Changes in Natural Silk Fibres by Hydration, Tensile Loading and Heating as Studied by (1)H NMR: Anisotropy in NMR Relaxation Times

B. mori silkworm natural silk is a fibrous biopolymer with a block copolymer design containing both hydrophobic and hydrophilic regions. Using (1)H NMR relaxation, this work studied B. mori natural silk fibres oriented at 0° and 90° to the static magnetic field B(0) to clarify how measured NMR parameters reflect the structure and anisotropic properties of hydrated silk fibres. The FTIR method was applied to monitor the changes in the silk I and β-sheet conformations. Unloaded B. mori silk fibres at different hydration levels (HL), the silk threads before and after tensile loading in water, and fibres after a stepped increase in temperature have been explored. NMR data discovered two components in T(1) and T(2) relaxations for both orientations of silk fibres (0° and 90°). For the slower T(2) component, the results showed an obvious anisotropic effect with higher relaxation times for the silk fibres oriented at 90° to B(0). The T(1) component (water protons, HL = 0.11) was sequentially decreased over a range of fibres: 0° oriented, randomly oriented, silk B. mori cocoon, 90° oriented. The degree of anisotropy in T(2) relaxation was decreasing with increasing HL. The T(2) in silk threads oriented at 0° and 90° also showed anisotropy in increased HL (to 0.42 g H(2)O/g dry matter), at tensile loading, and at an increasing temperature towards 320 K. The changes in NMR parameters and different relaxation mechanisms affecting water molecular interactions and silk properties have been discussed. The findings provide new insights relating to the water anisotropy in hydrated Bombyx mori silk fibres at tensile loading and under a changing HL and temperature.