Propriétés hydrodynamiques des conformations ordonnées et desordonnées du poly(γ-L-glutamate de benzyle): dimensions et flexibilité de la chaîne

Abstract

Theoretically we can determine the disordered or ordered structure of polypeptides and their dimensions in dilute solutions from hydrodynamic properties. We have presently a wealth of theories for random coil chains and a limited but sufficient number of theories for ordered chains for interpreting experimental results.Viscosity data for seven poly(γ-benzyl-L-glutamate) samples in 1,2-dichloroethane at 25 °C are analyzed and the length per monomeric residue (h) is calculated according to the equivalent ellipsoid approach. The degree of flexibility or rigidity is characterized by calculating Ns, the number of monomer units in a rigid segment or a Kuhn statistical segment; the determination of Ns is made by applying Yamakawa and Fujii's equation modified by Vitovskaya and Tsvetkov.Values obtained for h assuming the solute molecule to be a rigid, stiff chain, range between 1.3 to 2 Å. One notices that the h value close to 1.5 Å is found for the three following molecular weights: 1.8 × 105, 1.7 × 105, and 1.5 × 105. They are, in fact, the samples having a length in good quantitative agreement with that of the rigid segment determined by the method of Vitovskaya and Tsvetkov. This rigid segment corresponds to a sample of 700 ± 100 monomer units.The analysis of the experimental data of poly(γ-benzyl-L-glutamate) in dichloroacetic acid indicates that, in addition to the formation of hydrogen bonds, other interactions between the polypeptide and the solvent are present.In summary, we may conclude that the study of the helix–coil transition using hydrodynamic measurements is judged satisfactory but the determination of characteristic dimensions used to describe exactly the conformation of the macromolecule is somewhat ambiguous. One major problem is the degree of flexibility encountered with high molecular weight chains. However, to get around this difficulty, we propose, according to our results, a method which consists in determining the number of monomer units within a rigid segment from the different values found for h and then the dimensions from the samples for which the chain length is in good agreement with that of a rigid segment thus determined.