Helical Structures Formed upon Adsorption

Adsorption of unfolded structures at interfaces can also induce the formation of helical conformations. Hydrophobins are among the best biosurfactants and

Fig. 13.9 Self-assembly and conformational equilibria of hydrophobin SC3 in different environments and at different interfaces (Tween = detergent poly(ethylene glycol) sorbitan monolaurate with n @ 20) [22]. (Adapted from ref. [22].)

show interesting behavior at interfaces. The hydrophobin SC3 is a fungal protein composed of 136 amino acid residues [21]. In solution, these proteins adopt no specific secondary structure. However, when adsorbed at a hydrophobic Teflon surface, an a-helical structure has been observed (Fig. 13.9) [22]. It should be noted that this a-helical structure is not the most stable global minimum state but can be transformed into a b-sheet structure by heating at 80 °C in the presence of a detergent (Tween). These structural transitions of hydrophobins are also observed at the air-water interface. Adsorption of SC3 first leads to an intermediate a-helical state, which is spontaneously converted into an amorphous b-sheet state. This state subsequently reorganizes into a b-sheet state with a rodlet structure, which could be visualized by AFM after transferring the monolayer from the air-water interface onto a carbon film. Although it is clear that these structural transitions of hydrophobins are caused by adsorption at hydrophobic interfaces, the molecular background of these transitions remains to be uncovered.

In a related context, a recent modeling study suggests that DNA naturation, i.e. the formation of the double-stranded helix, can also be stimulated by adsorption at interfaces [23]. This modeling study shows that after adsorption of single stranded DNA at a surface the entropy loss created by dimerization of these two strands will be much smaller than in solution, thereby decreasing the free energy for dimerization. Hence, the dimerization process that leads to helix formation is stimulated by the interface and the nucleation barrier is lowered.

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