Light‐scattering and potentiometric‐titration studies of poly‐<scp>L</scp>‐tyrosine in aqueous solution

Abstract

Abstract Aqueous solutions of poly‐ L ‐tyrosine were studied by potentiometric titrations, light‐scattering measurements, and infrared spectroscopy in order to characterize the conformational changes the macromolecules exhibit when the hydrogen‐ion concentration of the solutions is varied. It was found that when the pH of the system at 25°C is lowered from a value of about 13 the poly‐ L ‐tyrosine molecules undergo a random‐coil to β‐structure conformation change at pH 11.5. It appears, that under the experimental conditions used, the formation of the β structure is an intramolecular process which involves the folding of the polymer backbone into several hairpins of antiparallel β structure. On further lowering of the pH of the solutions aggregation (pH 11.35) and subsequently precipitation (pH 11.2) takes place. Since the apparent pK of the polymer does not show a drastic change during the random‐coil to β‐structure conformation change, it was concluded, that the therinodynamic driving force for this conformation change is mainly due to the electrostatic effect of the partially charged side chains of the poly‐ L ‐tyrosine molecules.