Mismatch tilt angle

Fig. 5. Responses of a TM protein to hydrophobic mismatch. The hydrophobic regions of a TM protein (black regions) may be too long for the lipid core, creating a mismatch. To help reduce this stress, the protein may change its tilt angle or undergo more favorable associations. The protein may associate with a specific lipid, with a different tail length or curvature, or with another protein to reduce the lipid-facing surface area.

The entire thermodynamic system of the membrane and TM protein must be considered to understand how the protein and bilayer achieve their native state. We have summarized four of the mechanisms, hydrophobic matching, tilt angles, and specific protein/lipid and protein/protein interactions that are important in determining the stability (Fig. 5). Other important factors, such as the stability of lipid/lipid interactions, have been left out of our protein-centric view. We describe a hydrophobic mismatch as an unfavorable interaction that can be relieved by the other three processes, but we would expect all these properties of the system to interact. We could easily describe the same equilibria by saying that a strain in curvature is relieved by a hydrophobic mismatch or that strong protein/protein packing interactions might help relieve the hydrophobic mismatch or curvature stress. The complex interplay between all these interactions is at the heart of what determines membrane protein stability and will no doubt be difficult to quantify. 

Park SH, Opella SJ (2005) Tilt angle of a trans-membrane helix is determined by hydrophobic mismatch. J Mol Biol 350 310-318... 

Figure 10 (a) A smaller tilt angle of 21° of F6-DPPC would involve unfavorable interactions because of mismatch between... 

If we imagine the diffraction of a plane wave from epilayers we see that there will in general be differences of diffraction angle between die layer and the substrate, whether these are caused by tilt or mismatch f Double or multiple peaks will therefore arise in the rocking curve. Peaks may be broadened... 

Fig. 2 Schematic representation of potential changes in integral membrane protein structure that could be imposed by a micellar environment (left hand side of each panel), compared to the native structure in bilayers (right). Possible distortions include (a) micelle-induced curvature in the TM helix or amphipathic helix (b) monomeric detergent molecules bound to a solvent-exposed region, in this case an aqueous cavity close to the micelle surface (c) altered relative orientations of amphipathic vs TM helices (d) loss of tilt relative to other TM segments. In this scenario hydrophobic mismatch between the TM helix and micelle are minimized by distortions in micelle structure that allow hydrophobic protein surfaces to remain in the hydrophobic phase. In the bilayer environment hydrophobic mismatch induces tilt, favoring a non-zero inter-helical crossing angles...

Our results confirmed the tendency of Lys-flanked peptides to compensate the positive mismatch between peptide and membrane hydrophobic core by tilting. Some of the peptides, however, prodnce superhelical donble-twisted structure. This only occurs in the membrane in the gel phase, where only a small hydrophobic mismatch exists. The peptide also alters certain properties of the surrounding hpids snch as membrane ordering, the amount of dihedral angles in tram conformation and the nnmber of transitions between tram and gauche conformation. It is likely that these effects shonld provide some preferable stmctnral state of the peptides in a membrane. The lipid stmctural state around the peptide is probably between gel and hqnid-ciystalline state. This effect depends on peptide amino acid composition. Amino acids with large side chains branched at (He, Val) produce hehx, which has more side chains finctuates than that of a poly-Len helix. This holds also for small side chains (Ala). 

The addition of homopolymers further enhances the tolerance of mismatching and allows chemical epitaxy of stractures more complex than the typical bulk phases of block copolymers. Stoykovich et al. introduced a ternary blend to further extend their surface-directed method. Such a blend system enables the block copolymer lamellae to conform to substrate stripe arrays with sharp bends. In the imblended block copolymer S3 tem, a high strain builds up in the polymer film at sharp comers of the chemical pattern because the comer-to-comer distance is much larger than the natural periodicity of the block copolymer. Successful replication of arrays of tilt boundaries with 45° and 90° angles was observed as a result of the redistribution of the homopolymer. (The polymer blend includes 20% PMMA homopolymer, 20% PS homopolymer, and 60% symmetric PS-b-PMMA.) Homopolymers are depleted above commensurate regions and concentrated above the distorted regions of the pattern to reduce the strain from incommensurability. 

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