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.
Highlights
The photophysical properties of XF73 could have been altered after the uptake of C. albicans cells
The detection of singlet oxygen by its luminescence is a great tool to show the action of singlet oxygen even in cells or bacteria
In this context it is important to have a detection procedure that provides reliable data from inside such cells, in particular when knowing that cellular constituents can substantially affect singlet oxygen luminescence
Summary
The fast development of multiresistant patterns against antibiotics of many species of bacteria has led to novel antibacterial strategies like the antibacterial photodynamic therapy (aPDT).[1,2] A lot of work has been done to develop molecular structures and their derivatives that are able to generate reactive oxygen species (ROS), which are the active agents for killing microorganisms.[3,4,5,6,7] The search for photosensitizers (PSs) for aPDT has caused the synthesis of various porphyrin molecules, which have been investigated regarding their photophysics and antimicrobial activity.[4,8,9] Naturally occurring porphyrins can be found endogenously, e.g., the protoporphyrin IX that is in the prosthetic group of the hemoglobin or the chlorophylls based on the chlorine structure. The porphyrin TMPyP has been frequently used for cell staining in order to investigate generation and decay of 1O2.11–13
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