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Abstract:
The interaction of the potential-sensitive styryl dye RH421 with dimyristoylphosphatidylcholine vesicles has been investigated above and below the gel-liquid crystal-phase transition temperature by the fluorescence stopped-flow method. The observed kinetic behavior is consistent with a disaggregation of dye aggregates in the aqueous solution and the incorporation of dye monomers into the vesicle membrane. Upon incorporation the dye undergoes a decrease in its pK(a) of over 2 pH units. This can be attributed to the change of the electronic environment of the dye. The magnitude of the pK(a) shift is discussed in terms of the local membrane dielectric constant, the dipole potential, and possible dye reorientation, which may all affect the free energy change of the protonation reaction.
