopyrimidoindole deoxynucleoside (1a: dC(PPI)) and its derivatives 2a and 3a substituted with electron-donating and -withdrawing groups, respectively, were synthesized according to the phosphoramidite approach. The photophysical properties and quenching efficiencies of oligonucleotides incorporating dC(PPI) derivatives were studied in detail. The thermal denaturation experiments and molecular dynamics simulation of DNA duplexes incorporating dC(PPI) suggested that a modified base of dC(PPI) Could form base pairs With guanine and adenine in canonical Watson-Crick and reverse-wobble geometries, respectively. The fluorescence of oligonucleotides incorporating dC(PPI) derivatives increased upon binding to the counter strands, except when dC(PPI) and guanine formed a base pair. It was revealed that dGMP quenched the fluorescence of the cyano derivative 3a most effectively, whereas it affected that of the methoxy derivative 2a least effectively. The involvement of the electron transfer from guanine to the dC(PPI) derivatives in the fluorescence quenching was Supported by energy considerations.