The adsorption of biological peptides and proteins at the oil/water interface. A potentially important but largely unexplored field

Abstract

This review focuses on some new techniques to study the behavior of peptides and proteins bound to oil droplets. We will show how model peptides e.g., amphipathic alpha helices (AalphaH) and amphipathic beta strand (AbetaS) and some apolipoproteins adsorb to triacylglycerol (TAG) droplets and how they behave once adsorbed to the interface. While most of the studies described involve peptides and proteins at an oil/water interface, studies can also be carried out when the surface has been partially covered with phospholipids. This work is important because it examines biophysical changes that take place at lipid droplet interfaces and how this may relate to the metabolism of lipoproteins and lipid droplets.

Fig. 1.

Amphipathic α helical (AαH) Peptide adsorption to the triolein/water (TO/W) interface and the response to stress compression and reexpansion with peptide in the aqueous phase (left) and after peptide has been washed out (right). Examples of behavior of three different peptides are shown in A and C (peptide at concentration noted). The peptide solution is removed between “start” to “stop” (B and D). A: [1-44] apoA-I, the N-terminal G* type (29) helix of apoA-I (32). A droplet of TO (area = 30 mm²) is formed in the peptide solution. Surface tension γ falls rapidly from 32 mN/m reaching equilibrium (γ_eq) at point 1. At point 2 area is decreased by 10%, producing a small decrease in γ that rapidly returns to γ_eq. Thus, on sensing the increased π, the peptide ejects from the surface into the bulk. Point 3, the area is reexpanded to 30 mm², immediately producing a spike in γ, but as the peptide rapidly readsorbs from the bulk γ_eq is reestablished. In points 4 and 5, larger compression and reexpansion show similar behavior. However, the larger compression (point 4) pushes more peptide off the surface and reexpansion at point 5 produces more free area resulting in the higher γ immediately after the reexpansion. Peptide then readsorbs to reestablish γ_eq. B: [1-44]ApoA-I after peptide is exchanged out of solution. Starting at γ_eq (∼19 mN/m) the peptide is exchanged out between “start” and “stop.” During exchange γ_eq rises quickly and establish a new γ_eq at ∼22 mN/m (point 1). Region 2 shows compression and reexpansion after peptide removal. Each compression produces a fall in γ but on reexpansion γ is increased above γ_eq. The larger the compression the greater the postexpansion γ. After expansion no change occurs in γ because there is virtually no peptide in the aqueous phase to readsorb (compare with points 3, 5, Fig. 1A). The final γ approaches 32 mN/m, like pure TO. However, large area oscillations (region 3, far right) causes γ to oscillate between ∼32 and 25 mN/m, indicating that even when γ approaches 32 mN/m after the last compression some peptide remains bound. To push this peptide off γ must be decreased below γ_eq of 22 mN/m (i.e., π > 10 mN/m). C: CSP, a consensus 44aa amphipathic α helix of A type (29). It consists of two identical 20 a.a. helices linked by a proline turn (2, 34). The peptide adsorbs and reaches equilibrium at γ of ∼17 mN/m. A small compression lowers γ_eq to ∼14 mN/m (point 1), which slowly moves back toward equilibrium. On reexpansion (point 2) γ immediately increases, above γ_eq to about 23 mN/m. This shows that molecules that were displaced by compression (point 1), readsorb after reexpansion moving γ back to γ_eq. Larger compressions (right) show larger changes of similar nature. In these experiments, the post compression increase in γ could be due to peptide molecules leaving the interface or to a conformational change in the peptide at the interface. At the smallest compression we think CSP stays bound but undergoes a conformational change that is rapidly reversible on reexpansion. At all the larger compressions (beyond point 1), CSP is ejected into the aqueous phase, joins bulk peptide, and must diffuse back to the surface to bind. D: CSP is exchanged out between “stop” and “start.” Point 1, no significant change in γ (∼15 mN/m) occurs during or after exchange (compare with Fig. 1B above). Four sets of two identical compressions were made. Point 2, a small compression decreases γ to ∼12 mN/m and on reexpansion (point 3) γ rises to ∼21 mN/m. This shows that compression pushed some peptide off. A second identical compression (point 4) reduces a γ to ∼16 mN/m, i.e., ≅ γ_eq. On reexpansion (point 5) γ returns to ∼21 mN/m. Thus, the second compression does not push more peptide off the surface but simply compresses it. Larger compression (right side) show that the first compression dislodges peptide but the second does not. Thus, if compression does not push γ below γ_eq no peptide is desorbed.

Fig. 2.

Amphipathic β peptides (AβS) A: P12, 12aa β strand (acetyl-LSLSLNADLRLK-amide) consensus sequence from TAG recruiting domains of apoB (4). At γ_eq, (∼14 mN/m)γ a moderate compression (point 1) decreases γ but there is no change after the compression. When the compression is released (point 2) returns to γ_eq. A larger compression (right) decreases the γ further, but γ on reexpansion returns to γ_eq. Such behavior indicates that the peptide does not leave the interface or have a major conformation change when compressed. B: P12 after the peptide is removed. During exchange no peptide desorbs from the interface and compression reexpansion experiments are almost identical to Fig. 1E. P12 is irreversibly bound and behaves as an elastic solid.