Quest for novel fluorogenic xanthene dyes: Synthesis, spectral properties and stability of 3-imino-3H-xanthen-6-amine (pyronin) and its silicon analog

To expand the range of primary aniline fluorophores available and suitable for the design of fluorogenic protease probes, the synthesis of 3-imino-3H-xanthen-6-amine (known as pyronin) and its silicon analog (Si-pyronin) was explored and presented here. A comprehensive photophysical study of these two fluorescent anilines, confirms the effectiveness of the heteroatom-substitution approach (O → SiMe2) to yield dramatic red-shifts in absorption and fluorescence maxima of the xanthene scaffold (+85 nm). However, it also revealed its adverse effect on the hydrolytic stability of the Si-pyronin, especially at physiological pH. The pro-fluorescent character and utility of these two fluorogenic (hetero)xanthene dyes are also proved by the preparation and in vitro validation of activatable fluorescence "turn-on" probes for penicillin G acylase (PGA).