Formation of Nickel(II)-Silk Sericin (Bombyx mori) Complexes and Their Secondary Structures

Abstract

The complexation between Ni(II) ion and silk sericin (Bombyx mori) and the secondary structure of the Ni(II)-silk sericin complexes were studied in order to find practical applications. The preparation of Ni(II)-silk sericin (Bombyx mori) complex was carried out under various conditions of pH and [Ni(II)]T/[HL]T (molality ratio of Ni(II) to sericin unit) in an aqueous solution at 25°C The complexation in an aqueous solution was determined by measurements of pH titration, electronic spectra, and the reduced viscosity. The secondary structure of the complexed sericin in a solid state was observed with FT-IR spectrometer and X-ray diffraction. The pH titration showed that four protons against a Ni(II) ion was released from sericin. Absorption peaks at 435 and 240 nm began to appear above about pH 9–9.5, and were saturated above pH 11.5. At constant pH of 11.5 the complexation was found to proceed stoichiometrically when the ratio of Ni(II) ions to a sericin unit was below 0.07. In the solid sample of the Ni(II)-silk sericin complex obtained from the solution at pH 11.5, measurements of a FT-IR spectrum and a X-ray diffraction pattern showed that the secondary structure was changed from the β-structure to the random coil. And the DSC measurement indicated that the thermal stability of silk sericin was increased as a result of the complexation with Ni(II) ions. © 1994, The Society of Polymer Science, Japan. All rights reserved.