Viscoelastic properties of rigid and semiflexible particles in solution. II. Application to rodlike molecules such as poly(γ‐benzyl‐L‐glutamate) in the helical state

Abstract

AbstractThe theory presented in Part I [Iwata, K. (1979) J. Chem. Phys. 71, 931–943] is applied to calculation of complex viscosities of the following three models which are designed to simulate various motions of polypeptides in helicogenic solvents. Model I is composed of identical spheres which are arranged regularly along a Kratky‐Porod chain and is adopted for describing flexural deformation of polypeptide helices. Model II is composed of two kinds of spheres, mobile and immobile, which are arranged in a pillar form with a multilayer structure and is adopted for estimating sidegroup motion of polypeptide helices. The mobile spheres are assumed movable in their individual intramolecular potential field under the influences of hydrodynamic resistance by solvent and of “internal friction” caused by other spheres in the same molecule. When the mobile and immobile spheres are arranged alternatively in a mosaic form (model IIa), the model exhibits a local motion of the side groups. When all the spheres are mobile (model IIb), their motions are correlated with each other through the internal friction.The complex viscosities of these models are computed and compared with those measured by Ookubo et al. [(1976) Biopolymers 15, 929–949] for poly(γ‐benzyl‐L‐glutamate)‐[Glu(OBzl)]n in m‐cresol. The results are as follows: (1) The dynamic viscosity [η′] of [Glu(OBzl)]n with molecular weight 75,000 is explained well not by model IIa or IIb, but by model I, and the relaxational mode B observed at 760 kHz is assigned to the flexural deformation of the [Glu(OBzl)]n helix. (2) [η′] of [Glu(OBzl)]n with molecular weight 120,000 deviates largely from that predicted by model I; it is therefore deduced that the interruption of the helix becomes predominant between Mr 75,000 and Mr 120,000. (3) The persistence length of [Glu(OBzl)]n in m‐cresol is estimated to be 1600 Å and the flexural rigidity of the helix to be 6.7 × 10−19 dyn cm2 from the relaxation time of mode B. The present estimate of the persistence length is larger than the previous ones (700–1100 Å), which were obtained by neglecting the possibility of helix interruption.