Poly Polypeptides, conformational stability

If the a hydrogen is substituted as, for example, in a-amino isobutyric acid (Aib), then the conformational energy (< >,t j) map is very restricted, and the preferred form of Aib peptides is computed to be the 310 rather than the a-helical form.150 This prediction has been verified by NMR and infrared spectroscopic measurements on solutions of oligomers of Aib.151 The stability of the 310-helix for short poly(Aib-L-alanine) polypeptides and the increased stabilization of the a-helical form with a lengthening of the chain have been demonstrated recently.152... 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

Azo-modified polypeptides were also investigated in connection with their photochromic behaviour caused by the trans to cis photoisomerization of the azo groups present in the side chains. In methanol/water solvent mixture, the 20% azo-poly(L-lysine) adopts the a-helix conformation. The helix stability was found to be higher when the azo chains are in cis than when they are in trans configuration. Irradiation at 340 nm (trans to cis isomerization) and alternately at 450 nm (cis to trans isomerization) produced reversible variations of the a-helix content. In hexafluoro-2-propanol/water/sodium dodecylsulfate mixture, the 43% azo-poly(L-lysine) adopted a P-structure and promoted the a-helix conformation. The effect was reversed upon irradiation at 450 nm. The photoinduced P to helix change was explained on the basis of the different geometry and hydrophobic character of the trans and the cis... 

Stability of a-helix has been studied extensively both from an experimental as well as from a theoretical point of view. Work on the stability of alanine based polypeptides has been pushed forward mainly by Baldwin and his collaborators [69-71]. Poly-alanine itself is not soluble in aqueous buffers but, when hydrophilic residues such as Glu, Gin, Asp, Asn, and Lys are inserted in every fifth or so position, the resulting polypeptides are readily soluble. All these alanine based polypeptides form a-helical conformation with a moderately high propensity as determined by CD spectrum. For relatively short polypeptides, the temperature induced transition between helical and random coil states is rather a gradual one. In aqueous mixture with trifluoroethanol (TFE), these polypeptides achieve progressively higher content of a-helical conformation. 

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