Glutamic acid helix-coil transition

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. 

These severe requirements are met principally by ionic, water-soluble polypeptides in which there is little solvent change over the helix-coil transition and in which X = Xn. Poly-L-glutamic acid and poly-n-lysine, which... 

Fig. 11. Comparison of theoretical curves, which describe the thermal helix-coil transition of a polypeptide system as a function of chain length, with experimental points obtained for poly-->-benzyl-i,-glutamate polymers of varying degrees of polymerization, n. In this solvent mixture, ethylene dichloride and dichloroacetic acid, the helical form, characterized by the more positive rotations, is stable at higher temperatures. The manner in which increased length sharpens the transition, both in theory and in actuality, is here clearly illustrated. For comparative purposes, Tc is defined as the temperature at the mid-point of the transition for the sample of highest molecular weight. (Zimm et al., 1959.)...

Fig. 12. Helix-coil transitions of equimolar oopoly-L-lysine-L-glutamic acid and ovalbumin induced by variation in solvent composition. The specific rotation is plotted as a function of the per cent of trifluoroacetic acid in the 2-chioroethanol-trifluoroacetic acid solvent system. (Doty et al., 1958.)...

Our present knowledge of the helix-coil transition in synthetic polypeptides, with particular reference to the poly-y-benzyl-h-glutamate-dichloroacetic acid-1,2-dichloroethane system, is briefly reviewed. Recent results concerning the effect of solvent composition and of polypeptide and solvent deuteration on the thermodynamic properties of the transition show that both the thermodynamics and presumably the molecular mechanism of the transition are generally more complicated than had been previously supposed. 

The helix-coil transition in the copolymer clearly shows that the glutamic acid residues form the dominant helix (Table VII). There is no helix formation at alkaline pH owing to the lysine residues either by polarization of fluorescence or by optical rotation measurements in fact, both p and [ql]d decrease. The degree of polarization at pH 4 can be 85% reduced by 9M urea in both poly Glu97Lys3 and poly Glu63Lys37 (No. 3) this reflects destruction of the glutamic acid helix. 

Additionally, Ti data for smaller oligomers of lysine have also been measured (Saito and Smith, 1974). Similarly, titration curves have been applied to the study of helix-coil transitions in aqueous solutions of poly-L-glutamic acid (Lyerla et al., 1973). 

Figure 18.5 Helix-coil transitions of poly(L-benzyl glutamate) in a mixture of dichloroacetic acid and heptane showing the variation of the apparent molecular weight (M in their notation) and the radius of gyration. Source Reprinted with permission from Cowie JMG. Pure Appl Chem 1970 23 355 [33], Copyright 1970 International Union of Pure and Applied Chemistry (after Marchal E, Strazielle C. Compt Rendu 1968 C267 135. Academie Franjaise de Science).

The specific rotation of poly(y-benzyl-L-glutamate) in a mixture of, for example, ethylene dichloride and dichloroacetic acid first increases slightly as more CHCbCOOH is added, then remains constant over a wide range of mixture compositions, and finally falls sharply to negative values at a 75% CHCbCOOH content (Figure 4-26). Since ethylene dichloride is a helicogenic solvent, the initial increase is considered to be a change in the helix structure (an expansion ), but the decrease results from a helix/coil transition. 

Fig. 3.16 The helix-coil transition in poly-L-glutamic acid as it is affected by the variation in pH. (From Doty (55))...

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]. 

---