Novel emissive bio-inspired non-proteinogenic coumarin-alanine amino acid: fluorescent probe for polyfunctional systems

Abstract

Two new bio-inspired non-proteinogenic compounds L1 and L2, containing coumarin and/or acridine chromophores and bearing as spacer an alanine amino acid were successfully synthesized and fully characterized by elemental analysis, (1)H and (13)C NMR, infrared spectroscopy (KBr discs), melting point, ESI-TOF (electrospray ionization-time of flight-mass), UV-vis absorption and emission spectroscopy, fluorescence quantum yields and lifetime measurements. A relative fluorescence quantum yield of 0.02 was determined for both compounds. In L2 the presence of an intramolecular energy transfer from the coumarin to the acridine unit was observed. L1 and L2 are quite sensitive to the basicity of the environment. At alkaline values both compounds show a strong quenching in the fluorescence emission, attributed to the photoinduced electron transfer (PET). However, both deprotonated forms recover the emission with the addition of Zn(2+), Cd(2+) and Al(3+) metal ions. As multifunctional emissive probes, the titration of L1 and L2 with lanthanides (III), Eu(3+) and Tb(3+) was also explored as new visible bio-probes in the absence and in the presence of liposomes. In a liposomal environment a lower energy transfer was observed.