Solid-state NMR studies on the supersecondary structures in fibrous proteins

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^<13>C CP/MAS NMR spectra of tropomyosin were measured in order to elucidate the higher order structure through the ^<13>C NMR chemical shifts of the amino acid residues and their mobility. The higher order structure of tropomyosin is a coiled-coil structure, and contains two different sites which are characterized as the internal and external sites. The ^<13>C signal which appear at 15-17 ppm can be easily assigned to the Ala C^<beta> carbons, and it can be seen that this signal was consisted of two peaks. The peak at 15.8 ppm have a longer T_1 value than those at 16.7 ppm. This is indicated that the former carbons are more mobile than the latter carbons. Therefore, the two peaks at 15.8 and 16.7 ppm are assigned to the Ala C^<beta> carbons in the extermal and internal sites of the coiled-coil structure, respectively. Wool keratin can be divided into three main fractions after reducing disulfide bonds and protection of the resulting thiol groups with iodoacetic acid to form S-carboxymethyl kerateine (SCMK). From the ^<13>C CP/MAS NMR spectra of wool and SCMK,it was suggested that the coiled-coil structure exists because the ^<13>C chemical shift values of Ala C^<beta> carbons in these samples coincide in experimental error with that of the Ala residue located in the internal site of the coiled-coil structure in tropomyosin. The conformational analysis using the conformation-dependent ^<13>C CP/MAS NMR chemical shifts were applied to estimate the conformational transition of SCMK by stretching, heating, or steam-treating. The beta-sheet form appears by stretching and steam-treating. For the heated SCMK,it can be said that the random coil form appears. However, the fact that the a-helix form remains to an appreciable extent in the heated one although the random coil form appears, suggests that only the packing of the coiled-coil structure in the SCMK is disrupted by heating, but secondary structure being retained.