Laser Raman spectra of calf thymus histones H1, H2A, and H2B

Laser Raman spectra of the calf thymus histones H1, H2A, and H2B in aqueous solutions are presented. The amide III band in the spectrum of the very lysine-rich histone H1 in aqueous solution appears at 1245 cm-1, which is almost at the same frequency as the corresponding vibration of the ionized form of poly(L-lysine). Upon increasing the NaCl concentration to 1 M, the frequency of the amide III vibration shifts to 1250 cm-1 as a result of the formation of a more compact disordered structure of at least the N-terminal region of the protein. Changing the pH from 3 to 5 induces the same frequency shift. The amide III regions of the Raman spectra of the slightly lysine-rich histones H2A and H2B shows two bands at 1247 and 1265 cm-1 for H2A, and at 1254 and 1265 cm-1 for H2B. These doublets are attributed to vibrations involving the backbone of at least two structurally distinct parts of the histone molecules. The low frequency component is assigned to the random-coil regions of the proteins which appear to have similar conformations for H1 and H2A. The frequency of this component also suggest that the structure of the disordered regions of H2B are more compact and less extended. These conclusions confirm the conformation predictions based on the primary structures of these proteins. The high frequency component at 1265 cm-1 is assigned to the alpha-helical and rigid disordered structures of H2A and H2B, since this band increases in intensity upon addition of NaCl. The amide I' region of the histone spectra is also presented but appears to be much less sensitive to the conformation than the amide III region. The intensity of the bands due to the single bond C-C stretching modes, as well as the intensity ratio of the tyrosine Fermi doublet at 855 and 830 cm-1, are also discussed.