Figure 4.

Comparison of the mobility of biomolecules of different dimensions on lipid monolayers at 70 Å²/molecule by point FCS. (A) Shown are autocorrelation curves obtained by point FCS for 0.01 mol% Atto655-DOPE, 10 nM of the membrane proximal external region (MPER) of the envelope glycoprotein gp41 of HIV-1 and G_M1-bound CtxB, both labeled with Alexa488, and 40 pM DNA structure X5 three-fold labeled with Atto488 in a DMPC monolayer at 70 Å²/molecule. Fits with single diffusional component and respective residuals are shown. (B) Diffusion coefficients obtained for each analyzed biomolecule in a DMPC monolayer at 70 Å²/molecule (filled bars). The studied biomolecules cover a range of sizes and number of membrane insertion points. The average of typically three independent samples and respective standard deviations are shown. Diffusion coefficients determined by others for Atto655- DOPE (23.5°C) (42), G_M1-bound CtxB (23.5°C) (77), and DNA structure X5 (27.5°C; A.K., J.M., Henri G. Franquelim, and P.S., unpublished data) in 1,2-dioleoyl-sn-glycero-3-phosphocholine free-standing lipid bilayers are shown for comparison (shadowed bars). (C) Surface viscosity η_s of DMPC monolayers based on FCS experiments on CtxB (open symbols, Fig. 3C) and ATTO655-DOPE (gray line with 95% confidence intervals, Fig. 2D; Table S2) are shown. η_s was obtained numerically by finding the zero between predicted and measured diffusion coefficients using Newton’s method. As expected, η_s increases with increasing lipid packing. Empirically, the η_s roughly follows a bi-exponential: ${\eta }_S=a_1\exp \left(-a_2\cdot \text{MMA}\right)+a_3\exp \left(-a_4\cdot \text{MMA}\right)$ with $a_1=1.2\cdot {10}^{-8}\text{Pa}\text{s}\text{m}$, $a_2=0.104{\text{Å}}^{-2}$, $a_3=1.7\cdot {10}^{-10}\text{Pa}\text{s}\text{m}$, $a_4=0.025{\text{Å}}^{-2}$. All values correspond to 30°C. To see this figure in color, go online.