ELECTROCHEMICAL MEASUREMENT OF LATERAL DIFFUSION-COEFFICIENTS OF UBIQUINONES AND PLASTOQUINONES OF VARIOUS ISOPRENOID CHAIN LENGTHS INCORPORATED IN MODEL BILAYERS

The long-range diffusion coefficients of isoprenoid quinones in a mode l of lipid bilayer were determined by a method avoiding fluorescent pr obe labeling of the molecules. The quinone electron carriers were inco rporated in supported dimyristoylphosphatidylcholine layers at physiol ogical molar fractions (<3 mol%), The elaborate bilayer template conta ined a built-in gold electrode at which the redox molecules solubilize d in the bilayer were reduced or oxidized. The lateral diffusion coeff icient of a natural quinone like UQ(10) or PQ(9) was 2.0 +/- 0.4 x 10( -8) cm(2) s(-1) at 30 degrees C, two to three times smaller than the d iffusion coefficient of a lipid analog in the same artificial bilayer. The lateral mobilities of the oxidized or reduced forms could be dete rmined separately and were found to be identical in the 4-13 pH range. For a series of isoprenoid quinones, UQ(2) or PQ(2) to UQ(10) the dif fusion coefficient exhibited a marked dependence on the length of the isoprenoid chain. The data fit very well the quantitative behavior pre dicted by a continuum fluid model in which the isoprenoid chains are t aken as rigid particles moving in the less viscous part of the bilayer and rubbing against the more viscous layers of lipid heads. The prese nt study supports the concept of a homogeneous pool of quinone located in the less viscous region of the bilayer.