Distinctive influence of two hexafluoro solvents on the structural stabilization of Bombyx mori silk fibroin protein and its derived peptides:: 13C NMR and CD studies

Employing high-resolution C-13 solution NMR and circular dichroism (CD) spectroscopic techniques, the distinctive influence of two intimately related hexafluoro solvents, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and hexafluoroacetone trihydrate (HFA), on the structural characteristics of Bombyx mori (B. mori) silk fibroin, the chymotrypsin precipitate (C-p) fraction, and two synthetic peptides, (AGSGAG)(5) and (AG)(15), is described. The observed C-13 solution NMR and CD spectra of these polypeptides in HFIP and HFA revealed a distinctive influence on their conformational characteristics. The C-13 NMR spectra, as analyzed from the unique chemical shifts of C-alpha and C-beta resonances of constituent residues revealed that fibroin largely assumes helical conformation(s) in both solvents. However, the peak shifts were greater for the samples in HFIP, indicating that the types of helical structure(s) may be different from the one populated in HFA. Similar structural tendencies of these polypeptides were reflected in CD spectra. The observed CD patterns, i.e., a strong positive band at similar to 190 nm and negative bands at similar to 206 and 222 nm, have been attributed to the preponderance of helical structures. Of the two prevalent helical structures, alpha-helix and 3(10)-helix, the evidence emerged for the fibroin protein in favor of 3(10)-helical structure stabilization in HFIP and its significant disruption in HFA, as deduced from the characteristic R1 (=[theta](190)/[theta](202)) and R2 (=[theta](222)/[theta](206)) ratios, determined from the CD data. Conversely, the native polypeptides and synthetic peptide fragments derived from highly crystalline regions of the silk fibroin protein sustained predominantly an unordered structure in HFA solvent.