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- W191735270 abstract "Spectroscopic studies have been made of whole histone and histone fractions from calf thymus and chicken erythrocyte. Optical rotatory dispersion studies show that in water the lysine rich fractions F1 and Ib are random while the moderately lysine rich and arginine rich fractions have a low helix content (of the order of 10%). Increasing the salt molarity (NaCl) or pH of aqueous solutions causes a marked increase in helix content to about 25–30% from the fractions other than the lysine rich fractions which show an increase of only 10%. The fractions are capable of a higher helix content as is shown by the much enhanced value of 66% and 85% for solutions in 2-chloroethanol and trifluorethanol respectively. Again the effect was much less marked for the lysine rich fractions and it concluded that these fractions show a much reduced tendency to form helical conformations in solution. This is supported by infrared studies of films of the histone fractions cast from the helix promoting solvents which show the absence of extended β-material for all fractions except the lysine rich fractions. For the latter a high proportion of β-material was present in all samples.High resolution nuclear magnetic resonance spectroscopic studies also indicate an increase in rigid conformations in salt solutions for all histone fractions except the lysine-rich fraction. Resonances at approximately 9 r, due to hydrocarbon sidechain terminal methyl groups in leucines and the smaller number of valine and isoleucines, and at 8.7r due to methylene hydrogens of leucine and the methyl hydrogens of alanine are greatly reduced in area on increasing salt molarity of aqueous solution. Since this effect is accompanied by an increase in helix content to about 25–30% it is suggested that the apolar residues such as alanine, leucine, isoleucine and valine are involved in helical regions, and furthermore since the increase in helix content with salt molarity is presumably due to a reduction in the electrostatic repulsion energy of like charged groups there must be regions of the polypeptides chains involving a high proportion of the apolar groups mentioned above and charged groups.Spectroscopic and deuteration studies of partial nucleohistones were also made. Deuteration of nucleohistone has revealed a slowly exchanging fraction of the labile protons which have been ascribed to imide protons in the polypeptide backbone shielded by being in some stable regular structure. The intensity of the N-H stretching band at 3300 cm-1 has been taken as a measure of the amount of the slowly exchanging amide protons and this has been determined for several series of partial nucleoproteins. It is found that the intensity of this band does not diminish until 20–25% of the histone has been removed from the complex. From this it is evident that in the first 20–25% of the histone to be removed, which form the lysine rich fraction, none of the labile protons are shielded in any way. Hence this fraction is non-helical and possibly takes up an extended form to facilitate cross linking in the nucleohistone complex.From polarized infrared studies of orientated films of partial nucleohistone it is found that the residual N-H stretching band of the slowly deuterating protein component is almost non-dichroic and exhibits only a very low parallel dichroism. This indicates that if the helical components of the histone are ordered relative to the axis of the DNA molecule then they must lie at an angle of about 54° to this axis and probably lie in one of the grooves of the DNA molecule. It is found, also, with the most tightly bound histones that the rapidly deuterating protein component also gives rise to dichoric bands. The dichroism of the amide ‘I’ band is parallel and would indicate that there is an extended portion of polypeptide chain which runs more parallel to the DNA axis than the helical component. These studies demonstrate that there is a definite spatial relationship between DNA and the most tightly bound histones." @default.
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- W191735270 date "1967-01-01" @default.
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- W191735270 title "Conformational Studies of Histones and Nucleohistones" @default.
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- W191735270 doi "https://doi.org/10.1016/b978-1-4832-2843-3.50014-5" @default.
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