Influence of vesicle curvature on fluorescence relaxation kinetics of fluorophores

The effect of membrane curvature on the fluorescence decay of 2-p-toluidinyl-naphthalene-6-sulfonic acid (TNS), 2-(9-anthroyloxy) stearic acid (2-AS) and 12-(9-anthroyloxy)-stearic acid (12-AS) was investigated for egg lecithin vesicles of average diameter dm = 22 nm and 250 nm. The biexponential fluorescence decay of TNS at the red edge of the emission spectrum was analysed according to the model of Gonzalo and Montoro [1]. Over the entire temperature range (1–40°C) the small TNS labelled vesicles showed significantly shorter solvent relaxation times τr than their larger counterparts (e.g. 1.3 ns compared with 2.1 ns at 5°C), indicating a higher mobility of the hydrated headgroups in the highly curved, small vesciles. The fluorescence decay of both AS derivatives is also biexponential. While the shorter decay times (1–3 ns) are practically identical for small and large vesicles, the longer decay times (5–14 ns) are identical only for 12-AS but not for 2-AS. This indicates that the microenvironment is similar in small and large vesicles deep in the membrane in spite of the differences in curvature