NMR OF SILK FIBROIN 11. ¹H NMR ANALYSIS OF WATER ORIENTATION IN POROUS SILK FIBROIN MEMBRANE

Abstract

The physical state and orientation of the water trapped in porous Bombyx mori silk fibroin membranes were examined in the absence and presence of alkali metal salts, NaCl, KCl and LiCI by means of NMR spectroscopy. In spite of the high content of water (93%) in the membrane, doublet ¹H NMR peaks of the water molecules were observed and its separation depended strongly on the angle, θ, between the membrane surface and the magnetic field. This trend was independent of the presence of alkali metal ions in the water. From the relationship between the separation of the doublet and the angle, θ, the orientation of the proton-proton axis of the water molecule was found, on the average, parallel to the surface of the membrane. ¹H NMR peaks of the water and also, ²³Na NMR peaks of the NaCl aqueous solution trapped in the membrane shifted to lower field with increasing angle, θ, indicating the anisotropy of the silk fibroin membrane itself. The diamagnetic susceptibility of the membrane was calculated to be -0.64ppm.