General remarks: All commercially available chemicals were reagent grade and used without further purification. NMR spectra were measured with a Bruker Avance 300 spectrometer ( 1 H: 300 MHz, 13 C: 75 MHz) at 25 °C; chemical shifts are given in ppm (δ) values. Multiplicities of 13 C NMR signals were determined from DEPT-135 experiments. The melting points were determined in open-end capillaries with a digital melting point instrument (SMP30, Stuart).

Fluorescence properties of an emissive guanine surrogate, thienoguanine (thGN, 2-aminothieno[3,4-d]pyrimidin-4(3H)-one), were exploited to design two real-time chemosensors of O6-methylguanine-DNA-methyltransferase (MGMT), key DNA repair enzyme involved in the resistance to DNA-alkylating...