Three dimensional diffusion of singlet oxygen in lipid suspensions using Monte Carlo Simulations

Jürgen Baier, Thomas Fuß, Christopher Wiesmann, Johannes Schwanzl, Max Maier, Wolfgang Bäumler
(J. Baier, T. Fuß, C. Wiesmann, J. Schwanzl, M. Maier and W. Bäumler)

Among others, singlet oxygen can be generated by an energy transfer of a light absorbing photosensitizer. The non-radiative deactivation of singlet oxygen is accompanied by radiative deactivation leading to infrared luminescence at 1270 nm, which is widely employed for singlet oxygen detection. 
The lifetime of single oxygen depends on the environment in which the singlet oxygen decays, which can be described by a theoretical system of differential equations. But usually no diffusion of singlet oxygen during different environments is considered. 
When adding phosphatidylcholine to water, droplets of the fatty acid were formed. It has been shown that ATMPn is exclusively localised in lipids when added to aqueous suspensions of fatty acids. By exciting the suspension singlet oxygen will be generated only in lipid. However, singlet oxygen can escape into the aqueous environment. In that way the lifetime of singlet oxygen depends on the exchange of singlet oxygen between water and lipid. This could explain the measured intermediate lifetime of 10 µs of singlet oxygen in the luminescence experiments of the suspension of ATMPn, which is shorter than the value for pure phosphatidylcholine (14 µs) and larger than the value of water (3.5 µs). 
To understand this process of diffusion of singlet oxygen Monte Carlo simulations were made for lipid suspension, which consists of two different environments for singlet oxygen. It could be shown that the diffusion length of singlet oxygen must be approximately equal to the dimension of the lipid areas. Moreover the droplets of lipid must be composed on bilayer of fatty acids which were surrounded by water. Simulations with solid droplets of lipid could not produce the experimental data in acceptable conditions. 

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