Proton-Dependent Switching of a Novel Amino Chlorin Derivative as a Fluorescent Probe and Photosensitizer for Acidic Media

Abstract

A novel chlorin derivative (TPCF20-NMe2) has been synthesized as a syn adduct of a pyrrolidine-fused chlorin bearing a C-linked N,N-dimethylaminophenyl residue. The absorption spectrum of TPCF20-NMe2 is essentially identical to that of TPCF20 in N,N-dimethylformamide, indicating a very weak interaction between the chlorin macrocycle and the amine group in the ground state. However, the fluorescence emission of the chlorin moiety in TPCF20-NMe2 is effectively quenched by the attached amine unit. Moreover, TPCF20-NMe2 is unable to attain a triplet excited state or to photosensitize singlet molecular oxygen. Spectroscopic and redox properties indicate that intramolecular photoinduced electron transfer can take place from the N,N-dimethylaminophenyl group to the chlorin macrocycle. Thus, in an acid medium, protonation of the amino group leads to a considerable increase in the fluorescence emission, triplet excited-state formation, and singlet molecular oxygen production. Photodynamic inactivation of Escherichia coli sensitized by TPCF20-NMe2 is negligible at neutral pH. However, this chlorin becomes highly effective in inactivating E. coli cells under acidic conditions. Therefore, these results indicate that TPCF20-NMe2 is an interesting molecular structure, in which protonation of the amino group can be used as an off/on molecular switch activating red fluorescence emission and photodynamic activity capable of eradicating bacteria.