Enzymically generated electron-excited species increase the membrane order parameter without changing the rotational relaxation time of TMA-DPH

The effect of enzymically generated triplet acetone produced from 60 mmol/l 2-methylpropanal (MPAL) in the presence of 2 mu mol/l horseradish peroxidase on the properties of artificial unilamellar liposomes was studied. The concentration of malondialdehyde, membrane lateral pressure (membrane order parameter), lifetime distribution of the excited state of TMA-DPH incorporated into liposomes, and rotational relaxation time of TMA-DPH were determined. The concentration of MDA increased with the MPAL concentration, and the membrane lateral pressure increased linearly with the concentration of MDA. The mean lifetime values changed (p < 0.01) as did the limiting anisotropy (p < 0.005), the widths of the main peak of the lifetime distribution and the rotational relaxation time remaining unchanged. This indicates that there is no significant difference between the widths and rotational relaxation times in the control and triplet-acetone-treated sample: i.e. peroxidation by MPAL increased the lipid order of the bilayer but the effect of neighboring lipid molecules on the dynamics of the probe was negligible.