Abstract
This study describes the design, syntheses, and evaluation of two new near-infrared (NIR) carbocyanine dyes as candidate photosensitizing agents for antimicrobial photodynamic therapy (aPDT). In aPDT, light activates photosensitizers to produce cytotoxic reactive oxygen species (ROS), to remediate treatment-resistant microbial infections. The carbocyanines feature terminal indolium groups connected by a heptamethine chain, enabling absorption of light in the tissue-penetrating NIR region. They also incorporate strategically positioned halogen atoms for improving hydrophobicity, chemical stability, and triplet-state yields. To enhance interactions with negatively charged bacterial cell walls and DNA, the dyes have different net charges at pH 7.0 (+1 vs. +2) and quaternary ammonium contents (QAC = 0 vs. 1). Across all experiments, the di-cationic carbocyanine (QAC = 1) outperformed its mono-cationic counterpart (QAC = 0), showing markedly greater stability in neutral aqueous solutions and generating higher levels of direct plasmid DNA strand breakage and hydroxyl radical/singlet oxygen ROS production under both 780 nm and broad-spectrum (707-759 nm) NIR irradiation. Spectral analyses and competitive binding assays indicated that monomeric and aggregated forms of the di-cationic carbocyanine bind within the minor groove of B-form DNA. At sub-micromolar concentrations, the di-cationic dye exhibited minimal dark toxicity toward cultured E. coli cells. However, upon 780 nm illumination, it was approximately tenfold more effective at inhibiting growth of this Gram-negative bacterium than its mono-cationic counterpart and the established aPDT agents indocyanine green and methylene blue (pH 7.0). These findings highlight the importance of developing di-cationic NIR carbocyanine dyes for aPDT applications.