PH-DEPENDENCE OF THE COILED-COIL STRUCTURE OF KERATIN INTERMEDIATE FILAMENT IN HUMAN HAIR BY C-13 NMR-SPECTROSCOPY AND THE MECHANISM OF ITSDISRUPTION

C-13 NMR spectra of the low sulfur Fraction in S-(carboxymethyl)kerati ne (SCMKA) which corresponds to the hard-keratin intermediate filament (KIF) in human hair have been observed as a function of pH to clarify its disruption mechanism. The assignment was performed by the amino a cid composition, both the distortionless enhancement by polarization t ransfer (DEFT) spectra and the chemical shift values of the SCMKA samp le in 8 M urea solution. C-13 NMR spectra at pH 5.0 and 6.0 contain es sentially no peaks from the amino acid residues in the rod domain, whi ch is due to its coiled-coil structure having highly restricted mobili ty. The coiled-coil structure was disrupted between pH 6.0 and 7.0 alo ng with great great increase in peak intensities, which indicated the random coil structure occurred. During this disruption process, single chains with a helical form could not exist because there were no heli cal peaks in the spectra, or they existed for only a very short time, even if they did. Especially, the peak intensities of the side chains of the negatively charged amino acids, Glu and Asp, and those of the p ositively charged amino acids, Lys and Arg, increased abruptly at arou nd pH 7.0, and these side chains formed ion-pairing interactions maint aining the coiled-coil structure in the rod domain. The peak intensiti es of the side chains of Leu and lie also increased abruptly, indicati ng that hydrophobic interactions among these side chains in the coiled -coil structure were weakened. When the pH of the SCMKA solution was r eadjusted to pH 6.0 from 9.6, the C-13 NMR Spectrum was almost identic al to that obtained originally at pH 6.0. Thus, both interactions are considered to contribute to the stability of the coiled-coil structure in the rod domain.