Diffusion-controlled photochemical reactions in membranes. Photodimerization of 6-(9-anthroyloxy) stearic acid in POPC bilayers under steady-state irradiation
Mj. Moreno et E. Melo, Diffusion-controlled photochemical reactions in membranes. Photodimerization of 6-(9-anthroyloxy) stearic acid in POPC bilayers under steady-state irradiation, J PHYS CH B, 103(48), 1999, pp. 10711-10717
We present a detailed study of the models for diffusion controlled bimolecu
lar reactions in phospholipid membranes analyzing the case of the photodime
rization of long lived (200 ns) and short lived (10 ns) excited species und
er steady state irradiation. It is shown that only when using the 2D formal
ism for the diffusion-controlled rate constant, k(diff), do the parameters
retrieved have physical meaning. In certain cases the use of a 3D law inste
ad of the 2D specific law does not lead to significantly different results.
However, this agreement between the two models is misleading and depends o
n the specific system, namely on the way the 3D concentration is calculated
and on the position of the reactant relative to the center of the bilayer.
The models were tested for the photodimerization of 6-(9-anthroyloxy)stear
ic acid (6-AS) in a membrane model system of 1-palmitoyl-2-oleoyl-sn-glycer
o-3-phosphocholine (POPC). The quenching of the excited state is diffusion
controlled but only a fraction of quenching, gamma, leads to dimerization.
Since we know from independent measurements that gamma must vary between 0.
05 and 0.1 we may conclude that the value of gamma = 0.3 obtained with the
3D law without considering the time dependence of k(diff) is out of range,
while gamma approximate to 0.1 obtained for both 2D and 3D complete laws is
acceptable. We further discuss the physical meaning of the application of
the 3D law to this system and the method used for the calculation of the 3D
concentration. The lateral diffusion coefficient of the reactants, necessa
ry for the calculations, was independently measured in a model system using
the technique of fluorescence recovery after photobleaching.