Poly membrane helix

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). 

E Pefferkorn, A Schmitt, R Varoqui. Helix-coil transition of poly(a,L-glutamic acid) at an interface Correlation with static and dynamic membrane properties. Biopolymers 21 1451-1463, 1982. 

Thus the low molecular weight PHB-polymer (19-20 monomer units) can be used effectively for preparation of artificial ion channels in cell membranes mimicking natural ones. The model of channels proposed contains two helices the outer one containing poly-(R)-3 hydroxybutanoate, complexed by hydrogen bonding with the inner helix of poly(-Ca phosphonate) (Fig 1). 

The first example of chiral polymer from a disubstituted acetylene is a polyd-trimethylsilyl-l-propyne)-based polymer, poly(46), which was synthesized in moderate yields using TaCls-PhaBi (112). Poly(46) displays small optical rotations, and its molar ellipticities of the Cotton effects are up to a few hundreds. The main chain of poly(46) is, therefore, not a well-ordered helix. This is probably because of the less controlled geometrical structure (cis and trans) of the polymer backbone. However, the free-standing film of this polymer achieves an enantioselective permeation of various racemates including alcohols and amino acids. For example, the concentration-driven permeation of an aqueous solution of tryptophan by poly(46) gives 81% enantiomeric excess (ee) of the permeate at the initial stage. A characteristic of the membrane of poly(46) is its ability to enantioselectively recognize 2-butanol and 1,3-butanediol, because the direct resolution of these racemates by hplc is impossible. 

Poly(L-glutamic acid) shows reversible helix-coil transition wifli pH changes, fri an alkaline solution, the gel swells violently due to static repulsion by the carboxylic group and flie shape of flie membrane cannot be maintained. If a block copolymer is synfiiesized wifli hydrophobic L-lysine, a cylindrical microphase separation is observed. This material avoids macroscopic deformation. Thus, a membrane wifli molecular level deformation can be manufactured [64]. This membrane shows not only control of permeation by pH changes but also nonlinear responses like vibration of potential by salt concentration difference [65] and nonlinear resistance upon voltage application [66]. 

The rapid disintegration of the helical portion exposed to water is somewhat surprising, considering that short poly-alanine peptides in water were found to contain a large fraction of 3io-helix [95]. Furthermore, simulations of an a-helix formed by a similar, but presumably less stable, peptide, the undecamer of polyleucine, in water revealed that the helix does not unravel on a few nanoseconds timescale [81]. It is possible that the interfacial water-membrane environment accelerates the denaturation of marginally stable, ordered protein structures but this issue requires further, systematic studies. 

The authors also reported on A-B-A type block copolymers composed of poly(y-benzyl-L-glutamate) as the A component and polyisoprene as the B component. By using wide-angle X-ray diffraction it could be shown that the block copolymers exhibit mesophase behavior in different solvents. In this case polyisoprene chains are in a random coil conformation and form domains embedded in the matrix phase consisting of poly(y-benzyl-L-glutamate) chains in the a-helix conformation. Model analysis of the complex modulus of the membrane cast from solution suggested the occurrence of spherical and cylindrical domain structures in the membrane [73]. Morphological study of these triblock copolymers... 

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