Ion-induced stacking of photosensitizer molecules can remarkably affect the luminescence detection of singlet oxygen in Candida albicans cells

Abstract

Singlet oxygen (¹O₂) is an important reactive intermediate in photodynamic reactions, particularly in antimicrobial PDT (aPDT). The detection of ¹O₂ luminescence is frequently used to elucidate the role of ¹O₂ in various environments, particularly in microorganisms and human cells. When incubating the fungus, Candida albicans, with porphyrins XF73 (5,15-bis-[4-(3-Trimethylammonio-propyloxy)-phenyl]-porphyrin) or TMPyP (5,10,15,20-Tetrakis(1-methyl-4-pyridinio)-porphyrin tetra(p-toluenesulfonate)), the ¹O₂ luminescence signals were excellent for TMPyP. In case of XF73, the signals showed strange rise and decay times. Thus, ¹O₂ generation of XF73 was investigated and compared with TMPyP. Absorption spectroscopy of XF73 showed a change in absorption cross section when there was a change in the concentration from 1×10⁻⁶M to 1×10⁻³  M indicating an aggregation process. The addition of phosphate buffered saline (PBS) substantially changed ¹O₂ luminescence in XF73 solution. Detailed experiments provided evidence that the PBS constituents NaCl and KCl caused the change of ¹O₂ luminescence. The results also indicate that Cl- ions may cause aggregation of XF73 molecules, which in turn enhances self-quenching of ¹O₂ via photosensitizer molecules. These results show that some ions, e.g., those present in cells in vitro or added by PBS, can considerably affect the detection and the interpretation of time-resolved luminescence signals of ¹O₂, particularly in in vitro and in vivo. These effects should be considered for any other photosensitizer used in photodynamic processes.