ESR study on the structure and molecular motion of spin-labelled silk fibroins.
Abstract
The ESR spectra of the Bombyx mori and Philosamia cynthia ricini silk fibroins spin-labelled the tyrosine side chain groups with nitroxide compounds are observed. The activation energy of the B. mori silk fibroin in aqueous solution obtained from the plot of the rotational correlation times, τc, determined from the ESR spectra against temperature-1 is 4.46 Kcal mol-1 in the temperature range of 20_??_90°C. The activation energy is also determined as 4.55 Kcal mol-1 for P. c. ricini silk fibroin in aqueous solution in the temperature range of -5_??_40°C, while a new broad peak component (τc>10-8 s) with the activation energy, 4.05 Kcal mol-1 is observed above 50°C other than main peak component. Such a broad component corresponds to the β-structure of the silk fibroin, indicates the ease of occurrence of the β-structure in P. c. ricini silk fibroin compared with B. mori silk fibroin. For the membranes of these silk fibroins, the activation energies of the rotational orrelation time are obtained as 4.53 Kcal mol-1 (for B. mori) and 2.74 Kcal mol-1 (for P. c. ricini), indicating the thermal stability of the P. c. ricini silk fibroin membrane compared with B. mori silk one. The structures and thermal properties of these membranes are essentially unchanged by the spin-labelling treatment.