The Raman spectra of aqueous solutions of histones H3 and H4 from calf thymus and from rye reflect the high degree of conservation from species to species of the primary and secondary structures of these proteins. The amount of β-sheet structure is estimated at 40 ± 5% in H4 and at 33 ± 5% in H3 from the intensities of the amide I and amide III bands at 1663 and 1241 cm−1, respectively, in the spectra. These values are independent of the salt concentration of the solutions, most likely because of the high histone concentration (~3 mM) required to obtain the spectra, which results in some aggregation of the proteins. The intensity ratio of the tyrosine doublet at 852 and 826 cm−1 indicates that the four tyrosine residues in H4 are relatively exposed to the solvent or weakly bound to positively charged groups of basic amino acids, whereas in H3 at least one tyrosine is buried inside the protein and tightly bound to a carboxylate group. The results also show that the secondary structure of H3 is slightly influenced by the state of oxidation of the two cysteine residues it contains.
Domain swapping of a llama VHH domain builds a crystal-wide β-sheet structure