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Spectroscopic studies (nuclear magnetic resonance, circular dichroism and infrared) have been carried out on chicken erythrocyte histone H5 and on three peptides cleaved therefrom: 1-31, 32-197 and 58-197. It is shown that at ionic strengths above o.1M part of the H5 molecule takes up a globular conformation containing 14% alpha helix but no beta sheet structure. Several details of the circular dichroism and nuclear magnetic resonace spectra indicate that the globular region is located in the N-terminal half of the molecule and this proposal is supported by the observation that the peptide 32-197 is largely incapable of folding and the peptide 59-197 is completely incapable of folding. Structural similarities and differences between histone H5 and histone H1 are discussed.

The conformation and stability of artificial complexes between chicken erythrocyte DNA and homologous histones FV and F2a2 was studied by circular dichroism (CD) and thermal denaturation followed by both absorbance and CD measurements. The complexes are made after a stepwise potassium fluoride gradient dialysis without urea and studied at low ionic strength (10−3 M). 1. 1) No structural changes of the DNA can be detected up to r = 0.2 with FV and r = 0.6 for F2a2. With FV at higher values of r the CD spectrum is altered, indicating the organization of DNA and histones in some kind of aggregate. 2. 2) The conformation of histone molecules inside the complexes is not related to the ionic strength of the medium but to an effective ionic environment close to 0.1 M. This ionic strength would also correspond to the melting temperature of histone-covered DNA. 3. 3) From the analysis of the absorbance melting profile the length of DNA covered with an histone molecule can be estimated. A good agreement is found between the negative charge of this piece of DNA and the net positive charge of the histone. 4. 4) Since the CD transition at 227 nm occurs before the second absorbance transition at 280 nm, the DNA is stabilized no longer by native histone but partially or fully denatured histones. The helical regions of the histone molecule are not involved in the binding process, which appears to be almost purely coulombian and most likely related to some structural fit between the pattern of negative charges in the DNA helix and that of positive charges along the peptide chain.

Reconstituted nucleohistones were obtained by mixing in given conditions acid extracted histones and eukaryotic DNA. The histone/DNA ratio (w/w) was in the range 0.35 - 0.95. With the four histones (H2A2B) we have been able to obtain subunits (nucleosomes or upsilon-bodies). The variation of cirsular dichroism signal with temperature at 280 nm was measured to follow structural changes of the DNA inside the complex. The true change of ellipticity (see article) of histone-bound DNA regions, is similar for reconstituted nucleohistone and H1-depleted chromatin, and is therefore a physical probe of the presence of nucleosomes.