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ISSN 1998-9539

Comparison of Photodynamic Activities of Molecular Oxygen and Porphyrins

А. А. Krasnovsky,@ A. S. Kozlov, A. S. Benditkis
Federal Research Center of Biotechnology of the Russian Academy of Sciences, 119071 Moscow, Russia

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DOI: 10.6060/mhc190659k
Macroheterocycles 2019 12(2) 171-180
An ideathat low level laser therapy (LLLT) by visible and infrared light results from direct excitation of oxygen molecules by laser radiation attracts keen attention of medical community as an alternative for classic photodynamic therapy (PDT) with porphyrin photosensitizers. Present paper deals with testing this idea based on experimental analysis of the rates of singlet oxygen generation upon direct and porphyrin-photosensitized excitation of O2 dissolved in aerated organic solvents and water under normal conditions. In aerated solvents having no porphyrins, singlet oxygen generation was observed under laser excitation at the wavelengths corresponding to the triplet-singlet transitions in monomeric oxygen molecules. The strongest effect was observed under excitation at 1273 and 765 nm, weaker activity was detected under excitation at 1070 and 690 nm. Under excitation at 630 nm (≤300 mW/cm2) corresponding to the absorption band of oxygen dimols, no reliable singlet oxygen generation was found. Kinetic analysis of the rates of singlet oxygen generation allowed estimation of absorbance and molar absorption coefficients corresponding to the absorption maxima of dissolved oxygen molecules. The obtained results were compared with the absorption coefficients of metal-free protoporphyrin IX. Analysis indicates that under the action of red light on porphyrins, which are always present in normal cells at a concentration of 10–50 nM, singlet oxygen should be produced with the efficiency, which is at least by two orders higher than that under the action of red light on dissolved oxygen in the porphyrin lacking systems. The increase in the concentration of porphyrins that occurs as a result of inflammation, porphyrias or application in PDT, leads to an increase of the efficiency of photodynamic action by another 2–4 orders of magnitude. Hence, photosensitization by porphyrins should contribute incomparably more than photosensitization by oxygen to biological action of red and visible light incomparably more than photosensitization by oxygen. Lacking the IR absorption bands in the absorption spectra of porphyrins excludes the involvement of porphyrins in photosensitization of biomedical processes caused by IR light. In this case oxygen molecules may serve as IR photosensitizers. However, the efficiency of the photosensitization by oxygen was shown to be very low, therefore a real contribution of this process to biomedical action of IR light cannot be considered to be fully established, so, further studies are needed.


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