Charge density distribution effect in pyrrolidine-fused chlorins on microbial uptake and antimicrobial photoinactivation of microbial pathogens

Abstract

Two novels structurally related pyrrolidine-fused chlorins were synthesized from 5,10,15,20-tetrakis(pentafluorophenyl)chlorin by nucleophilic aromatic substitution of the para-fluoro groups. The reaction with 2-dimethylaminoethanol produced TPCF16-NMe2 in 77% yield, while TPCF16-NBu was obtained using butylamine in 87% yield. The latter was extensively methylated to form TPCF16-N+Bu in 92% yield. The synthetic strategy was designed to compare the effect of charge density distribution on chlorin in the efficacy to induce photodynamic inactivation of pathogens. TPCF16-NMe2 has five tertiary amines that can acquire positive charges in aqueous medium by protonation. Furthermore, four of the cationic groups are located in amino groups linked to the chlorine macrocycle by an aliphatic structure of two carbon atoms, which gives it greater movement capacity. In contrast, TPCF16-N+Bu presents intrinsic positive charges on aromatic rings. Absorption and fluorescence emission properties were not affected by the peripheral substitution on the chlorin macrocycle. Both photosensitizers (PSs) were able to form singlet molecular oxygen and superoxide anion radical in solution. Uptake and photodynamic inactivation mediated by these chlorins were examined on Staphylococcus aureus and Escherichia coli. Both phototherapeutic agents produced efficient photoinactivation of S. aureus. However, only TPCF16-NMe2 was rapidly bound to E. coli cells and this chlorin was effective to photoinactivate both strains of bacteria using lower concentrations and shorter irradiation periods. Our outcomes reveal that the charge density distribution is a key factor to consider in the development of new PSs. Accordingly, this work stands out as a promising starting point for the design of new tetrapyrrolic macrocycles with application in PDI.