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1.
Fig. 4.

Fig. 4. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

The change in Π immediately after a compression was dependent on the size of the compression. This relationship can be used to calculate the value of , which is equal to the value of AO/AC as ΠSATC approaches zero. The value of for C46.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
2.
Fig. 2.

Fig. 2. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

Postwashout compressions caused Π to rapidly rise and then recovered to a new equilibrium value near the precompression Π. When the surface was reexpanded, Π was lower than the precompression Π. This indicates that some of the peptide was expelled from the surface by compression. In other words, the nonexchangeable state can convert to the exchangeable state of the peptide by a compression.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
3.
Fig. 3.

Fig. 3. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

Two-surface state thermodynamic model to explain the partial exchangeability of C46 before washout (left) and after washout (right). The soluble form (S) is in an adsorption-desorption equilibrium with the exchangeable form (Ex). On the surface, the Ex form and a nonexchangeable form (NE) are in a Π-dependent equilibrium. When the S form is removed by a washout (right), some of the Ex form of C46 desorbs from the surface and Π fall. The fall in Π encourages the NE conformation of C46.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
4.
Fig. 9.

Fig. 9. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

Depiction of the Π-dependent structure of C46 according to model 2. In model 2 (see Table 1), only helix 3 (H3) interacts in the exchangeable form. When Π is lower, helix 2 (H2) binds, making the peptide irreversibly bound until it is compressed. The equilibrium between these two conformations is dependent on the surface equilibrium constant.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
5.
Fig. 7.

Fig. 7. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

(A) The Π-A relationship of a TO/C46/W interface was determined by linearly compressing the drop. The peptide was adsorbed to the surface, washed out of the bulk (black bar), and then the drop was linearly expanded and compressed. When the surface was compressed, Π increased until ΠSAT, where Π plateaued. Area is represented by the red curve, and Π represented by the black curve. (B) Linear compression isotherm of C46 at a TO/W interface during a compression at a rate of 2.7 mm2/min. The isotherm was constructed by plotting Π against area during the compression.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
6.
Fig. 5.

Fig. 5. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

Surface Gibbs free energy as a function of time during the adsorption of C46 to the surface (far right), a washout (black bar), and after a rapid postwashout compression (arrow on the far right). The area was kept constant for the first 47 min of the experiment at 30 mm2. At 47 min (arrow), the surface was rapidly reduced by ∼30% to 20.6 mm2. After the compression, peptide slowly desorbed from the surface. Gibbs free energy is defined as: Pressure is traced in black and Gibbs free energy is traced in gray.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
7.
Fig. 8.

Fig. 8. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

Lipid binding of C46 induces the formation of AαHs. C46 is predicted to form three potential helices separated by prolines at aa 209 and 220. In full-length apoA-I, the first helix (198–209) likely extends to aa ∼189. The C-terminal helix, helix 3 aa 220–238, is the longest and most hydrophobic in C46. The number of amino acids in the hydrophobic face (#AA) and the Gibbs free energy of transfer from water to oil (ΔGW O) of each helix were determined based on the helical wheel diagrams based on the White/Wimley (W/W) and GES hydrophibicity scales.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
8.
Fig. 6.

Fig. 6. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

The two-state thermodynamic model validated by modeling the change in Π during a washout (A) and after a rapid postwashout compression (B). To determine the desorption constant (kD), the kinetic model of Π(t) after a washout (equation 15) was regressed to experimental data (black points). The regression is shown as a dotted line in panel A (R2 = 0.85). The regressed value is kD = 0.31 min−1. This value of kD was used to predict the Π(t) after a compression (equation 16). The prediction is shown as a solid line in Panel B (R2 = 0.81). In an alternative method, the value of kD was regressed from the postcompression Π(t) with a value of kD = 0.24 min−1, which is shown as a dotted line in panel B (R2 = 0.88). This value was used to predict the Π(t) during a washout, shown as the solid line in panel A (R2 = 0.83). The good agreement between experimental data and the model predictions suggests the two-state thermodynamic model is an adequate description of AαH partial exchangeability.

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.
9.
Fig. 1.

Fig. 1. From: Surface pressure-dependent conformation change of apolipoprotein-derived amphipathic ?-helices.

(A) When C46 was adsorbed to a TO/W interface at a bulk concentration of 4.2 μg/ml, the surface pressure (Π) rose to ∼16 mN/m. After reaching an equilibrium Π, C46 was washed out of the bulk (indicated by the black bar). The washout caused Π to decrease to a new steady state Π. When C46 was readded after the washout at a concentration of ∼4.2 μg/ml (indicated by the arrow on the far right), Π returned to the prewashout Π. Note that the y axis is inverted. The area was kept constant throughout this experiment. (B) The prewashout surface tension (γ) of C46 was dependent on the bulk concentration (18), but the postwashout γ was independent of the prewashout bulk concentration. The prewashout γ was higher at a higher bulk C46 concentration, e.g., at 16.6 μg/ml (blue) the equilibrium γ was 1.5 mN/m lower than that at 0.16 μg/ml (purple). (C and D) The prewashout Π was dependent on bulk concentration (black diamonds, top). Before the washout, Π was related to [C46] by the relation Π = 1.12log([C46])+15.162 (R2 = 0.946). Π was saturated at a bulk concentration >10 μg/ml. The Π at the saturated concentration is defined as ΠSAT. The postwashout γ was not dependent on bulk concentration (gray squares, bottom). The Π after a washout is defined as ΠWO. Π was calculated by subtracting the γ of a TO/C46/W interface from the γ of a clean TO/W interface (Π = 32 − γ).

Matthew A. Mitsche, et al. J Lipid Res. 2013 June;54(6):1578-1588.

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