Non-newtoniam viscosity of polypeptides in the helix-coil transition region

Abstract

The non-Newtonian intrinsic viscosities[$\eta$] of poly($\gamma$-methyl L-glutamate) were measured in the helixcoil transition region under various conditions in this work. The helix content $f_H$, which represents the degree of conformational transition, was obtained by using a polarimeter. The experimental results show that the non-Newtonian behavior of the polypeptide is markedly affected by its conformation, i.e., the non-Newtonian effect becomes larger as $f_H$ increases. The effect of external pressure $\Delta$p on [$\eta$] was studied carefully. [$\eta$] increases with $f_H$ when $\Delta p<1.5 psi$, but it decreases when $\Delta p>1.5 psi$ and $f_H>0.8$ (Fig. 8). The reason for this result was considered in the text. In order to find the further evidence for the non-Newtonian flow behavior, the non-Newtonian viscosities of poly (L-proline) were measured in the helix-helix transition region and various salt solutions. The experimental results for poly(L-proline) show that the non-Newtonian behavior is not significantly affected by its $[\alpha]_{D^\prime}$ which represents the degree of helix-helix transition, while $\eta_{sp}$/C is remarkably changed by $CaCl_2$ concentrations. These interesting results must be attributed to the unique structure of poly(L-proline), which is studied in this paper. In Part Two, the experimental data of $[\eta]^f/[\eta]^O$ are express by the following equation: $$\frac{[\eta]^f}{[\eta]^O} = 1- \frac{A}{[\eta]^O} [1- \frac{\sin h^{-1} \beta_2 f/\eta_0}{\beta_2 f/\eta_O} + \frac{1}{kT \beta_2} \frac{C_2 f}{\eta_O}]\; (A1)$$ where $[\eta]^f$ and $[\eta]^O$ are the intrinsic viscosity at shear stress f and zero, respectively, $A=\lim_{c \rightarrow 0} [(1/C)(x_2/\alpha_2)(\beta_2/\eta_0)]$, $\eta_0=$ viscosity of the solvent, $\beta_2$ is the relaxation time of flow unit 2, $C_2$ is a constant related to the orientation effect. The theoretical derivation of Eq.(A1) is given in the text. The experimental results for poly($\gamma...