Elongational flow field as a tool for investigating helix–coil transition: Observation of helix–coil transition in poly(L-glutamic acid) induced by pH change

Elongational flow technique has been used to investigate helix–coil transition of poly(L -glutamic acid) (PGA) induced by pH change. The elongational flow field was generated by a four-roller mill apparatus of the type originally used by G. I. Taylor¹⁶. The flow-induced nonlocalized birefringence was observed all over the pH range measured. The birefringence, Δn₁₇₆, at a fixed strain rate ε = 176 s^?1 decreases rapidly within a narrow pH range with increasing pH. The Δn₁₇₆ value is considered to contain information about both helix content, θ, of the system and the degree of orientation of polymers in the elongational flow field. From the Δn₁₇₆ value, using rotational diffusion coefficient, D_r, the helix content, θ, was estimated at each iso-pH measurement. pH dependence of θ was obtained and was found to correspond well to those determined by optical rotary dispersion (ORD) studies. Even in a hinged-rod conformation, the PGA molecule is expected to be in an extended structure. On the basis of the hydrodynamic considerations about the molecular conformation, it is concluded that the elongational flow technique is useful to investigate conformational transitions of biopolymers. © 1995 John Wiley & Sons, Inc.