ИСТИНА

Direct laser excitation of oxygen molecules: application to studies of oxygen photonics in systems of biomedical importance* A.A. Krasnovsky Federal Center for Biotechnology, A.N. Bach Institute of Biochemistry, Russian Academy of Science, Moscow, 119071, Russia phoal@mail.ru Abstract—Oxygenation rates of singlet oxygen traps were compared upon direct laser excitation of dissolved oxygen molecules using continuous and pulse laser radiation and under photosensitization by porphyrins. Novel procedure of data processing was developed and accurate absorption coefficients were obtained for the main IR absorption maxima of molecular oxygen under ambient conditions. Biomedical importance of the data is discussed. Keywords— molecular oxygen, laser excitation, diode lasers, pulse lasers, oxygen absorbance, singlet states of oxygen, laser therapy, biological action I. INTRODUCTION It is known that molecular oxygen has the triplet ground state (3Σg-) and two relatively low-lying singlet states 1g and 1Σg+. The 1g state is highly reactive. The 1Σg+ state is non-reactive, but in the solution-phase, it is rapidly converted into the 1g state. In air-saturated systems, singlet oxygen is known to be efficiently produced by energy transfer to 3O2 from the excited triplet molecules of dyes. Approximately 10 years ago we have shown that population of singlet oxygen occurs in air-saturated systems also without dyes upon direct laser excitation of dissolved oxygen molecules under ambient conditions. This process leads to readily observed oxygenation of the singlet oxygen traps. Further studies were devoted to application of this effect to investigation of the absorption spectra and absorption coefficients of dissolved oxygen, which cannot be measured in oxygen solutions using conventional spectrophotometers. Procedure of analysis has been improved step by step during several years. Finally, we arrived to the most accurate results which are shortly described below. II. METHODS, RESULTS AND DISCUSSION The most accurate analytical method is based on comparison of photooxyge¬na¬tion rates of singlet oxygen traps of different nature (1,3-diphenylisobenzofurane, tetracene, rubrene and uric acid) upon direct excitation of oxygen by IR laser radiation and upon singlet oxygen formation photosensitized by porphyrin. The action spectra of the photooxygenation reactions and the correction coefficients accounting for the overlapping of the oxygen absorption spectrum and the spectrum of laser radiation were taken into account. As a result, the absorption spectra, optical densities, molar absorption coeffici¬ents () and absorption cross sections () corresponding to the maxima of oxygen absorption bands at 765, 1073 and 1273 nm were accurately determined in air-saturated organic solvents and in water and aqueous detergent dispersions. The obtained absorption coefficients were markedly different from those obtained at high pressure, when dimers (dimols) of oxygen molecules dominate. In agreement with the prior publications, ε1273 and σ1273 markedly decreased on going from non-polar solvents to water proportionally to the decrease of the rates of radiative deactivation of the 1Δg-state, whereas ε765 and σ765 are less sensitive to solvents and slightly increased with the increase of solvent polarity. The data suggest that laser radiation at 765 nm is more appropriate for oxygen excitation in biomedical systems since its efficiency is almost equal to that of radiation at 1273 nm, but the dark red light causes weaker heating of water. These conclusions are supported by experiments on damage of living cells by IR laser radiation. REFERENCES [1] Krasnovsky A.A., Drozdova N.N., Ivanov A.V., Ambartzumian R.V. Biochemistry (Moscow), 2003, vol. 68, 963-966. [2] Krasnovsky A.A., Drozdova N.N., Ivanov A.V., Ambartzumian R.V. Chinese Opt. Letters, 2005, vol. 3 (supplement), S1-S4. [3] Krasnovsky A.A., Biochemistry Moscow, 2007, vol. 72, 1311-1331. [4] Krasnovsky A.A., Kryukov I.V., Sharkov A.V., Proc. SPIE, 2007, vol. 6535, 65351Q1- Q5. [5] Krasnovsky A.A., J. Photochem. Photobiol. A: Chem. 2008, vol. 196, 210-218. [6] Krasnovsky A.A., Rоumbal Ya.V., Strizhakov A.A., Chem. Phys. Lett., 2008, vol. 458, 195-199. [7] Krasnovsky A.A., Kozlov A.S., Rоumbal Ya.V. Photochem. Photobiol. Sciences, 2012, vol. 11, 988-997. [8] Krasnovsky A.A., Kozlov A.S., Biofizika, 2014, vol. 59, 250-257.